[InstCombine] combine mul+shl separated by zext

This appears to slightly help patterns similar to what's
shown in PR42874:
https://bugs.llvm.org/show_bug.cgi?id=42874
...but not in the way requested.

That fix will require some later IR and/or backend pass to
decompose multiply/shifts into something more optimal per
target. Those transforms already exist in some basic forms,
but probably need enhancing to catch more cases.

https://rise4fun.com/Alive/Qzv2

llvm-svn: 367891

1 files changed, 13 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index a30bcbd64ca..c0a1df6b9a7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -715,14 +715,25 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
     unsigned ShAmt = ShAmtAPInt->getZExtValue();
     unsigned BitWidth = Ty->getScalarSizeInBits();
 
-    // shl (zext X), ShAmt --> zext (shl X, ShAmt)
-    // This is only valid if X would have zeros shifted out.
     Value *X;
     if (match(Op0, m_OneUse(m_ZExt(m_Value(X))))) {
       unsigned SrcWidth = X->getType()->getScalarSizeInBits();
+      // shl (zext X), ShAmt --> zext (shl X, ShAmt)
+      // This is only valid if X would have zeros shifted out.
       if (ShAmt < SrcWidth &&
           MaskedValueIsZero(X, APInt::getHighBitsSet(SrcWidth, ShAmt), 0, &I))
         return new ZExtInst(Builder.CreateShl(X, ShAmt), Ty);
+
+      // shl (zext (mul MulOp, C2)), ShAmt --> mul (zext MulOp), (C2 << ShAmt)
+      // This is valid if the high bits of the wider multiply are shifted out.
+      Value *MulOp;
+      const APInt *C2;
+      if (ShAmt >= (BitWidth - SrcWidth) &&
+          match(X, m_Mul(m_Value(MulOp), m_APInt(C2)))) {
+        Value *Zext = Builder.CreateZExt(MulOp, Ty);
+        Constant *NewMulC = ConstantInt::get(Ty, C2->zext(BitWidth).shl(ShAmt));
+        return BinaryOperator::CreateMul(Zext, NewMulC);
+      }
     }
 
     // (X >> C) << C --> X & (-1 << C)