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digitalmars.D - Missed optimisation case - internal use of STCin

reply "Iain Buclaw" <ibuclaw gdcproject.org> writes:
Hi,

I'm currently testing out a GCC optimisation that allows you to 
set call argument flags.  The current assumptions being:

in parameters  =>  Assume no escape, no clobber (read-only).
ref parameters, classes and pointers  =>  Assume worst case.
default  =>  Assume no escape.


See here for implementation details:
http://gcc.gnu.org/ml/fortran/2010-05/msg00032.html


The idea for the 'in' parameters being that if we have the 
following:
--
bool somefunc (in char[] a);


The compiler can assume that whatever parameters are passed, they 
do not changes.  So the compiler can more aggressively optimise 
the following case, for instance.

eg:
--
char[] foo = "bar";
assert(foo == "bar");
somefunc(foo);
assert(foo == "bar");


One problem I've found with this though, is that the front-end 
tends to set all library function parameters as STCin, so the 
optimisation is fundamentally broken:

eg:
--
int[3] arr = [2,3,1];
arr.sort;  // Defined internally as _adSort(in void[], in 
TypeInfo);
assert(arr[0] == 1);  // Compiler infers as false, throws assert.


I think it would be more than reasonable to fix the frontend here 
so that the internal representation better matches that found in 
the internal runtime library.  Just wanted to pass this by you 
guys first.


Regards
Iain.
Apr 19 2014
parent reply "Iain Buclaw" <ibuclaw gdcproject.org> writes:
On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to 
 set call argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


That should read:

ref parameters, inout parameters, classes and pointers.

The default of assuming no escape is an experiment - I may limit 
this to only scalar types, and parameters marked as 'scope'  (So 
long as no one plans on deprecating it soon :)
Apr 19 2014
next sibling parent Artur Skawina via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 
 That should read:
 
 ref parameters, inout parameters, classes and pointers.
 
 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


What does "assume no escape" actually mean?
[The above list doesn't really make sense. W/o context, it's
hard to even tell why, hence the question.]

Also, 'inout' is about constness -- doesn't affect lifetime and
reachability, but does imply no clobbering.

artur
Apr 19 2014
prev sibling next sibling parent Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]

 Also, 'inout' is about constness -- doesn't affect lifetime and
 reachability, but does imply no clobbering.


'out' parameters - I sometimes confuse the two when I have a head cold. :o)
Apr 19 2014
prev sibling next sibling parent Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]


Actually, I might change the default to assume worst case.  I've just
tried this out, which is still valid.

class C {
   int * p;
   this(int x) {
     p = &x; // escapes the address of the parameter.
   }
}


Worse, scope doesn't error on the general case either.

class D {
   int * p;
   this(scope int x) {
     p = &x; // escapes the address of the scope parameter.
   }
}


Do these examples give you a good example?
Apr 19 2014
prev sibling next sibling parent Artur Skawina via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]


 
 Actually, I might change the default to assume worst case.  I've just
 tried this out, which is still valid.
 
 class C {
    int * p;
    this(int x) {
      p = &x; // escapes the address of the parameter.
    }
 }


This might be currently accepted, but it is clearly invalid
(escapes local; the only way to make it work safely would
be to silently copy 'x' to the GC-managed heap, which would be
way too costly). 


   A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.

This is OK (even if ideally 'g' should be forbidden from escaping 'a').

Similarly:

   A f(A a) {
      auto o = register(&a); // can modify 'a'
      o.blah();              // ditto
      doneWith(o);           // ditto
      return a;
   }


What I was wondering was things like whether that "assume no escape"
property was transitive; if /locally/ escaping was disallowed, and
to what extent. What does "assume no escape" mean at all? In your
examples you're mentioning refs together with pointers, that would
only make sense if no-escape were transitive -- but then treating all
args as no-escape would be very wrong.



 Worse, scope doesn't error on the general case either.
 
 class D {
    int * p;
    this(scope int x) {
      p = &x; // escapes the address of the scope parameter.
    }
 }


D's "scope" isn't enforced in any way right now, which means
that code could exist that is invalid, but currently works. It
would break silently(!) when compiled with a decent compiler,
which still doesn't enforce scope.



 Do these examples give you a good example?


