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digitalmars.D.bugs - [Issue 6169] New: [CTFE] pure functions cannot compute constants using functions not marked as pure

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=6169

           Summary: [CTFE] pure functions cannot compute constants using
                    functions not marked as pure
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: timon.gehr gmx.ch


--- Comment #0 from timon.gehr gmx.ch 2011-06-16 17:09:25 PDT ---
With DMD 2.053:

string impure(){return ";";}

void main() pure{
    enum s = impure(); // fail (cannot call impure function 'impure')
    mixin(impure());   // ditto
}

Removing the pure attribute from 'main' or adding it to 'impure' makes the code
pass.
This restriction is nonsensical and should be removed.

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next sibling parent d-bugmail puremagic.com writes:
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bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc


--- Comment #1 from bearophile_hugs eml.cc 2011-06-16 18:42:49 PDT ---
I think you have just made D a bit more complex :-)

In D compile time and run time are fully separated (and computing an enum
inside a CTFE function spans a fully separated and fully enclosed
sub-computation), so there is no way compile-time constants can break the
purity of a pure function. So this looks OK regarding run-time purity.

Currently in D in a function the path of compile-time execution has to be pure,
even if the whole function is not pure, so you are right saying this program
has to compile:


int x = 10;
int foo(bool b) {
    if (b)
        x++;
    return 0;
}
pure void main() {
    enum y = foo(false);
}


If in future D compilers CTFE will be allowed to modify global variables too,
then I think your idea is in troubles. Otherwise at first sight it seems OK,
but more thinking is required because future D compilers are allowed to perform
pure-related optimizations on pure functions even in CTFE. Is nonpurity able to
cause troubles to pure functions at compile-time?

In this program spam is pure, and it calls bar, that's not pure, to compute z.
But this program doesn't cause troubles even if a smart D compiler applies pure
optimization of spam()+spam() replacing it with spam()*2 because z is computed
only once, because bar() is called in a sub-computation that's fully sealed:


pure nothrow int foo() {
    int x = 1;
    nothrow int bar(int y) { // nonpure
        x++;
        return x + y;
    }

    pure nothrow int spam() {
        enum z = bar(1); // calls a nonpure
        return z;
    }
    return spam() + spam();
}

enum r = foo();
void main() {}


So if I am right, then this proposal is safe :-)

One problem left is that this proposal introduces another special case in D,
because the rules of purity have to say a pure function is allowed to call an
impure one at compile-time. Is it worth it? I think it's acceptable.

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au


--- Comment #2 from Don <clugdbug yahoo.com.au> 2011-06-17 01:02:22 PDT ---
Applies to  safe as well.
CTFE enforces safety and purity, using more relaxed rules than  safe and pure
do.

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--- Comment #3 from timon.gehr gmx.ch 2011-06-17 06:57:25 PDT ---
Interestingly, it does not apply to nothrow unless interpreting the function
fails :). I think this is a diagnostics bug:

int foo(){return 0;}
int bar(){assert(0);}
void main() nothrow{
    enum f = foo(); // fine
    enum b = bar(); // assert(0) failed / bar is not nothrow 
                    // main is nothrow yet may throw
}



(In reply to comment #1)
 ...
 If in future D compilers CTFE will be allowed to modify global variables too,
 then I think your idea is in troubles.
 [snip.]

I think letting CTFE mutate static storage is a bad idea anyways, but actually that does not matter for this. You are just computing some manifest constant during compile time that is later used to influence the function's behavior. This cannot possibly make the function return different results when passed the same arguments (even during compile time), ergo it is still pure.
 ...
 One problem left is that this proposal introduces another special case in D,
 because the rules of purity have to say a pure function is allowed to call an
 impure one at compile-time. Is it worth it? I think it's acceptable.
 [snip.]

Actually I think this is the same 'special case' as the one that says manifest constants are evaluated during compile time. The behavior we have now is a special case of this 'special case'. I think we remove some special casing (and therefore complexity) by fixing this. Oh, and this should work too: string impure(){return ";";} void main() pure{static s = impure();} -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                 CC|                            |yebblies gmail.com
         AssignedTo|nobody puremagic.com        |yebblies gmail.com


--- Comment #4 from yebblies <yebblies gmail.com> 2012-01-29 15:27:06 EST ---
https://github.com/D-Programming-Language/dmd/pull/652

Please test and review if you're interested.

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--- Comment #5 from yebblies <yebblies gmail.com> 2012-04-28 15:02:07 EST ---
*** Issue 7994 has been marked as a duplicate of this issue. ***

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monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra gmail.com


--- Comment #6 from monarchdodra gmail.com 2013-01-31 02:27:40 PST ---
Just wanted to add that I just hit this bug. Any news on the progression of
this bug fix?

