[X86] Post process the DAG after isel to remove vector moves that were added to zero upper bits.

We previously avoided inserting these moves during isel in a few cases which is implemented using a whitelist of opcodes. But it's too difficult to generate a perfect list of opcodes to whitelist. Especially with AVX512F without AVX512VL using 512 bit vectors to implement some 128/256 bit operations. Since isel is done bottoms up, we'd have to check the VT and opcode and subtarget in order to determine whether an EXTRACT_SUBREG would be generated for some operations.

So instead of doing that, this patch adds a post processing step that detects when the moves are unnecesssary after isel. At that point any EXTRACT_SUBREGs would have already been created and appear in the DAG. So then we just need to ensure the input to the move isn't one.

Differential Revision: https://reviews.llvm.org/D44289

llvm-svn: 327724

1 files changed, 0 insertions, 1 deletions

diff --git a/llvm/test/CodeGen/X86/bitcast-setcc-128.ll b/llvm/test/CodeGen/X86/bitcast-setcc-128.ll
index 5164d4f4a26..599e907251b 100644
--- a/llvm/test/CodeGen/X86/bitcast-setcc-128.ll
+++ b/llvm/test/CodeGen/X86/bitcast-setcc-128.ll
@@ -658,7 +658,6 @@ define i64 @v16i8_widened_with_zeroes(<16 x i8> %a, <16 x i8> %b) {
 ; AVX2-LABEL: v16i8_widened_with_zeroes:
 ; AVX2:       # %bb.0: # %entry
 ; AVX2-NEXT:    vpcmpeqb %xmm1, %xmm0, %xmm0
-; AVX2-NEXT:    vmovdqa %xmm0, %xmm0
 ; AVX2-NEXT:    vpmovmskb %ymm0, %eax
 ; AVX2-NEXT:    vzeroupper
 ; AVX2-NEXT:    retq