I'm worried about a) invalid assumptions making it into GDC;
b) certain valid assumptions making into GDC. The latter because
it could mean that code that's incorrect, but still accepted by
the other compilers could silently break when compiled with GDC.


artur
Apr 19 2014
prev sibling next sibling parent reply Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 Hi,

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]


 Actually, I might change the default to assume worst case.  I've just
 tried this out, which is still valid.

 class C {
    int * p;
    this(int x) {
      p = &x; // escapes the address of the parameter.
    }
 }


 This might be currently accepted, but it is clearly invalid
 (escapes local; the only way to make it work safely would
 be to silently copy 'x' to the GC-managed heap, which would be
 way too costly).


    A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.

 This is OK (even if ideally 'g' should be forbidden from escaping 'a').

 Similarly:

    A f(A a) {
       auto o = register(&a); // can modify 'a'
       o.blah();              // ditto
       doneWith(o);           // ditto
       return a;
    }


 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.



 Worse, scope doesn't error on the general case either.

 class D {
    int * p;
    this(scope int x) {
      p = &x; // escapes the address of the scope parameter.
    }
 }


 D's "scope" isn't enforced in any way right now, which means
 that code could exist that is invalid, but currently works. It
 would break silently(!) when compiled with a decent compiler,
 which still doesn't enforce scope.


People should get bug fixing soon then.  =)



 Do these examples give you a good example?


 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


Invalid assumptions rarely make it into GDC.  The testsuite is a good
bench for this, as well as several projects (now I've got dub set-up)
to test it in the wild.

Saying that, we have had to revert some optimisation cases as D's
schizophrenic nature of enforcing attributes and behaviours is
becoming increasingly dismal.

eg:
- nothrow has *no* guarantee, period, because it still allows
unrecoverable errors being thrown, and allows people to catch said
unrecoverable errors.
- pure is a tough nut to crack also.  The theory should allow you to
be able to cache return values, but in practise...
- The nature of debug statements breaks guarantees of both nothrow and
pure, possibly many more.
- Defining reliable strict aliasing rules, it turns out, is not that
simple (this is something that Walter has mentioned about D should
have good guarantees for, ie: D arrays).

I'm just in investigating all avenues, as I usually do.  There is no
reason why 'in' shouldn't have more powerful guarantees IMO, and what
I've found is not a problem with user code, it's a problem with the
compiler implementation.


For reference, this is the declaration of _adSort in rt.qsort:

extern (C) void[] _adSort(void[], TypeInfo);

This is how the compiler frontend defines the function internally for
its own use:

extern (C) T[] _adSort(in void[], in TypeInfo);
Apr 19 2014
parent reply "Dicebot" <public dicebot.lv> writes:
On Saturday, 19 April 2014 at 14:21:23 UTC, Iain Buclaw via 
Digitalmars-d wrote:

 eg:
 - nothrow has *no* guarantee, period, because it still allows
 unrecoverable errors being thrown, and allows people to catch 
 said
 unrecoverable errors.


Hm, it is hard to find clear answer in spec but I _think_ that 
replacing error throwing with program halting in release mode for 
nothrow function should be valid behavior.
Apr 19 2014
parent reply Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 19 April 2014 15:36, Dicebot via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On Saturday, 19 April 2014 at 14:21:23 UTC, Iain Buclaw via Digitalmars-d
 wrote:

 eg:
 - nothrow has *no* guarantee, period, because it still allows
 unrecoverable errors being thrown, and allows people to catch said
 unrecoverable errors.



 Hm, it is hard to find clear answer in spec but I _think_ that replacing
 error throwing with program halting in release mode for nothrow function
 should be valid behavior.


Nope.  I can't recall the exact code, but an example is in the
testsuite.  It was discovered when porting to ARM, which was found to
omit unwind directives for D nothrow functions, causing runtime hangs
when said 'nothrow' functions infact threw an Error.
Apr 19 2014
parent "Dicebot" <public dicebot.lv> writes:
On Saturday, 19 April 2014 at 15:00:26 UTC, Iain Buclaw via 
Digitalmars-d wrote:

 Nope.  I can't recall the exact code, but an example is in the
 testsuite.  It was discovered when porting to ARM, which was 
 found to
 omit unwind directives for D nothrow functions, causing runtime 
 hangs
 when said 'nothrow' functions infact threw an Error.


