[InstCombine] don't try SimplifyDemandedInstructionBits from zext/sext because it's slow and unnecessary

This one seems more obvious than D30270 that it can't make improvements because an extension always needs
all of the incoming bits. There's one specific transform in SimplifyDemandedInstructionBits of converting
a sext to a zext when the sign-bit is known zero, but that is handled explicitly in visitSext() with
ComputeSignBit().

Like D30270, there are no IR differences (other than instruction names) for the case in PR32037:
https://bugs.llvm.org//show_bug.cgi?id=32037
...and no regression test differences.

Zext/sext are a smaller part of the profile, but this still appears to shave off another 0.5% or so from
'opt -O2'.

Differential Revision: https://reviews.llvm.org/D30280

llvm-svn: 296129

1 files changed, 0 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 80b2870f893..e60687fa7ad 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -838,11 +838,6 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
 
-  // See if we can simplify any instructions used by the input whose sole
-  // purpose is to compute bits we don't care about.
-  if (SimplifyDemandedInstructionBits(CI))
-    return &CI;
-
   Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType(), *DestTy = CI.getType();
 
@@ -1124,11 +1119,6 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
   if (Instruction *I = commonCastTransforms(CI))
     return I;
 
-  // See if we can simplify any instructions used by the input whose sole
-  // purpose is to compute bits we don't care about.
-  if (SimplifyDemandedInstructionBits(CI))
-    return &CI;
-
   Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType(), *DestTy = CI.getType();