[X86] combineSelect - remove X86ISD::BLENDV if the condition signbit is known

[RKSimon]

Fixes part of #162812

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Changes

Fixes part of #162812

---

Full diff: https://github.com/llvm/llvm-project/pull/163974.diff

2 Files Affected:

• (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+9)
• (modified) llvm/test/CodeGen/X86/pr162812.ll (+16-34)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index ff8abb3bb5814..2feee053ea813 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -47726,6 +47726,15 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
                                                            DL, DAG, Subtarget))
       return V;
 
+  // If the sign bit is known then BLENDV can be folded away.
+  if (N->getOpcode() == X86ISD::BLENDV) {
+    KnownBits KnownCond = DAG.computeKnownBits(Cond);
+    if (KnownCond.isNegative())
+      return LHS;
+    if (KnownCond.isNonNegative())
+      return RHS;
+  }
+
   if (N->getOpcode() == ISD::VSELECT || N->getOpcode() == X86ISD::BLENDV) {
     SmallVector<int, 64> CondMask;
     if (createShuffleMaskFromVSELECT(CondMask, Cond,
diff --git a/llvm/test/CodeGen/X86/pr162812.ll b/llvm/test/CodeGen/X86/pr162812.ll
index 4ea3101ce8456..cec093c3df743 100644
--- a/llvm/test/CodeGen/X86/pr162812.ll
+++ b/llvm/test/CodeGen/X86/pr162812.ll
@@ -34,61 +34,43 @@ define <32 x i8> @PR162812(<32 x i8> %a, <32 x i8> %mask) {
 ;
 ; SSE42-LABEL: PR162812:
 ; SSE42:       # %bb.0:
-; SSE42-NEXT:    movdqa %xmm2, %xmm5
-; SSE42-NEXT:    movdqa %xmm0, %xmm2
+; SSE42-NEXT:    movdqa %xmm0, %xmm4
+; SSE42-NEXT:    psrlw $2, %xmm2
+; SSE42-NEXT:    movdqa {{.*#+}} xmm5 = [8224,8224,8224,8224,8224,8224,8224,8224]
+; SSE42-NEXT:    pand %xmm5, %xmm2
+; SSE42-NEXT:    paddb %xmm2, %xmm2
+; SSE42-NEXT:    paddb %xmm2, %xmm2
 ; SSE42-NEXT:    movdqa %xmm0, %xmm6
-; SSE42-NEXT:    psllw $2, %xmm6
-; SSE42-NEXT:    movdqa {{.*#+}} xmm7 = [252,252,252,252,252,252,252,252,252,252,252,252,252,252,252,252]
-; SSE42-NEXT:    pand %xmm7, %xmm6
-; SSE42-NEXT:    psrlw $2, %xmm5
-; SSE42-NEXT:    movdqa {{.*#+}} xmm4 = [8224,8224,8224,8224,8224,8224,8224,8224]
-; SSE42-NEXT:    pand %xmm4, %xmm5
+; SSE42-NEXT:    paddb %xmm0, %xmm6
+; SSE42-NEXT:    movdqa %xmm2, %xmm0
+; SSE42-NEXT:    pblendvb %xmm0, %xmm6, %xmm4
+; SSE42-NEXT:    psrlw $2, %xmm3
+; SSE42-NEXT:    pand %xmm3, %xmm5
 ; SSE42-NEXT:    paddb %xmm5, %xmm5
-; SSE42-NEXT:    movdqa %xmm5, %xmm0
-; SSE42-NEXT:    pblendvb %xmm0, %xmm6, %xmm2
-; SSE42-NEXT:    movdqa %xmm2, %xmm6
-; SSE42-NEXT:    paddb %xmm2, %xmm6
 ; SSE42-NEXT:    paddb %xmm5, %xmm5
+; SSE42-NEXT:    movdqa %xmm1, %xmm2
+; SSE42-NEXT:    paddb %xmm1, %xmm2
 ; SSE42-NEXT:    movdqa %xmm5, %xmm0
-; SSE42-NEXT:    pblendvb %xmm0, %xmm6, %xmm2
-; SSE42-NEXT:    movdqa %xmm1, %xmm5
-; SSE42-NEXT:    psllw $2, %xmm5
-; SSE42-NEXT:    pand %xmm7, %xmm5
-; SSE42-NEXT:    psrlw $2, %xmm3
-; SSE42-NEXT:    pand %xmm3, %xmm4
-; SSE42-NEXT:    paddb %xmm4, %xmm4
-; SSE42-NEXT:    movdqa %xmm4, %xmm0
-; SSE42-NEXT:    pblendvb %xmm0, %xmm5, %xmm1
-; SSE42-NEXT:    movdqa %xmm1, %xmm3
-; SSE42-NEXT:    paddb %xmm1, %xmm3
-; SSE42-NEXT:    paddb %xmm4, %xmm4
+; SSE42-NEXT:    pblendvb %xmm0, %xmm2, %xmm1
 ; SSE42-NEXT:    movdqa %xmm4, %xmm0
-; SSE42-NEXT:    pblendvb %xmm0, %xmm3, %xmm1
-; SSE42-NEXT:    movdqa %xmm2, %xmm0
 ; SSE42-NEXT:    retq
 ;
 ; AVX2-LABEL: PR162812:
 ; AVX2:       # %bb.0:
-; AVX2-NEXT:    vpsllw $2, %ymm0, %ymm2
-; AVX2-NEXT:    vpand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %ymm2, %ymm2
+; AVX2-NEXT:    vpaddb %ymm0, %ymm0, %ymm2
 ; AVX2-NEXT:    vpsrlw $2, %ymm1, %ymm1
 ; AVX2-NEXT:    vpand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %ymm1, %ymm1
 ; AVX2-NEXT:    vpaddb %ymm1, %ymm1, %ymm1
-; AVX2-NEXT:    vpblendvb %ymm1, %ymm2, %ymm0, %ymm0
-; AVX2-NEXT:    vpaddb %ymm0, %ymm0, %ymm2
 ; AVX2-NEXT:    vpaddb %ymm1, %ymm1, %ymm1
 ; AVX2-NEXT:    vpblendvb %ymm1, %ymm2, %ymm0, %ymm0
 ; AVX2-NEXT:    retq
 ;
 ; AVX512-LABEL: PR162812:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpsllw $2, %ymm0, %ymm2
-; AVX512-NEXT:    vpandd {{\.?LCPI[0-9]+_[0-9]+}}(%rip){1to8}, %ymm2, %ymm2
+; AVX512-NEXT:    vpaddb %ymm0, %ymm0, %ymm2
 ; AVX512-NEXT:    vpsrlw $2, %ymm1, %ymm1
 ; AVX512-NEXT:    vpandd {{\.?LCPI[0-9]+_[0-9]+}}(%rip){1to8}, %ymm1, %ymm1
 ; AVX512-NEXT:    vpaddb %ymm1, %ymm1, %ymm1
-; AVX512-NEXT:    vpblendvb %ymm1, %ymm2, %ymm0, %ymm0
-; AVX512-NEXT:    vpaddb %ymm0, %ymm0, %ymm2
 ; AVX512-NEXT:    vpaddb %ymm1, %ymm1, %ymm1
 ; AVX512-NEXT:    vpblendvb %ymm1, %ymm2, %ymm0, %ymm0
 ; AVX512-NEXT:    retq