[llvm] [AArch64] Allow unrolling of scalar epilogue loops (PR #151164)

[Ahmad Yasin via llvm-commits]

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@@ -4905,14 +4905,17 @@ void AArch64TTIImpl::getUnrollingPreferences(
   // Disable partial & runtime unrolling on -Os.
   UP.PartialOptSizeThreshold = 0;
 
-  // No need to unroll auto-vectorized loops
-  if (findStringMetadataForLoop(L, "llvm.loop.isvectorized"))
-    return;
-
   // Scan the loop: don't unroll loops with calls as this could prevent
-  // inlining.
+  // inlining. Don't unroll auto-vectorized loops either, though do allow
+  // unrolling of the scalar remainder.
+  bool IsVectorized = getBooleanLoopAttribute(L, "llvm.loop.isvectorized");
   for (auto *BB : L->getBlocks()) {
     for (auto &I : *BB) {
+      // Both auto-vectorized loops and the scalar remainder have the
+      // isvectorized attribute, so differentiate between them by the presence
+      // of vector instructions.
+      if (IsVectorized && I.getType()->isVectorTy())
+        return;
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ayasin-a wrote:

setting an attribute or a field to the Loop instance telling "remainder of a loop that has isvectorized attribute" would help. Each of the loop variant its run through `getUnrollingPreferences()` by its own. 
This way any later-on check, for example in a target-specific tuning, would leverage the same without re-iterating over each and every instruction in the loop.


https://github.com/llvm/llvm-project/pull/151164