Now I am pretty sure _this_ is a druntime bug. I don't know about 
being able to catch actual exception but the fact that unwinding 
is not guaranteed for Errors is mentioned either on dlang.org or 
in TDPL.
Apr 19 2014
prev sibling next sibling parent Artur Skawina via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]


 Actually, I might change the default to assume worst case.  I've just
 tried this out, which is still valid.

 class C {
    int * p;
    this(int x) {
      p = &x; // escapes the address of the parameter.
    }
 }


 This might be currently accepted, but it is clearly invalid
 (escapes local; the only way to make it work safely would
 be to silently copy 'x' to the GC-managed heap, which would be
 way too costly).


    A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.

 This is OK (even if ideally 'g' should be forbidden from escaping 'a').

 Similarly:

    A f(A a) {
       auto o = register(&a); // can modify 'a'
       o.blah();              // ditto
       doneWith(o);           // ditto
       return a;
    }


 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.



 Worse, scope doesn't error on the general case either.

 class D {
    int * p;
    this(scope int x) {
      p = &x; // escapes the address of the scope parameter.
    }
 }


 D's "scope" isn't enforced in any way right now, which means
 that code could exist that is invalid, but currently works. It
 would break silently(!) when compiled with a decent compiler,
 which still doesn't enforce scope.


 
 People should get bug fixing soon then.  =)


Until some kind of diagnostics appear, most of those bugs won't
even be found. It's too easy to write "auto f (in A a)" and then
forget about the implicit 'scope' when modifying the function body.



 Do these examples give you a good example?


 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 
 Invalid assumptions rarely make it into GDC.  The testsuite is a good


AFAICT what you're proposing *is* invalid. I can't be sure because
it's not clear what that "no-escape" property means; that's why I
asked about it twice already...
Clearly, escaping objects reachable indirectly via function arguments
is perfectly fine (eg string slicing), yet you wanted to treat args as
no-escape by default.

Also, treating /some/ types specially wouldn't be ideal;

   struct A { int a; /* no pointers or classes */ }
   struct B { int* b; /*...*/ }

   f(A); // should be treated similarly to 'f(int)'
   f(B); // should be treated similarly to 'f(int*)'

Yes, not doing it is "just" a missed optimization, but in practice
it means that wrapping types becomes even more expensive in D (it's
already almost prohibitively so - eg returning small one-element
structs from functions needlessly happens by ref). 


 bench for this, as well as several projects (now I've got dub set-up)
 to test it in the wild.


These problems will result in invalid optimizations, so can be hard
to trigger and may come and go away randomly.



 Saying that, we have had to revert some optimisation cases as D's
 schizophrenic nature of enforcing attributes and behaviours is
 becoming increasingly dismal.
 
 eg:
 - nothrow has *no* guarantee, period, because it still allows
 unrecoverable errors being thrown, and allows people to catch said
 unrecoverable errors.
 - pure is a tough nut to crack also.  The theory should allow you to
 be able to cache return values, but in practise...


D's "pure" doesn't have much in common with the "normal" pure concept;
exposing gcc's pure via an attribute and completely ignoring D's
version is probably the only practical solution, anything else would
be too costly, result in too small gains, and be too hard to get right. 


 - The nature of debug statements breaks guarantees of both nothrow and
 pure, possibly many more.
 - Defining reliable strict aliasing rules, it turns out, is not that
 simple (this is something that Walter has mentioned about D should
 have good guarantees for, ie: D arrays).


Short term, disabling strict aliasing is the only option. I was scared
of the impact it had before you even started to add support for it a
few years ago (the codegen was already different w/ -fno-strict-aliasing
before, which meant that I immediately had to disable it everywhere...)
There probably does not exist a single D program that respects strict
aliasing rules, other than by chance. The perf gains are minimal globally,
but the potential for silent data corruption is huge.


 I'm just in investigating all avenues, as I usually do.  There is no
 reason why 'in' shouldn't have more powerful guarantees IMO, and what


Of course "scope" (which is part of "in) should be taken advantage of.
I'm concerned about args not marked as "scope" being treated wrongly.

