[LV][NFC] Simplify initialization of MinProfitableTripCount

[Mel-Chen]

Iteration runtime check confirms whether the trip count is greater than VFxUF at least. Therefore, there is no need to adjust the MinProfitableTripCount to VF if it is zero.
Retaining the original MinProfitableTripCount information is also beneficial for supporting more profitable runtime checks in the future.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-vectorizers

Author: Mel Chen (Mel-Chen)

Changes

Iteration runtime check confirms whether the trip count is greater than VFxUF at least. Therefore, there is no need to adjust the MinProfitableTripCount to VFxUF if it is zero.
Retaining the original MinProfitableTripCount information is also beneficial for supporting more profitable runtime checks in the future.

---

Full diff: https://github.com/llvm/llvm-project/pull/113445.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/LoopVectorize.cpp (+2-6)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 60a94ca1f86e42..804b541e8ffdec 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -469,18 +469,14 @@ class InnerLoopVectorizer {
                       LoopVectorizationCostModel *CM, BlockFrequencyInfo *BFI,
                       ProfileSummaryInfo *PSI, GeneratedRTChecks &RTChecks)
       : OrigLoop(OrigLoop), PSE(PSE), LI(LI), DT(DT), TLI(TLI), TTI(TTI),
-        AC(AC), ORE(ORE), VF(VecWidth), UF(UnrollFactor),
+        AC(AC), ORE(ORE), VF(VecWidth),
+        MinProfitableTripCount(MinProfitableTripCount), UF(UnrollFactor),
         Builder(PSE.getSE()->getContext()), Legal(LVL), Cost(CM), BFI(BFI),
         PSI(PSI), RTChecks(RTChecks) {
     // Query this against the original loop and save it here because the profile
     // of the original loop header may change as the transformation happens.
     OptForSizeBasedOnProfile = llvm::shouldOptimizeForSize(
         OrigLoop->getHeader(), PSI, BFI, PGSOQueryType::IRPass);
-
-    if (MinProfitableTripCount.isZero())
-      this->MinProfitableTripCount = VecWidth;
-    else
-      this->MinProfitableTripCount = MinProfitableTripCount;
   }
 
   virtual ~InnerLoopVectorizer() = default;

[Mel-Chen]

Ping

[david-arm]

On the surface the change seems fine. Do you have any examples of where this would be useful?

[fhahn]

LGTM, thanks! There should be no need to handle the case where it is zero separately.

[Mel-Chen]

On the surface the change seems fine. Do you have any examples of where this would be useful?

We encountered a case where vectorization requires a lot of overflow checks, resulting in the need for a higher trip count for vectorization to be profitable. We hope to emit a profitable check for such cases when MinProfitableTripCount is not zero, in order to reduce unprofitable vectorized code be executed.