[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))),
+                   m_OneUse(m_SpecificCmp(ICmpInst::ICMP_UGT,
+                                          m_ExtractValue<0>(m_Deferred(WOV)),
+                                          m_APInt(C2))))) &&
+      WO->getIntrinsicID() == Intrinsic::umul_with_overflow &&
+      match(WO->getRHS(), m_APInt(C1)) && !C1->isZero() && WO->hasNUses(2)) {
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nikic wrote:

For the same reason as the others, I'd drop the use check on WO as well. Even if there are other uses, we still at least simplify or+icmp to icmp.

(More than two uses would be somewhat unusual anyway, because it means there are redundant extracts. May happen if they don't dominate each other.)

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