[llvm] [InstCombine] Fold umul.overflow(x, c1) | (x*c1 > c2) to x > c2/c1 (PR #147327)

[Nikita Popov via llvm-commits]

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@@ -3717,6 +3717,30 @@ Value *InstCombinerImpl::reassociateDisjointOr(Value *LHS, Value *RHS) {
   return nullptr;
 }
 
+/// Fold Res, Overflow = (umul.with.overflow x c1); (or Overflow (ugt Res c2))
+/// --> (ugt x (c2/c1)). This code checks whether a multiplication of two
+/// unsigned numbers (one is a constant) is mathematically greater than a
+/// second constant.
+static Value *foldOrUnsignedUMulOverflowICmp(BinaryOperator &I,
+                                             InstCombiner::BuilderTy &Builder,
+                                             const DataLayout &DL) {
+  const WithOverflowInst *WO;
+  const Value *WOV;
+  const APInt *C1, *C2;
+  if (match(&I,
+            m_c_Or(m_ExtractValue<1>(
+                       m_CombineAnd(m_WithOverflowInst(WO), m_Value(WOV))),
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nikic wrote:

Could replace `m_WithOverflowInst(WO)` with `m_Intrinsic<Intrinsic::umul_with_overflow>(m_Value(X), m_APInt(C1))` here.

https://github.com/llvm/llvm-project/pull/147327