[llvm] [InstCombine] Fold Minimum over Trailing/Leading Bits Counts (PR #90402)

[Nikita Popov via llvm-commits]

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@@ -1633,6 +1633,39 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
       Value *Cmp = Builder.CreateICmpNE(I0, Zero);
       return CastInst::Create(Instruction::ZExt, Cmp, II->getType());
     }
+    // umin(cttz(x), const) --> cttz(x | (1 << const))
+    Value *X;
+    const APInt *Y;
+    Value *Z;
+    if (match(I0, m_OneUse(m_Cttz(m_Value(X), m_Value(Z)))) &&
+        match(I1, m_APInt(Y))) {
+      Value *CttzOp = X;
+      if (Y->ult(I1->getType()->getScalarType()->getIntegerBitWidth())) {
+        auto One = APInt::getOneBitSet(
+            I1->getType()->getScalarType()->getIntegerBitWidth(), 0);
+        Value *NewConst = ConstantInt::get(I1->getType(), One << *Y);
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nikic wrote:

If you're going to use getOneBitSet(), you should actually pass the set bit (Y) to it. If you want to express it as 1 << Y then use the normal APInt constructor to create the 1.

https://github.com/llvm/llvm-project/pull/90402