I'm hitting this on a rather trivial use case, where I'm just trying to
generate a compile-time-known error message:

//----
import std.string : format;

struct S
{
    int* p;
    ref inout(int) get() inout nothrow pure  safe
    {
        enum message = format("Called %s on null %s.", "get", S.stringof);
        assert(p, message);
        return *p;
    }
}
//----

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--- Comment #7 from yebblies <yebblies gmail.com> 2013-01-31 23:03:40 EST ---
This bug is exactly where it has been for the last year - awaiting approval
from Walter.

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh quickfur.ath.cx


--- Comment #8 from yebblies <yebblies gmail.com> 2013-02-17 12:47:19 EST ---
*** Issue 9517 has been marked as a duplicate of this issue. ***

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--- Comment #9 from github-bugzilla puremagic.com 2013-05-04 09:48:17 PDT ---
Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/42d1af1635fec1c43bbda6c4a3894e148ca6fc22
Fix Issue 6169 - [CTFE] pure functions cannot compute constants using functions
not marked as pure

When running semantic on an expression used anywhere that forces compile time
evaluation, use a scope flag to prevent purity and safety checks on function
calls.
This allows better purity/safety inferrence as well.

https://github.com/D-Programming-Language/dmd/commit/15a57832d0a9ed18eb619c58261db6c2eedc663e
Merge pull request #652 from yebblies/issue6169

Issue 6169 - [CTFE] pure functions cannot compute constants using functions not
marked as pure

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--- Comment #10 from bearophile_hugs eml.cc 2013-05-04 11:11:40 PDT ---
(In reply to comment #1)
 pure nothrow int foo() {
     int x = 1;
     nothrow int bar(int y) { // nonpure
         x++;
         return x + y;
     }
 
     pure nothrow int spam() {
         enum z = bar(1); // calls a nonpure
         return z;
     }
     return spam() + spam();
 }
 
 enum r = foo();
 void main() {}

A reduction of that code: int foo() { int x = 0; int bar() { return x; } enum y = bar(); return 0; } enum r = foo(); void main() {} It gives: temp.d(4): Error: variable x cannot be read at compile time temp.d(6): called from here: bar() temp.d(9): called from here: foo() -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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--- Comment #11 from yebblies <yebblies gmail.com> 2013-05-11 20:51:44 EST ---
(In reply to comment #10)
 
 A reduction of that code:
 
 
 int foo() {
     int x = 0;
     int bar() {
         return x;
     }
     enum y = bar();
     return 0;
 }
 enum r = foo();
 void main() {}
 
 
 It gives:
 
 temp.d(4): Error: variable x cannot be read at compile time
 temp.d(6):        called from here: bar()
 temp.d(9):        called from here: foo()

That appears to be correct. While both 'foo' and 'bar' are running at compile time, they are in different compile time evaluation contexts. The evaluation of 'y' _must_ be independent of the running of 'foo'. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
May 11 2013
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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #12 from yebblies <yebblies gmail.com> 2013-07-01 15:33:39 EST ---
This was fixed ages ago.

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich gmail.com


--- Comment #13 from yebblies <yebblies gmail.com> 2013-07-01 15:34:45 EST ---
*** Issue 10506 has been marked as a duplicate of this issue. ***

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monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |


--- Comment #14 from monarchdodra gmail.com 2013-07-01 00:39:34 PDT ---
Not fully fixed for  safe. The conditions to reproduce are a bit complicated
actually. It requires attribute inference, mixin and default args (!) I'm not
sure which it is that it producing the problem:

--------
string bar(string op = "+")  property
{
    return "a" ~ op ~ "b";
}

void foo()()
{
    int a, b;
    int c = mixin(bar);
}

 safe void main()
{
    foo!()();
}
--------
main.d(14): Error: safe function 'D main' cannot call system function
'main.foo!().foo'
--------

Observations:
1) The problem is only with  safe, not pure.
2) Calling "min(bar("+"))" also makes the problem go away.

Built with HEAD from 30-06-2013

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--- Comment #15 from Kenji Hara <k.hara.pg gmail.com> 2013-07-01 08:01:17 PDT
---
(In reply to comment #14)
 Not fully fixed for  safe. The conditions to reproduce are a bit complicated
 actually. It requires attribute inference, mixin and default args (!) I'm not
 sure which it is that it producing the problem:

https://github.com/D-Programming-Language/dmd/pull/2290 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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--- Comment #16 from github-bugzilla puremagic.com 2013-07-07 21:09:32 PDT ---
Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/c2647ba31e7a60ccc354e67aad3a05ceaf7b755e
Remain fix for issue 6169

Add ctfeResolveProperties() which avoids purity and safety check

https://github.com/D-Programming-Language/dmd/commit/d895e4e8463de991c7b3ea723803374d6f9bf36a
Merge pull request #2290 from 9rnsr/fix6169

Remain fix for issue 6169

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED


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