[LoopFusion] Restrict loop fusion to rotated loops.

Summary:
This patch restricts loop fusion to only consider rotated loops as valid candidates.
This simplifies the analysis and transformation and aligns with other loop optimizations.

Reviewers: jdoerfert, Meinersbur, dmgreen, etiotto, Whitney, fhahn, hfinkel

Reviewed By: Meinersbur

Subscribers: ormris, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71025

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopFuse.cpp b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
index 2bbbd7c..e5ea914 100644
--- a/llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
@@ -91,6 +91,7 @@ STATISTIC(FusionNotBeneficial, "Fusion is not beneficial");
 STATISTIC(NonIdenticalGuards, "Candidates have different guards");
 STATISTIC(NonEmptyExitBlock, "Candidate has a non-empty exit block");
 STATISTIC(NonEmptyGuardBlock, "Candidate has a non-empty guard block");
+STATISTIC(NotRotated, "Candidate is not rotated");
 
 enum FusionDependenceAnalysisChoice {
   FUSION_DEPENDENCE_ANALYSIS_SCEV,
@@ -319,6 +320,11 @@ struct FusionCandidate {
       return reportInvalidCandidate(NotSimplifiedForm);
     }
 
+    if (!isRotated()) {
+      LLVM_DEBUG(dbgs() << "Loop " << L->getName() << " is not rotated!\n");
+      return reportInvalidCandidate(NotRotated);
+    }
+
     return true;
   }