[PATCH] [IR] Make {extract, insert}element accept an index of any integer type.

[Michael Spencer]

CCing Chris, who apparently doesn't have a phab account.

- Michael Spencer

On Fri, Apr 25, 2014 at 7:17 PM, Michael Spencer <bigcheesegs at gmail.com> wrote:
> Given the following C code llvm currently generates suboptimal code for
> x86-64:
>
> __m128 bss4( const __m128 *ptr, size_t i, size_t j )
> {
>     float f = ptr[i][j];
>     return (__m128) { f, f, f, f };
> }
>
> =================================================
>
> define <4 x float> @_Z4bss4PKDv4_fmm(<4 x float>* nocapture readonly %ptr, i64 %i, i64 %j) #0 {
>   %a1 = getelementptr inbounds <4 x float>* %ptr, i64 %i
>   %a2 = load <4 x float>* %a1, align 16, !tbaa !1
>   %a3 = trunc i64 %j to i32
>   %a4 = extractelement <4 x float> %a2, i32 %a3
>   %a5 = insertelement <4 x float> undef, float %a4, i32 0
>   %a6 = insertelement <4 x float> %a5, float %a4, i32 1
>   %a7 = insertelement <4 x float> %a6, float %a4, i32 2
>   %a8 = insertelement <4 x float> %a7, float %a4, i32 3
>   ret <4 x float> %a8
> }
>
> =================================================
>
>         shlq    $4, %rsi
>         addq    %rdi, %rsi
>         movslq  %edx, %rax
>         vbroadcastss    (%rsi,%rax,4), %xmm0
>         retq
>
> =================================================
>
> The movslq is uneeded, but is present because of the trunc to i32 and then
> sext back to i64 that the backend adds for vbroadcastss.
>
> We can't remove it because it changes the meaning. The IR that clang
> generates is already suboptimal. What clang really should emit is:
>
>   %a4 = extractelement <4 x float> %a2, i64 %j
>
> This patch makes that legal. A separate patch will teach clang to do it.
>
> http://reviews.llvm.org/D3519
>
> Files:
>   docs/LangRef.rst
>   lib/Bitcode/Reader/BitcodeReader.cpp
>   lib/Bitcode/Writer/BitcodeWriter.cpp
>   lib/IR/Constants.cpp
>   lib/IR/Instructions.cpp
>   test/CodeGen/X86/vec_splat.ll
>   test/Feature/instructions.ll