[llvm] 3f8be15 - [LoopInterchange] Handle lcssa PHIs with multiple predecessors

[via llvm-commits]

Author: Congzhe Cao
Date: 2021-05-11T21:30:54-04:00
New Revision: 3f8be15f2911a3d3645030911be83a115bfe9e5c

URL: https://github.com/llvm/llvm-project/commit/3f8be15f2911a3d3645030911be83a115bfe9e5c
DIFF: https://github.com/llvm/llvm-project/commit/3f8be15f2911a3d3645030911be83a115bfe9e5c.diff

LOG: [LoopInterchange] Handle lcssa PHIs with multiple predecessors

This is a bugfix in the transformation phase.

If the original outer loop header branches to both the inner loop
(header) and the outer loop latch, and if there is an lcssa PHI
node outside the loop nest, then after interchange the new outer latch
will have an lcssa PHI node inserted which has two predecessors, i.e.,
the original outer header and the original outer latch. Currently
the transformation assumes it has only one predecessor (the original
outer latch) and crashes, since the inserted lcssa PHI node does
not take both predecessors as incoming BBs.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D100792

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/LoopInterchange.cpp
    llvm/test/Transforms/LoopInterchange/lcssa.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 58b456d5615f..c63bf160706e 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1529,6 +1529,13 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
       PHINode *NewPhi = dyn_cast<PHINode>(P.clone());
       NewPhi->setIncomingValue(0, P.getIncomingValue(0));
       NewPhi->setIncomingBlock(0, OuterLatch);
+      // We might have incoming edges from other BBs, i.e., the original outer
+      // header.
+      for (auto *Pred : predecessors(InnerLatch)) {
+        if (Pred == OuterLatch)
+          continue;
+        NewPhi->addIncoming(P.getIncomingValue(0), Pred);
+      }
       NewPhi->insertBefore(InnerLatch->getFirstNonPHI());
       P.setIncomingValue(0, NewPhi);
     }

diff  --git a/llvm/test/Transforms/LoopInterchange/lcssa.ll b/llvm/test/Transforms/LoopInterchange/lcssa.ll
index d7b6fdc148db..8dd449ba588b 100644
--- a/llvm/test/Transforms/LoopInterchange/lcssa.ll
+++ b/llvm/test/Transforms/LoopInterchange/lcssa.ll
@@ -298,3 +298,35 @@ for.exit:                                         ; preds = %outer.inc
 for.end16:                                        ; preds = %for.exit
   ret void
 }
+
+; Should not crash when the outer header branches to
+; both the inner loop and the outer latch, and there
+; is an lcssa phi node outside the loopnest.
+; REMARK: Interchanged
+; REMARK-NEXT: lcssa_08
+define i64 @lcssa_08([100 x [100 x i64]]* %Arr) {
+entry:
+  br label %for1.header
+
+for1.header:                                         ; preds = %for1.inc, %entry
+  %indvars.iv23 = phi i64 [ 0, %entry ], [ %indvars.iv.next24, %for1.inc ]
+  br i1 undef, label %for2, label %for1.inc
+
+for2:                                        ; preds = %for2, %for1.header
+  %indvars.iv = phi i64 [ 0, %for1.header ], [ %indvars.iv.next.3, %for2 ]
+  %arrayidx = getelementptr inbounds [100 x [100 x i64]], [100 x [100 x i64]]* %Arr, i64 0, i64 %indvars.iv, i64 %indvars.iv23
+  %lv = load i64, i64* %arrayidx, align 4
+  %indvars.iv.next.3 = add nuw nsw i64 %indvars.iv, 1
+  %exit1 = icmp eq i64 %indvars.iv.next.3, 100
+  br i1 %exit1, label %for1.inc, label %for2
+
+for1.inc:                                ; preds = %for2, %for1.header
+  %indvars.iv.next24 = add nuw nsw i64 %indvars.iv23, 1
+  %exit2 = icmp eq i64 %indvars.iv.next24, 100
+  br i1 %exit2, label %for1.loopexit, label %for1.header
+
+for1.loopexit:                                 ; preds = %for1.inc
+  %sum.outer.lcssa = phi i64 [ %indvars.iv23, %for1.inc ]
+  ret i64 %sum.outer.lcssa
+}
+