[llvm] [LV] Ignore some costs when loop gets fully unrolled (PR #106699)

[Florian Hahn via llvm-commits]

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@@ -2663,6 +2663,33 @@ static Value *getExpandedStep(const InductionDescriptor &ID,
   return I->second;
 }
 
+/// Knowing that loop \p L would be fully unrolled after vectorisation, add
+/// instructions that will get simplified and thus should not have any cost to
+/// \p InstsToIgnore
+static void AddFullyUnrolledInstructionsToIgnore(
+    Loop *L, const LoopVectorizationLegality::InductionList &IL,
+    SmallPtrSetImpl<Instruction *> &InstsToIgnore) {
+  auto *Cmp = L->getLatchCmpInst();
+  if (!Cmp)
+    return;
+  InstsToIgnore.insert(Cmp);
+  for (const auto &[IV, IndDesc] : IL) {
+    // Get next iteration value of the induction variable
+    Instruction *IVInst =
+        cast<Instruction>(IV->getIncomingValueForBlock(L->getLoopLatch()));
+    bool IsSimplifiedAway = true;
+    // Check that this value used only to exit the loop
+    for (auto *UIV : IVInst->users()) {
+      if (UIV != IV && UIV != Cmp) {
+        IsSimplifiedAway = false;
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fhahn wrote:

would be good to have a test case for this

https://github.com/llvm/llvm-project/pull/106699