[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 +1847,30 @@ 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())) &&
+          !C->isZero() && match(B, m_OneUse(m_Add(m_Specific(X), m_One())))) {
+        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);
+      }
+      return nullptr;
+    };
----------------
goldsteinn wrote:

nit: I think its easier to read if you use early returns as opposed to mashing it all together into a single condition ie:

```
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);
};
```

That being said this is purely stylistic, so do as you do.

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