aqjune added inline comments.
================
Comment at: clang/test/CodeGen/X86/avx-builtins.c:182
// CHECK-LABEL: test_mm256_castsi128_si256
- // CHECK: shufflevector <2 x i64> %{{.*}}, <2 x i64> %{{.*}}, <4 x i32> <i32
0, i32 1, i32 undef, i32 undef>
+ // CHECK: shufflevector <2 x i64> %{{.*}}, <2 x i64> %{{.*}}, <4 x i32> <i32
0, i32 1, i32 poison, i32 poison>
return _mm256_castsi128_si256(A);
----------------
aqjune wrote:
> RKSimon wrote:
> > efriedma wrote:
> > > This change might be visible to user code.
> > Yes the length changing casts are worrying me as well - we could update the
> > header to insert zero into the upper elements I suppose, in many cases
> > these would be folded away by AVX ops implicitly zeroing the 128-bits. But
> > we'd definitely have the potential for regressions.
> I quickly skimmed through the headers in clang/lib/Headers and listed the
> functions calling `__builtin_shufflevector` with at least one -1 mask operand.
> It seems there aren't very many, which is good news; I found 17 functions
> only ({F19257445}).
>
> But, correctly fixing these headers seems to require a lot of work.
> Since using the zero vector can cause performance regressions, we need to use
> a frozen poison (undef) vector to encode a vector having unspecified bits.
> A few months ago, I created D104790 to start using freeze(vector poison) for
> `mm*_undefined*` intrinsics. However, teaching the existing codebase to
> successfully deal with the frozen poison vector was a pretty tough job.
> When it comes to fixing the headers, there is even no C intrinsic function
> that represents a frozen poison vector AFAIK.
>
> I'll appreciate any idea or help in addressing this issue. :/
Okay, D130339 has finally been merged.
I will make a patch that updates the `mm256_castsi128_si256` and its family
functions to emit shufflevector with freeze(poison) operand.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D103874/new/
https://reviews.llvm.org/D103874
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