[cfe-dev] Inefficient code generation for _mm_test{z, c, nzc} (SSE4.1)

[Evan Cheng]

On Apr 11, 2012, at 2:41 AM, Florian Pflug wrote:

> Hi
> 
> I've stumbled over a deficiency in clang's codegen for the SSE4.1 _mm_test* intrinsics. These intrinsics are supposed to map to the PTEST instruction, which sets the ZF (zero flag) and CF (carry flag) depending on whether the bitwise AND (or ANDNOT for CF) of two SSE registers is all zero or not. The construct
> 
>  if (_mm_test{z,c,nzc}_si128(v, m))
>    …
> 
> should thus produce a PTEST instruction followed by a branch instruction (JZ for _mm_testz_si128, JC fr _mm_testc_si128, JNBE for _mm_testnzc_si128). Clang, however, instead produces something like
> 
>  PTEST …
>  SETE %al
>  MOVZBL %al, %eax
>  TEST %eax, %eax
>  JNE ...
> 
> Also, the LLVM bitcode looks a tad strange. For
> 
>  if (_mm_testz_si128(v,v))
>    body();
> 
> Clang generates
> 
>  %2 = tail call i32 @llvm.x86.sse41.ptestz(<4 x float> %1, <4 x float> %1) nounwind
>  %3 = icmp eq i32 %2, 0
>  br i1 %3, label %5, label %4
> ; <label>:4                                       ; preds = %0
>  tail call void (...)* @body() nounwind
>  br label %5
> ; <label>:5                                       ; preds = %4, %0
>  ret void
> 
> Since _mm_testz_si128 uses __m128i (the integer SSE type), *not* __m128 (the single-precision float SSE type), it seems strange that the corresponding LLVM intrinsic takes parameters of type float.
> 
> I'm not sure whether fixing this involves changing Clang or LLVM (or both?), which is why I haven't filed a bug report so far, but instead posted this here.

IMO, this is strictly a LLVM codegen issue. I also recall someone has been working on this? Are you using trunk clang / llvm?

Evan

> 
> Funnily enough, GCC 4.2 (at least the OSX version) has the same problem. Later GCC versions get it right, though.
> 
> best regards,
> Florian Pflug
> 
> 
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