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

[David Sherwood via llvm-commits]

================
@@ -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;
----------------
david-arm wrote:

Can this be simplified with something like?

```
  if (llvm::all_of(IVInst->users(), [&](const User *U) {
                     return U == IV || U == Cmp; }))
    InstsToIgnore.insert(IVInst);
```

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