[llvm] [InstCombine] Convert fshl(x, 0, y) to shl(x, y) or shl(X, and(Y, BitWidth - 1)) (PR #122362)

[Nikita Popov via llvm-commits]

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@@ -2229,6 +2229,19 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
         return BitOp;
     }
 
+    // fshal(X, 0, Y) --> shl(X, and(Y, BitWidth - 1))
+    // fshal(X, 0, Y) --> Shl(X, Y) if Y within the range 0 to type bit width
+    if (match(Op1, m_ZeroInt())) {
+      unsigned BitWidth = Ty->getScalarSizeInBits();
+      Value *Op2 = II->getArgOperand(2);
+      if (auto Range = II->getRange(); Range && Range->getLower().sge(0) &&
+                                       Range->getUpper().sle(BitWidth)) {
+        return BinaryOperator::CreateShl(Op0, Op2);
+      }
+      Value *And = Builder.CreateAnd(Op2, ConstantInt::get(Ty, BitWidth - 1));
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nikic wrote:

This transform needs to be limited to power of two bitwidths, otherwise this is incorrect.

https://github.com/llvm/llvm-project/pull/122362