[llvm] [InstCombine] Infer exact division from a known-zero remainder (PR #221509)

[via llvm-commits]

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@@ -1318,6 +1318,39 @@ static Value *foldIDivShl(BinaryOperator &I, InstCombiner::BuilderTy &Builder) {
   return nullptr;
 }
 
+/// Return true if a matching remainder is known to be zero at the division.
+static bool isKnownExactFromRem(const BinaryOperator &Div,
+                                const SimplifyQuery &Q) {
+  assert((Div.getOpcode() == Instruction::SDiv ||
+          Div.getOpcode() == Instruction::UDiv) &&
+         "Expected integer divide");
+
+  unsigned RemOpcode = Div.getOpcode() == Instruction::SDiv ? Instruction::SRem
+                                                            : Instruction::URem;
+
+  Value *Op0 = Div.getOperand(0);
+  Value *Op1 = Div.getOperand(1);
+
+  // A matching remainder uses both operands. Avoid scanning constants, whose
+  // users may be shared across functions or whose use-lists may be unavailable.
+  Value *V = isa<Constant>(Op0) ? Op1 : Op0;
+  if (isa<Constant>(V))
+    return false;
+
+  for (User *U : V->users()) {
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ZERICO2005 wrote:

I would appreciate some guidance on how to bound this search here.

https://github.com/llvm/llvm-project/pull/221509