[The scope-related bugs in D programs is a /language/ or frontend problem,
 not a reason to avoid doing the right thing in GDC. It's just that in
 practice what can happen is that GDC will be seen as miscompiling
 "working" D programs...]

artur
Apr 19 2014
prev sibling next sibling parent Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 19 April 2014 17:10, Artur Skawina via Digitalmars-d
<digitalmars-d puremagic.com> wrote:

 On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]


 Actually, I might change the default to assume worst case.  I've just
 tried this out, which is still valid.

 class C {
    int * p;
    this(int x) {
      p = &x; // escapes the address of the parameter.
    }
 }


 This might be currently accepted, but it is clearly invalid
 (escapes local; the only way to make it work safely would
 be to silently copy 'x' to the GC-managed heap, which would be
 way too costly).


    A f(A a) { g(&a); return a; } // likewise with ref instead of pointer.

 This is OK (even if ideally 'g' should be forbidden from escaping 'a').

 Similarly:

    A f(A a) {
       auto o = register(&a); // can modify 'a'
       o.blah();              // ditto
       doneWith(o);           // ditto
       return a;
    }


 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.



 Worse, scope doesn't error on the general case either.

 class D {
    int * p;
    this(scope int x) {
      p = &x; // escapes the address of the scope parameter.
    }
 }


 D's "scope" isn't enforced in any way right now, which means
 that code could exist that is invalid, but currently works. It
 would break silently(!) when compiled with a decent compiler,
 which still doesn't enforce scope.


 People should get bug fixing soon then.  =)


 Until some kind of diagnostics appear, most of those bugs won't
 even be found. It's too easy to write "auto f (in A a)" and then
 forget about the implicit 'scope' when modifying the function body.



 Do these examples give you a good example?


 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 Invalid assumptions rarely make it into GDC.  The testsuite is a good


 AFAICT what you're proposing *is* invalid. I can't be sure because
 it's not clear what that "no-escape" property means; that's why I
 asked about it twice already...
 Clearly, escaping objects reachable indirectly via function arguments
 is perfectly fine (eg string slicing), yet you wanted to treat args as
 no-escape by default.


Not wanted - experimented.  I could not think at the time a use case
where a parameter could escape.  And *I* have shown a very valid use
case to *disprove myself*.  I'm not sure of your continual persistence
of bringing this up as this is no longer relevant to me.  Other than
possibly the silent breaking of 'scope' or 'in' parameters that are
misused (accepts invalid bugs).



 bench for this, as well as several projects (now I've got dub set-up)
 to test it in the wild.


 These problems will result in invalid optimizations, so can be hard
 to trigger and may come and go away randomly.



 Saying that, we have had to revert some optimisation cases as D's
 schizophrenic nature of enforcing attributes and behaviours is
 becoming increasingly dismal.

 eg:
 - nothrow has *no* guarantee, period, because it still allows
 unrecoverable errors being thrown, and allows people to catch said
 unrecoverable errors.
 - pure is a tough nut to crack also.  The theory should allow you to
 be able to cache return values, but in practise...


 D's "pure" doesn't have much in common with the "normal" pure concept;
 exposing gcc's pure via an attribute and completely ignoring D's
 version is probably the only practical solution, anything else would
 be too costly, result in too small gains, and be too hard to get right.


Is what we do.  I think the current conditions for a pure in the C
sense function in D is more or less impossible to achieve.  The only
guarantee that we can cling onto is that the transitive nature of pure
means that functions can be const folded as deep as the pure rabbit
hole goes.


 - The nature of debug statements breaks guarantees of both nothrow and
 pure, possibly many more.
 - Defining reliable strict aliasing rules, it turns out, is not that
 simple (this is something that Walter has mentioned about D should
 have good guarantees for, ie: D arrays).


 Short term, disabling strict aliasing is the only option. I was scared
 of the impact it had before you even started to add support for it a
 few years ago (the codegen was already different w/ -fno-strict-aliasing
 before, which meant that I immediately had to disable it everywhere...)
 There probably does not exist a single D program that respects strict
 aliasing rules, other than by chance. The perf gains are minimal globally,
 but the potential for silent data corruption is huge.


I think the performance gains are worthy of note, most benchmarks tend
to suggest that code is at least 5% slower with strict aliasing
optimisations turned off.



 I'm just in investigating all avenues, as I usually do.  There is no

 reason why 'in' shouldn't have more powerful guarantees IMO, and what


 Of course "scope" (which is part of "in) should be taken advantage of.
 I'm concerned about args not marked as "scope" being treated wrongly.

