[llvm] [RISCV] Adjust unroll prefs for loops with vectors (PR #151525)

[Ahmad Yasin via llvm-commits]

================
@@ -2627,18 +2627,17 @@ void RISCVTTIImpl::getUnrollingPreferences(
   if (L->getNumBlocks() > 4)
     return;
 
-  // Don't unroll vectorized loops, including the remainder loop
-  if (getBooleanLoopAttribute(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");
   InstructionCost Cost = 0;
   for (auto *BB : L->getBlocks()) {
     for (auto &I : *BB) {
-      // Initial setting - Don't unroll loops containing vectorized
-      // instructions.
-      if (I.getType()->isVectorTy())
+      // 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;
----------------
ayasin-a wrote:

Thanks for introducing this enhancement!

It would help if the code would could add an attribute or a field to the Loop instance that this is actually the "remainder of a loop that has `isvectorized` attribute. 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.

@fhahn 

https://github.com/llvm/llvm-project/pull/151525