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

[Nikita Popov via llvm-commits]

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@@ -1974,6 +1975,133 @@ static bool foldFCmpToFPClassTest(CmpInst *Cmp, const TargetLowering &TLI,
   return true;
 }
 
+static bool isRemOfLoopIncrementWithLoopInvariant(Instruction *Rem,
+                                                  const LoopInfo *LI,
+                                                  Value *&RemAmtOut,
+                                                  PHINode *&LoopIncrPNOut) {
+  Value *Incr, *RemAmt;
+  // 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(Rem->getParent());
+  if (!L || !L->getLoopPreheader() || !L->getLoopLatch())
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nikic wrote:

IIRC getLoopPreheader is expensive, so maybe those this after the phi check.

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