Revert trunc(lshr (sext A), Cst) to ashr A, Cst

This reverts commit r246997, it introduced a regression (PR24763).

llvm-svn: 247180

1 files changed, 0 insertions, 20 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index fabf80b..768616f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -498,26 +498,6 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
   }
 
-  // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
-  // conversion.
-  // It works because bits coming from sign extension have the same value as
-  // sign bit of the original value; performing ashr instead of lshr
-  // generates bits of the same value as the sign bit.
-  if (Src->hasOneUse() &&
-      match(Src, m_LShr(m_SExt(m_Value(A)), m_ConstantInt(Cst))) &&
-      cast<Instruction>(Src)->getOperand(0)->hasOneUse()) {
-    const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
-    // This optimization can be only performed when zero bits generated by
-    // the original lshr aren't pulled into the value after truncation, so we
-    // can only shift by values smaller then the size of destination type (in
-    // bits).
-    if (Cst->getValue().ult(ASize)) {
-      Value *Shift = Builder->CreateAShr(A, Cst->getZExtValue());
-      Shift->takeName(Src);
-      return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
-    }
-  }
-
   // Transform "trunc (and X, cst)" -> "and (trunc X), cst" so long as the dest
   // type isn't non-native.
   if (Src->hasOneUse() && isa<IntegerType>(Src->getType()) &&