[llvm] [CodeGenPrepare] Folding `urem` with loop invariant value as remainder (PR #96625)

[Yingwei Zheng via llvm-commits]

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@@ -1974,6 +1975,160 @@ static bool foldFCmpToFPClassTest(CmpInst *Cmp, const TargetLowering &TLI,
   return true;
 }
 
+static bool isRemOfLoopIncrementWithLoopInvariant(
+    Value *Rem, const LoopInfo *LI, Value *&RemAmtOut,
+    std::optional<bool> &AddOrSubOut, Value *&AddOrSubOffsetOut,
+    PHINode *&LoopIncrPNOut) {
+  Value *Incr, *RemAmt;
+  if (!isa<Instruction>(Rem))
+    return false;
+  // NB: If RemAmt is a power of 2 it *should* have been transformed by now.
+  if (!match(Rem, m_URem(m_Value(Incr), m_Value(RemAmt))))
+    return false;
+
+  // Only trivially analyzable loops.
+  Loop *L = LI->getLoopFor(cast<Instruction>(Rem)->getParent());
+  if (L == nullptr || L->getLoopPreheader() == nullptr ||
+      L->getLoopLatch() == nullptr)
+    return false;
+
+  std::optional<bool> AddOrSub;
+  Value *AddOrSubOffset;
+  // Find out loop increment PHI.
+  PHINode *PN = dyn_cast<PHINode>(Incr);
+  if (PN != nullptr) {
+    AddOrSub = std::nullopt;
+    AddOrSubOffset = nullptr;
+  } else {
+    // Search through a NUW add/sub.
+    Value *V0, *V1;
+    if (match(Incr, m_NUWAddLike(m_Value(V0), m_Value(V1))))
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dtcxzyw wrote:

Isn't `RemAmt` a loop invariant?

https://github.com/llvm/llvm-project/pull/96625