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digitalmars.D - SIMD ideas for Rust

reply "bearophile" <bearophileHUGS lycos.com> writes:
A small blog post about SIMD ideas for Rust language:

http://blog.aventine.se/post/55669497784/my-vision-for-rust-simd

Some info on one of the discussed intrinsics, _mm_mul_epu32:
http://msdn.microsoft.com/en-us/library/f3d9e6fk%28v=vs.90%29.aspx

Bye,
bearophile
Jul 17 2013
parent reply Manu <turkeyman gmail.com> writes:
Interesting. Almost all his points are what we do already in D.
Always nice to see others come to the same conclusions :)


On 18 July 2013 11:15, bearophile <bearophileHUGS lycos.com> wrote:


 A small blog post about SIMD ideas for Rust language:

 http://blog.aventine.se/post/**55669497784/my-vision-for-**rust-simd<http://blog.aventine.se/post/55669497784/my-vision-for-rust-simd>

 Some info on one of the discussed intrinsics, _mm_mul_epu32:
 http://msdn.microsoft.com/en-**us/library/f3d9e6fk%28v=vs.90%**29.aspx<http://msdn.microsoft.com/en-us/library/f3d9e6fk%28v=vs.90%29.aspx>

 Bye,
 bearophile
Jul 18 2013
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Manu:


 Interesting. Almost all his points are what we do already in D.
 Always nice to see others come to the same conclusions :)


There is also some discussion here:
http://www.reddit.com/r/rust/comments/1igvye/vision_for_rust_simd/

Regarding SIMD, few days ago I have added some notes in this 
thread, that are significant:
http://forum.dlang.org/thread/kpt0ja$1fk6$1 digitalmars.com?page=2

Bye,
bearophile
Jul 18 2013
prev sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Manu:


 Interesting. Almost all his points are what we do already in D.
 Always nice to see others come to the same conclusions :)


While trying to write a multiplication of two complex numbers 
using SSE3 with LDC2 I have found about seven or more bugs, that 
I will discuss elsewhere. But regarding the syntax, in nice code 
like this D requires to add ".array" before all those subscripts 
(code adapted from Fog):


double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = [b.array[1], b.array[0]];
     double2 a_im = [a.array[1], a.array[1]];
     double2 a_re = [a.array[0], a.array[0]];
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;
     return [arb.array[0] - aib.array[0], arb.array[1] + 
aib.array[1]];
}


A line like this:

double2 b_flip = [b.array[1], b.array[0]];

becomes something like:

pshufd   $238,  %xmm1, %xmm3

Similarly all the other lines become single instructions (but the 
last one, because LDC2 misses to use a addsubpd).

I vaguely remember you saying that slow SIMD operations shouldn't 
have a too much short syntax to avoid giving an illusion of 
efficiency. But given that "often" the CPU executes such array 
subscripting and shuffling efficiently, isn't it nicer/enough to 
support a simpler syntax like this in D?

double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = [b[1], b[0]];
     double2 a_im = [a[1], a[1]];
     double2 a_re = [a[0], a[0]];
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;
     return [arb[0] - aib[0], arb[1] + aib[1]];
}


Bye,
bearophile
Jul 19 2013
parent reply Manu <turkeyman gmail.com> writes:
On 19 July 2013 19:33, bearophile <bearophileHUGS lycos.com> wrote:


 Manu:

  Interesting. Almost all his points are what we do already in D.

 Always nice to see others come to the same conclusions :)


 While trying to write a multiplication of two complex numbers using SSE3
 with LDC2 I have found about seven or more bugs, that I will discuss
 elsewhere. But regarding the syntax, in nice code like this D requires to
 add ".array" before all those subscripts (code adapted from Fog):


 double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = [b.array[1], b.array[0]];
     double2 a_im = [a.array[1], a.array[1]];
     double2 a_re = [a.array[0], a.array[0]];
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;
     return [arb.array[0] - aib.array[0], arb.array[1] + aib.array[1]];
 }


 A line like this:

 double2 b_flip = [b.array[1], b.array[0]];

 becomes something like:

 pshufd   $238,  %xmm1, %xmm3

 Similarly all the other lines become single instructions (but the last
 one, because LDC2 misses to use a addsubpd).