 [The scope-related bugs in D programs is a /language/ or frontend problem,
  not a reason to avoid doing the right thing in GDC. It's just that in
  practice what can happen is that GDC will be seen as miscompiling
  "working" D programs...]


Indeed.  However my approach may be the reverse here as my attention
span is short lived when it comes to working/moving around the various
areas of gdc (Implement the optimisation, then fix the accepts invalid
upstream), otherwise it may not be for another 2 years till I look at
this again.
Apr 19 2014
prev sibling next sibling parent Artur Skawina via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 17:10, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you to set call
argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit this to only
scalar types, and parameters marked as 'scope'  (So long as no one plans on
deprecating it soon :)


 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]













 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.









 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 Invalid assumptions rarely make it into GDC.  The testsuite is a good


 AFAICT what you're proposing *is* invalid. I can't be sure because
 it's not clear what that "no-escape" property means; that's why I
 asked about it twice already...
 Clearly, escaping objects reachable indirectly via function arguments
 is perfectly fine (eg string slicing), yet you wanted to treat args as
 no-escape by default.


 
 Not wanted - experimented.  I could not think at the time a use case
 where a parameter could escape.  And *I* have shown a very valid use
 case to *disprove myself*.  I'm not sure of your continual persistence
 of bringing this up as this is no longer relevant to me.  Other than


Several reasons. First, I did not realize that you had changed your mind.
Second, I'm just trying to figure out what that 'assume-no-escape'
property implies. From a C based compiler I would have expected a feature
that disallowed address-of on objects, but did not affect any other
object referenced from that one (for handling C's 'register' storage class).
Third, I'm paranoid when it comes to GDC regressions. Once upon the time
I had a working D compiler, one that was able to deliver maybe ~75% of
GCC functionality, and where the main issues were with the language and
frontend, not with the tool. This was a few years ago, after you'd fixed
the many small problems I had run into. The type of those issues made it
very clear that I was the only one that was trying to really use GDC.
Unfortunately what happened then was that things started to get worse, and
by worse I mean "unusable"; GDC ceased to be able to deliver functionality
and performance that was in the same league as GCC C/C++. LTO stopped
working (which is not only crucial for a language w/o macros and low-level
control, but was useful as a work-around for tool issues, like the lack
of cross module inlining). The accepted language changed in a way that
was completely backward incompatible and would have required an extra
preprocessing step (pragma-attributes were removed and became hard
errors). GCC attributes became inaccessible; things like  pure,  malloc,
 ifunc,  align,  cold,  regparm all used to work, but now didn't.  inline,
 noinline and  flatten were added back as /true/ D-style attributes later,
and seem to work great. Thanks for exposing those, but my concern is that
dealing with them one-by-one does not scale - that's why I haven't filed
any bug reports wrt the other ones. Your work on getting D support merged
into GCC is critically important for the survival of the language; things
that are merely optimizations can happen later, or will be mostly irrelevant
(if the merge never happens).
But the situation right now is that the latest GDC version that works is
the gcc4.6 one (IIRC 2.057 based) - I have tried to migrate to a newer one
maybe 5 or 6 times over the last 1.5 year, and always run into some kind of
problem immediately after trying a new build. Like the last time, with the
failure of inlining of certain trivial functions. So when I then read about
a proposed new optimization, which sounds dubious to me, I ask about
more details, as I'm horrified that I could encounter yet another new GDC
specific problem during the next round of the let's-upgrade-gdc experience...



 - Defining reliable strict aliasing rules, it turns out, is not that
 simple (this is something that Walter has mentioned about D should
 have good guarantees for, ie: D arrays).


 Short term, disabling strict aliasing is the only option. I was scared
 of the impact it had before you even started to add support for it a
 few years ago (the codegen was already different w/ -fno-strict-aliasing
 before, which meant that I immediately had to disable it everywhere...)
 There probably does not exist a single D program that respects strict
 aliasing rules, other than by chance. The perf gains are minimal globally,
 but the potential for silent data corruption is huge.


 
 I think the performance gains are worthy of note, most benchmarks tend
 to suggest that code is at least 5% slower with strict aliasing
 optimisations turned off.


