[llvm] [InstCombine] Fold umax(nuw_mul(x, C0), x + 1) into (x == 0 ? 1 : nuw_mul(x, C0)) (PR #123468)

[via llvm-commits]

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@@ -1847,6 +1847,33 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
         return CastInst::Create(Instruction::ZExt, NarrowMaxMin, II->getType());
       }
     }
+    // If C0 is not 0:
+    //   umax(nuw_shl(x, C0), x + 1) -> x == 0 ? 1 : nuw_shl(x, C0)
+    // If C0 is not 0 or 1:
+    //   umax(nuw_mul(x, C0), x + 1) -> x == 0 ? 1 : nuw_mul(x, C0)
+    auto foldMaxMulShift = [&](Value *A, Value *B) -> Instruction * {
+      const APInt *C;
+      Value *X;
+      if (!(match(A, m_NUWShl(m_Value(X), m_APInt(C)))) &&
+          !(match(A, m_NUWMul(m_Value(X), m_APInt(C))) && !C->isOne()))
+        return nullptr;
+      if (C->isZero())
+        return nullptr;
+      if (!match(B, m_OneUse(m_Add(m_Specific(X), m_One()))))
+        return nullptr;
+
+      Value *Cmp = Builder.CreateICmpEQ(X, ConstantInt::get(X->getType(), 0));
+      Value *NewSelect =
+          Builder.CreateSelect(Cmp, ConstantInt::get(X->getType(), 1), A);
+      return replaceInstUsesWith(*II, NewSelect);
+    };
+
+    if (IID == Intrinsic::umax) {
+      if (Instruction *I = foldMaxMulShift(I0, I1))
+        return I;
+      if (Instruction *I = foldMaxMulShift(I1, I0))
+        return I;
+    }
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goldsteinn wrote:

As a possible follow up if you are interested, you can also handle `umin`: https://alive2.llvm.org/ce/z/DAy-C5

https://github.com/llvm/llvm-project/pull/123468