[llvm] bc43078 - [LoopFlatten] Fix bug where SCEVCouldNotCompute object is used

[Sjoerd Meijer via llvm-commits]

Author: Sjoerd Meijer
Date: 2021-07-28T18:35:08+01:00
New Revision: bc43078fe835e19782a90ad2f464ebdc150911de

URL: https://github.com/llvm/llvm-project/commit/bc43078fe835e19782a90ad2f464ebdc150911de
DIFF: https://github.com/llvm/llvm-project/commit/bc43078fe835e19782a90ad2f464ebdc150911de.diff

LOG: [LoopFlatten] Fix bug where SCEVCouldNotCompute object is used

The SCEV method getBackedgeTakenCount() returns a SCEVCouldNotCompute
object if the backedge-taken count is unpredictable. This fix ensures
there is no longer an attempt to use such an object to find the trip
count.

Patch by: Rosie Sumpter.

Differential Revision: https://reviews.llvm.org/D106970

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/LoopFlatten.cpp
    llvm/test/Transforms/LoopFlatten/loop-flatten-negative.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LoopFlatten.cpp b/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
index f54289f85ef53..46f12aafb552d 100644
--- a/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopFlatten.cpp
@@ -170,8 +170,12 @@ static bool findLoopComponents(
   // icmp ult %inc, tripcount -> icmp ult %j, tripcount-1, then we don't flatten
   // the loop (yet).
   TripCount = Compare->getOperand(1);
-  const SCEV *SCEVTripCount =
-      SE->getTripCountFromExitCount(SE->getBackedgeTakenCount(L));
+  const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
+  if (isa<SCEVCouldNotCompute>(BackedgeTakenCount)) {
+    LLVM_DEBUG(dbgs() << "Backedge-taken count is not predictable\n");
+    return false;
+  }
+  const SCEV *SCEVTripCount = SE->getTripCountFromExitCount(BackedgeTakenCount);
   if (SE->getSCEV(TripCount) != SCEVTripCount) {
     if (!IsWidened) {
       LLVM_DEBUG(dbgs() << "Could not find valid trip count\n");

diff  --git a/llvm/test/Transforms/LoopFlatten/loop-flatten-negative.ll b/llvm/test/Transforms/LoopFlatten/loop-flatten-negative.ll
index e7c8697b14c68..3806414e1e503 100644
--- a/llvm/test/Transforms/LoopFlatten/loop-flatten-negative.ll
+++ b/llvm/test/Transforms/LoopFlatten/loop-flatten-negative.ll
@@ -637,5 +637,51 @@ for.cond.cleanup:
   ret void
 }
 
+; Backedge-taken count is not predictable.
+%struct.Limits = type { i16, i16 }
+define void @backedge_count(%struct.Limits* %lim) {
+entry:
+  %N = getelementptr inbounds %struct.Limits, %struct.Limits* %lim, i32 0, i32 0
+  %M = getelementptr inbounds %struct.Limits, %struct.Limits* %lim, i32 0, i32 1
+  %0 = load i16, i16* %N, align 2
+  %cmp20 = icmp sgt i16 %0, 0
+  br i1 %cmp20, label %for.cond2.preheader.preheader, label %for.cond.cleanup
+
+for.cond2.preheader.preheader:
+  %.pre = load i16, i16* %M, align 2
+  br label %for.cond2.preheader
+
+for.cond2.preheader:
+  %1 = phi i16 [ %3, %for.cond.cleanup6 ], [ %0, %for.cond2.preheader.preheader ]
+  %2 = phi i16 [ %4, %for.cond.cleanup6 ], [ %.pre, %for.cond2.preheader.preheader ]
+  %i.021 = phi i32 [ %inc9, %for.cond.cleanup6 ], [ 0, %for.cond2.preheader.preheader ]
+  %cmp417 = icmp sgt i16 %2, 0
+  br i1 %cmp417, label %for.body7, label %for.cond.cleanup6
+
+for.cond.cleanup:
+  ret void
+
+for.cond.cleanup6.loopexit:
+  %.pre22 = load i16, i16* %N, align 2
+  br label %for.cond.cleanup6
+
+for.cond.cleanup6:
+  %3 = phi i16 [ %.pre22, %for.cond.cleanup6.loopexit ], [ %1, %for.cond2.preheader ]
+  %4 = phi i16 [ %5, %for.cond.cleanup6.loopexit ], [ %2, %for.cond2.preheader ]
+  %inc9 = add nuw nsw i32 %i.021, 1
+  %conv = sext i16 %3 to i32
+  %cmp = icmp slt i32 %inc9, %conv
+  br i1 %cmp, label %for.cond2.preheader, label %for.cond.cleanup
+
+for.body7:
+  %j.018 = phi i32 [ %inc, %for.body7 ], [ 0, %for.cond2.preheader ]
+  tail call void bitcast (void (...)* @g to void ()*)()
+  %inc = add nuw nsw i32 %j.018, 1
+  %5 = load i16, i16* %M, align 2
+  %conv3 = sext i16 %5 to i32
+  %cmp4 = icmp slt i32 %inc, %conv3
+  br i1 %cmp4, label %for.body7, label %for.cond.cleanup6.loopexit
+}
+
 declare dso_local void @f(i32*)
 declare dso_local void @g(...)