I'd like to avoid something like the GCC C strict-aliasing disaster.
String-aliasing helps some code, but breaks some other (legacy) code.
Turning on strict-aliasing by default and recompiling the whole userspace
(hundreds of packages, thousands of sources) is not going to make any
noticeable difference for a desktop user, yet it is likely to trigger
data-corrupting bugs /somewhere/. So the result is practically no perf
gain, but a non-zero chance of corrupting user data, often by programs
that are not perf sensitive at all - that's not a good trade-off.
D doesn't have the MLOCs of legacy code, but I doubt that the D code
that exists was written with strict aliasing in mind. Especially since
the rules are not exactly well defined.

artur
Apr 20 2014
prev sibling next sibling parent Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 20 April 2014 13:19, Artur Skawina via Digitalmars-d <
digitalmars-d puremagic.com> wrote:

 On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 17:10, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you to


















set call argument flags. The current assumptions being:

 in parameters => Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers => Assume worst case.
 default => Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit
















this to only scalar types, and parameters marked as 'scope' (So long as no
one plans on deprecating it soon :)

 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]













 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.









 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 Invalid assumptions rarely make it into GDC. The testsuite is a good


 AFAICT what you're proposing *is* invalid. I can't be sure because
 it's not clear what that "no-escape" property means; that's why I
 asked about it twice already...
 Clearly, escaping objects reachable indirectly via function arguments
 is perfectly fine (eg string slicing), yet you wanted to treat args as
 no-escape by default.


 Not wanted - experimented. I could not think at the time a use case
 where a parameter could escape. And *I* have shown a very valid use
 case to *disprove myself*. I'm not sure of your continual persistence
 of bringing this up as this is no longer relevant to me. Other than


 Several reasons. First, I did not realize that you had changed your mind.
 Second, I'm just trying to figure out what that 'assume-no-escape'
 property implies. From a C based compiler I would have expected a feature
 that disallowed address-of on objects, but did not affect any other
 object referenced from that one (for handling C's 'register' storage


class).

 Third, I'm paranoid when it comes to GDC regressions. Once upon the time
 I had a working D compiler, one that was able to deliver maybe ~75% of
 GCC functionality, and where the main issues were with the language and
 frontend, not with the tool. This was a few years ago, after you'd fixed
 the many small problems I had run into. The type of those issues made it
 very clear that I was the only one that was trying to really use GDC.
 Unfortunately what happened then was that things started to get worse, and
 by worse I mean "unusable"; GDC ceased to be able to deliver functionality
 and performance that was in the same league as GCC C/C++. LTO stopped
 working (which is not only crucial for a language w/o macros and low-level
 control, but was useful as a work-around for tool issues, like the lack
 of cross module inlining).


I have not seen this, infact the opposite.

As for speed of runtime.  Many of the optimisations that were removed in
the last year have been found when porting to architectures that are less
forgiving than x86.

I still don't what's up with LTO. It's not easy to debug, and fails in a
way that doesn't seem to suggest a front-end problem (it chokes when
reading LTO information written by itself).


 The accepted language changed in a way that
 was completely backward incompatible and would have required an extra
 preprocessing step (pragma-attributes were removed and became hard
 errors). GCC attributes became inaccessible; things like  pure,  malloc,
  ifunc,  align,  cold,  regparm all used to work, but now didn't.  inline,
  noinline and  flatten were added back as /true/ D-style attributes later,
 and seem to work great.


Many of these attributes to which need a good reason to be hashed out.
Originally my intention was to only expose those that are impossible to
infer otherwise in D through existing language features.


 Thanks for exposing those, but my concern is that
 dealing with them one-by-one does not scale - that's why I haven't filed
 any bug reports wrt the other ones.


They were exposed one-by-one before too. The entire source of routines was
pretty much a copy-paste job from c-common.c. It has been suggested by the
gcc maintainers to put those attributes into a more common area that can be
shared between C/C++/ObjC and D - but there were too many subtle
differences to make that work at all.  So instead I lopped of the head of
the problem. The same as D iasm support, which was similarly dropped
completely.


 Your work on getting D support merged
 into GCC is critically important for the survival of the language; things
 that are merely optimizations can happen later, or will be mostly


irrelevant

 (if the merge never happens).
 But the situation right now is that the latest GDC version that works is
 the gcc4.6 one (IIRC 2.057 based) - I have tried to migrate to a newer one
 maybe 5 or 6 times over the last 1.5 year, and always run into some kind


of

 problem immediately after trying a new build.