 I vaguely remember you saying that slow SIMD operations shouldn't have a
 too much short syntax to avoid giving an illusion of efficiency. But given
 that "often" the CPU executes such array subscripting and shuffling
 efficiently, isn't it nicer/enough to support a simpler syntax like this in
 D?

 double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = [b[1], b[0]];
     double2 a_im = [a[1], a[1]];
     double2 a_re = [a[0], a[0]];
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;
     return [arb[0] - aib[0], arb[1] + aib[1]];
 }


The point about eliminating the index operator is because it implies a
vector->float cast.
You want to perform a shuffle(/swizzle), but you are only really performing
the operation incidentally.
What you're really doing is casting a bunch of vector components to floats,
and then rebuilding a vector, and LLVM can helpfully deal with that.

I would suggest calling a spade a spade and using a swizzle function to
perform a swizzle, instead of code like what you wrote.
Wouldn't this be better:

double2 complexMult(in double2 a, in double2 b) pure nothrow {
    double2 b_flip = b.yx; // or b.swizzle!"yx", if we don't want to
include an opDispatch in the basic type
    double2 a_im = a.yy;
    double2 a_re = a.xx;
    double2 aib = a_im * b_flip;
    double2 arb = a_re * b;

//    return [arb[0] - aib[0], arb[1] + aib[1]]; // this final line is
tricky... it's not very portable.

    // Maybe:
    return select([-1, 0], arb-aib, arb+aib);
    // Hopefully the x86 optimiser will generate the proper opcode. Or a
bunch of other options; a multi-vector shuffle, shift, swizzle, interleave.
}

I think that would be better. More portable, and it eliminates the code
that implies a vector->float->vector cast sequence, which I maintain,
should be syntactically discouraged at all costs.
You don't want to be giving people bad ideas that it's reasonable code to
write ;)
Jul 19 2013
next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Manu:


 What you're really doing is casting a bunch of vector 
 components to floats,
 and then rebuilding a vector, and LLVM can helpfully deal with 
 that.

 I would suggest calling a spade a spade and using a swizzle 
 function to
 perform a swizzle, instead of code like what you wrote.
 Wouldn't this be better:

 double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = b.yx; // or b.swizzle!"yx", if we don't 
 want to
 include an opDispatch in the basic type
     double2 a_im = a.yy;
     double2 a_re = a.xx;
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;


I see and you are right.

(If I turn the basic type into a struct containing a double2
aliased-this to the whole structure, the generated code becomes
awful).

A YMM that already contains 8 floats, and probably SIMD registers
will keep growing, maybe to become 1024 bits long. So the swizzle
item names like x y z w will not suffice and some more general
naming scheme is needed.



 //    return [arb[0] - aib[0], arb[1] + aib[1]]; // this final 
 line is
 tricky... it's not very portable.

     // Maybe:
     return select([-1, 0], arb-aib, arb+aib);
     // Hopefully the x86 optimiser will generate the proper 
 opcode. Or a
 bunch of other options; a multi-vector shuffle, shift, swizzle, 
 interleave.
 }

 I think that would be better. More portable, and it eliminates 
 the code
 that implies a vector->float->vector cast sequence, which I 
 maintain,
 should be syntactically discouraged at all costs.
 You don't want to be giving people bad ideas that it's 
 reasonable code to
 write ;)


My experience in writing such kind of code is limited. I will try
your select to see what kind of code LDC2-LLVM generates.

Bye,
bearophile
Jul 19 2013
parent Manu <turkeyman gmail.com> writes:
On 20 July 2013 03:43, bearophile <bearophileHUGS lycos.com> wrote:


 Manu:

  What you're really doing is casting a bunch of vector components to

 floats,
 and then rebuilding a vector, and LLVM can helpfully deal with that.

 I would suggest calling a spade a spade and using a swizzle function to
 perform a swizzle, instead of code like what you wrote.
 Wouldn't this be better:

 double2 complexMult(in double2 a, in double2 b) pure nothrow {
     double2 b_flip = b.yx; // or b.swizzle!"yx", if we don't want to
 include an opDispatch in the basic type
     double2 a_im = a.yy;
     double2 a_re = a.xx;
     double2 aib = a_im * b_flip;
     double2 arb = a_re * b;


 I see and you are right.

