[LLVMdev] AVX Status?

[Bruno Cardoso Lopes]

Hi Ralf

On Wednesday, June 1, 2011, Ralf Karrenberg <Chareos at gmx.de> wrote:
> Hi,
>
> The last time the AVX backend was mentioned on this list seems to be
> from November 2010, so I would like to ask about the current status. Is
> anybody (e.g. at Cray?) still actively working on it?

I don't think so!

> I have tried both LLVM 2.9 final and the latest trunk, and it seems like
> some trivial stuff is already working and produces nice code for code
> using <8 x float>.

Almost everything that could be matched in tablegen files only by
extending the 128-bit PatFrags and PatLeafs to their 256-bit
counterparts should work, but besides that (which is where the
interesting stuff happens) there's no support yet!

> Unfortunately, the backend gets confused about mask code as e.g.
> produced by VCMPPS together with mask operations (which LLVM requires to
> work on <8 x i32> atm) and corresponding bitcasts.
>
> Consider these two examples:
>
> define <8 x float> @test1(<8 x float> %a, <8 x float> %b, <8 x i32> %m)
> nounwind readnone {
> entry:
>    %cmp = tail call <8 x float> @llvm.x86.avx.cmp.ps.256(<8 x float> %a,
> <8 x float> %b, i8 1) nounwind readnone
>    %res = tail call <8 x float> @llvm.x86.avx.blendv.ps.256(<8 x float>
> %a, <8 x float> %b, <8 x float> %cmp) nounwind readnone
>    ret <8 x float> %res
> }
>
> This works fine and produces the expected assembly (VCMPLTPS + VBLENDVPS).
>
> On the other hand, this does not work:
>
> define <8 x float> @test2(<8 x float> %a, <8 x float> %b, <8 x i32> %m)
> nounwind readnone {
> entry:
>    %cmp = tail call <8 x float> @llvm.x86.avx.cmp.ps.256(<8 x float> %a,
> <8 x float> %b, i8 1) nounwind readnone
>    %cast = bitcast <8 x float> %cmp to <8 x i32>
>    %mask = and <8 x i32> %cast, %m
>    %blend_cond = bitcast <8 x i32> %mask to <8 x float>
>    %res = tail call <8 x float> @llvm.x86.avx.blendv.ps.256(<8 x float>
> %a, <8 x float> %b, <8 x float> %blend_cond) nounwind readnone
>    ret <8 x float> %res
> }
>
> llc (latest trunk) bails out with:
>
> LLVM ERROR: Cannot select: 0x2510540: v8f32 = bitcast 0x2532270 [ID=16]
>    0x2532270: v4i64 = and 0x2532070, 0x2532170 [ID=15]
>      0x2532070: v4i64 = bitcast 0x2510740 [ID=14]
>        0x2510740: v8f32 = llvm.x86.avx.cmp.ps.256 0x2510640, 0x2511340,
> 0x2510f40, 0x2511140 [ORD=3] [ID=12]
> ...
>
> The same counts for or and xor where VXORPS etc. should be selected.

Please file bug reports!

> There seems to be some code for this because
> xor <8 x i32> %m, %m
> works, probably because it can get rid of all bitcasts.
>
> Ideally, I guess we would want code like this instead of the intrinsics
> at some point:
>
> define <8 x float> @test3(<8 x float> %a, <8 x float> %b, <8 x i1> %m)
> nounwind readnone {
> entry:
>    %cmp = fcmp ugt <8 x float> %a, %b
>    %mask = and <8 x i1> %cmp, %m
>    %res = select <8 x i1> %mask, <8 x float> %a, <8 x float> %b
>    ret <8 x float> %res
> }

That would be nice indeed

> -> VCMPPS, VANDPS, BLENDVPS
>
> Nadav Rotem sent around a patch a few weeks ago in which he implemented
> codegen for the select for SSE, unfortunately I did not have time to
> look at it in more depth so far.
>
> Can anybody comment on the current status of AVX?

No codegen support yet (although some stuff works), but the assembler
support is complete!

>
> Best,
> Ralf
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-- 
Bruno Cardoso Lopes
http://www.brunocardoso.cc