[InstCombine] Move casts around shift operations

It is possible to perform a left shift before zero extending if the
shift would only shift out zeros.

llvm-svn: 290928

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index bc38c4aca34..5ad2a1c0e3e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -731,6 +731,25 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
   if (ConstantInt *Op1C = dyn_cast<ConstantInt>(I.getOperand(1))) {
     unsigned ShAmt = Op1C->getZExtValue();
 
+    // Turn:
+    //  %zext = zext i32 %V to i64
+    //  %res = shl i64 %V, 8
+    //
+    // Into:
+    //  %shl = shl i32 %V, 8
+    //  %res = zext i32 %shl to i64
+    //
+    // This is only valid if %V would have zeros shifted out.
+    if (auto *ZI = dyn_cast<ZExtInst>(I.getOperand(0))) {
+      unsigned SrcBitWidth = ZI->getSrcTy()->getScalarSizeInBits();
+      if (ShAmt < SrcBitWidth &&
+          MaskedValueIsZero(ZI->getOperand(0),
+                            APInt::getHighBitsSet(SrcBitWidth, ShAmt), 0, &I)) {
+        auto *Shl = Builder->CreateShl(ZI->getOperand(0), ShAmt);
+        return new ZExtInst(Shl, I.getType());
+      }
+    }
+
     // If the shifted-out value is known-zero, then this is a NUW shift.
     if (!I.hasNoUnsignedWrap() &&
         MaskedValueIsZero(I.getOperand(0),