 (If I turn the basic type into a struct containing a double2
 aliased-this to the whole structure, the generated code becomes
 awful).

 A YMM that already contains 8 floats, and probably SIMD registers
 will keep growing, maybe to become 1024 bits long. So the swizzle
 item names like x y z w will not suffice and some more general
 naming scheme is needed.



Swizzling bytes already has that problem. Hexadecimal swizzle strings work
nicely up to 16 elements, but past that, I'd probably require the template
receive a tuple of int's.
These are trivial details. .xyzw are particularly useful for 2-4d vectors.
They can be removed for anything higher. The nicest/most preferred
interface can be decided with experience.
As yet there's not a lot of practical experience with >128bit registers,
and the sorts of patterns that appear frequently.

 //    return [arb[0] - aib[0], arb[1] + aib[1]]; // this final line is

 tricky... it's not very portable.

     // Maybe:
     return select([-1, 0], arb-aib, arb+aib);
     // Hopefully the x86 optimiser will generate the proper opcode. Or a
 bunch of other options; a multi-vector shuffle, shift, swizzle,
 interleave.
 }

 I think that would be better. More portable, and it eliminates the code
 that implies a vector->float->vector cast sequence, which I maintain,
 should be syntactically discouraged at all costs.
 You don't want to be giving people bad ideas that it's reasonable code to
 write ;)


 My experience in writing such kind of code is limited. I will try
 your select to see what kind of code LDC2-LLVM generates.


It probably won't be good because I haven't paid attention to how it
optimises on SSE yet.
You need to encourage the compiler to generate ADDSUBPD for SSE, and any
(or none) of the possible expressions may result in it choosing the proper
opcode.
I'm apprehensive to add a helper function for that operation, since it's
dreadfully SSE-specific. It's the sort of thing where you might rather
carefully make sure the standard API will reliably encourage the optimiser
to do it.
If you can find a pattern of operations that optimises to ADDSUBPD, I'm
interested to know what the sequence(/s) are.
If not, we'll consider an explicit function. It can be emulated within
reason on other architectures, but I think it would be better to work a
different solution though. Ie, perform 2 (or 4) side by side (stream
processing)... That will work well on all architectures.
Jul 19 2013
prev sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Manu:


 //    return [arb[0] - aib[0], arb[1] + aib[1]]; // this final 
 line is
 tricky... it's not very portable.

     // Maybe:
     return select([-1, 0], arb-aib, arb+aib);
     // Hopefully the x86 optimiser will generate the proper 
 opcode. Or a
 bunch of other options; a multi-vector shuffle, shift, swizzle, 
 interleave.
 }

 I think that would be better. More portable, and it eliminates 
 the code
 that implies a vector->float->vector cast sequence, which I 
 maintain,
 should be syntactically discouraged at all costs.
 You don't want to be giving people bad ideas that it's 
 reasonable code to
 write ;)


Apparently that select() uses __builtin_ia32_pblendvb128, that is 
a SSE4.1 instruction. At the moment I have only SSE3 :-(

Bye,
bearophile
Jul 23 2013
parent Manu <turkeyman gmail.com> writes:
On 23 July 2013 17:05, bearophile <bearophileHUGS lycos.com> wrote:


 Manu:


  //    return [arb[0] - aib[0], arb[1] + aib[1]]; // this final line is

 tricky... it's not very portable.

     // Maybe:
     return select([-1, 0], arb-aib, arb+aib);
     // Hopefully the x86 optimiser will generate the proper opcode. Or a
 bunch of other options; a multi-vector shuffle, shift, swizzle,
 interleave.
 }

 I think that would be better. More portable, and it eliminates the code
 that implies a vector->float->vector cast sequence, which I maintain,
 should be syntactically discouraged at all costs.
 You don't want to be giving people bad ideas that it's reasonable code to
 write ;)


 Apparently that select() uses __builtin_ia32_pblendvb128, that is a SSE4.1
 instruction. At the moment I have only SSE3 :-(


It's probably better to use a shuf, or a shift for compatibility, since the
selection predicate is constant anyway.
Jul 23 2013