What projects are you having problems with. My experience says that there
are more frontend implementation problems than gdc glue when moving across
several major versions.


 Like the last time, with the
 failure of inlining of certain trivial functions. So when I then read


about

 a proposed new optimization, which sounds dubious to me, I ask about
 more details, as I'm horrified that I could encounter yet another new GDC
 specific problem during the next round of the let's-upgrade-gdc


experience...


The failure of inlining is not a blocker IMO - relying on 'hidden'
features, NRVO being another example, in user code is a bug. Regardless of
the validity.  This particular problem has been noted and it is being dealt
with by improving the relationship between differing variants of const and
mutable types.



 Apr 20 2014



prev sibling next sibling parent  Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com>  writes:


On 20 Apr 2014 13:19, "Artur Skawina via Digitalmars-d" <
digitalmars-d puremagic.com> wrote:

 On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 17:10, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you to


















set call argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may limit
















this to only scalar types, and parameters marked as 'scope'  (So long as no
one plans on deprecating it soon :)

 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]













 What I was wondering was things like whether that "assume no escape"
 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating all
 args as no-escape would be very wrong.









 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 Invalid assumptions rarely make it into GDC.  The testsuite is a good


 AFAICT what you're proposing *is* invalid. I can't be sure because
 it's not clear what that "no-escape" property means; that's why I
 asked about it twice already...
 Clearly, escaping objects reachable indirectly via function arguments
 is perfectly fine (eg string slicing), yet you wanted to treat args as
 no-escape by default.


 Not wanted - experimented.  I could not think at the time a use case
 where a parameter could escape.  And *I* have shown a very valid use
 case to *disprove myself*.  I'm not sure of your continual persistence
 of bringing this up as this is no longer relevant to me.  Other than


 Several reasons. First, I did not realize that you had changed your mind.


It wouldn't be an experiment if I didn't change my mind 30 times. :)


 Second, I'm just trying to figure out what that 'assume-no-escape'
 property implies. From a C based compiler I would have expected a feature
 that disallowed address-of on objects, but did not affect any other
 object referenced from that one (for handling C's 'register' storage


class).

This is an optimisation not built with a C compiler in mind, nor is it
leveraged by C/C++, so I don't think you should think of it in this way.

I was actually made aware of this particular attribute when someone spoke
to a GNU/Fortran maintainer and thought that INTENT(IN) - or at least the
no clobber aspect - could be used to improve optimisations around
'immutable' parameters. Something that gives us the one up that C-family
const could never guarantee (currently there is no difference between const
and immutable).  That is still on the table to do using this attribute
pending investigation.

The no escape aspect leverages on what 'scope' and 'in' should be enforcing.


 Third, I'm paranoid when it comes to GDC regressions. Once upon the time
 I had a working D compiler, one that was able to deliver maybe ~75% of
 GCC functionality, and where the main issues were with the language and
 frontend, not with the tool.


GCC functionality is not to be confused with C/C++ functionality.  And most
of the attributes exposed to C-family languages were nothing more an excess
baggage, ie:  artificial makes no sense when there is no inline keyword.

Anything that needs raising would ideally be in a bug report, as I can't
keep track of issues raised in threads.

Regards
Iain



 Apr 21 2014



prev sibling next sibling parent  Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com>  writes:


On 21 Apr 2014 09:56, "Iain Buclaw" <ibuclaw gdcproject.org> wrote:

 On 20 Apr 2014 13:19, "Artur Skawina via Digitalmars-d" <


digitalmars-d puremagic.com> wrote:

 On 04/20/14 03:00, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 17:10, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 16:21, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 14:33, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 14:37, Iain Buclaw via Digitalmars-d wrote:

 On 19 April 2014 13:02, Artur Skawina via Digitalmars-d
 <digitalmars-d puremagic.com> wrote:

 On 04/19/14 13:03, Iain Buclaw via Digitalmars-d wrote:

 On Saturday, 19 April 2014 at 10:49:22 UTC, Iain Buclaw wrote:

 I'm currently testing out a GCC optimisation that allows you




















to set call argument flags.  The current assumptions being:

 in parameters  =>  Assume no escape, no clobber (read-only).
 ref parameters, classes and pointers  =>  Assume worst case.
 default  =>  Assume no escape.


 That should read:

 ref parameters, inout parameters, classes and pointers.

 The default of assuming no escape is an experiment - I may


















limit this to only scalar types, and parameters marked as 'scope'  (So long
as no one plans on deprecating it soon :)

 What does "assume no escape" actually mean?
 [The above list doesn't really make sense. W/o context, it's
 hard to even tell why, hence the question.]













 What I was wondering was things like whether that "assume no












escape"

 property was transitive; if /locally/ escaping was disallowed, and
 to what extent. What does "assume no escape" mean at all? In your
 examples you're mentioning refs together with pointers, that would
 only make sense if no-escape were transitive -- but then treating












all

 args as no-escape would be very wrong.









 I'm worried about a) invalid assumptions making it into GDC;
 b) certain valid assumptions making into GDC. The latter because
 it could mean that code that's incorrect, but still accepted by
 the other compilers could silently break when compiled with GDC.


 Invalid assumptions rarely make it into GDC.  The testsuite is a










good

 AFAICT what you're proposing *is* invalid. I can't be sure because
 it's not clear what that "no-escape" property means; that's why I
 asked about it twice already...
 Clearly, escaping objects reachable indirectly via function arguments
 is perfectly fine (eg string slicing), yet you wanted to treat args








as

 no-escape by default.


 Not wanted - experimented.  I could not think at the time a use case
 where a parameter could escape.  And *I* have shown a very valid use
 case to *disprove myself*.  I'm not sure of your continual persistence
 of bringing this up as this is no longer relevant to me.  Other than


 Several reasons. First, I did not realize that you had changed your




mind.

 It wouldn't be an experiment if I didn't change my mind 30 times. :)


 Second, I'm just trying to figure out what that 'assume-no-escape'
 property implies. From a C based compiler I would have expected a




feature

 that disallowed address-of on objects, but did not affect any other
 object referenced from that one (for handling C's 'register' storage




class).

 This is an optimisation not built with a C compiler in mind, nor is it


leveraged by C/C++, so I don't think you should think of it in this way.

 I was actually made aware of this particular attribute when someone spoke


to a GNU/Fortran maintainer and thought that INTENT(IN) - or at least the
no clobber aspect - could be used to improve optimisations around
'immutable' parameters. Something that gives us the one up that C-family
const could never guarantee (currently there is no difference between const
and immutable).  That is still on the table to do using this attribute
pending investigation.

 The no escape aspect leverages on what 'scope' and 'in' should be


enforcing.

 Third, I'm paranoid when it comes to GDC regressions. Once upon the time
 I had a working D compiler, one that was able to deliver maybe ~75% of
 GCC functionality, and where the main issues were with the language and
 frontend, not with the tool.


 GCC functionality is not to be confused with C/C++ functionality.  And


most of the attributes exposed to C-family languages were nothing more an
excess baggage, ie:  artificial makes no sense when there is no inline
keyword.

 Anything that needs raising would ideally be in a bug report, as I can't


keep track of issues raised in threads.


As for the original topic.

STCin is an attribute ignored by the compiler (only talking about gdc) -
which is a shame because marking parameters as 'in' I see is still
encouraged.  And I would assume the same is true also for dmd and ldc,
otherwise someone would have noticed this bug by now.



 Apr 21 2014



prev sibling  parent  Artur Skawina via Digitalmars-d <digitalmars-d puremagic.com>  writes:


On 04/20/14 22:11, Iain Buclaw via Digitalmars-d wrote:

 The failure of inlining is not a blocker IMO


It is one in practice. A language with a compiler that can not even inline
this trivial function:

   test   %rdi,%rdi
   sete   %al
   retq   

is not a viable alternative to C.

Not everything can or should be an enum, and D doesn't have macros -- there
is no workaround.

[That's "array.empty". More context, for people not reading the GDC list:
 http://forum.dlang.org/thread/mailman.75.1396605155.19942.d.gnu puremagic.com ]

The bug may be hard to fix, and I really appreciate your effort, but this
definitely *is* a major problem.



 relying on 'hidden' features, NRVO being another example, in user code is a
bug.


Not if RVO is mandated by a spec. Yes, I had trouble with this in the past too.
:)
IIRC an auto-return-type related problem was fixed by Walter some time ago; but
I
still have to maintain the workaround, even after the bug has been fixed
upstream...

artur



 Apr 21 2014