[llvm] 2b089e9 - [SimplifyCFG] Try to merge edge block when threading (PR55765)

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2022-06-20T10:29:33+02:00
New Revision: 2b089e9ae0df0fa44831cc74253b55ea6d43cd41

URL: https://github.com/llvm/llvm-project/commit/2b089e9ae0df0fa44831cc74253b55ea6d43cd41
DIFF: https://github.com/llvm/llvm-project/commit/2b089e9ae0df0fa44831cc74253b55ea6d43cd41.diff

LOG: [SimplifyCFG] Try to merge edge block when threading (PR55765)

When threading, we always create a new block for the threaded edge
(even if the edge is not critical), which will later get folded back
into the predecessor if possible. Depending on precise processing
order, this separate block may break the detection of trivial
cycles in the threading code, which normally avoids infinite
threading of loops. Explicitly merge the created edge block into
the predecessor to avoid this.

Fixes https://github.com/llvm/llvm-project/issues/55765.

Differential Revision: https://reviews.llvm.org/D127216

Added: 
    llvm/test/Transforms/SimplifyCFG/pr55765.ll

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 330344b55bcf6..399fc3baf7aa4 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3145,6 +3145,12 @@ FoldCondBranchOnValueKnownInPredecessorImpl(BranchInst *BI, DomTreeUpdater *DTU,
       DTU->applyUpdates(Updates);
     }
 
+    // For simplicity, we created a separate basic block for the edge. Merge
+    // it back into the predecessor if possible. This not only avoids
+    // unnecessary SimplifyCFG iterations, but also makes sure that we don't
+    // bypass the check for trivial cycles above.
+    MergeBlockIntoPredecessor(EdgeBB, DTU);
+
     // Signal repeat, simplifying any other constants.
     return None;
   }

diff  --git a/llvm/test/Transforms/SimplifyCFG/pr55765.ll b/llvm/test/Transforms/SimplifyCFG/pr55765.ll
new file mode 100644
index 0000000000000..929994143f919
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/pr55765.ll
@@ -0,0 +1,63 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+
+; This used to infinitely thread between loop and loop.latch without reaching a
+; fixed point.
+
+declare void @dummy()
+
+define i32 @main(i1 %c1, i1 %c2, i32 %y) {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[EXIT:%.*]], label [[LOOP_PRE_PREHEADER:%.*]]
+; CHECK:       loop.pre.preheader:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[Y:%.*]], -1
+; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_PREHEADER:%.*]], label [[EXIT]]
+; CHECK:       loop.preheader:
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq i32 [[Y]], 0
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    br i1 [[C1]], label [[LOOP2:%.*]], label [[LOOP]]
+; CHECK:       loop.latch:
+; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT]]
+; CHECK:       loop2:
+; CHECK-NEXT:    br i1 [[CMP2]], label [[JOIN:%.*]], label [[IF:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    br label [[JOIN]]
+; CHECK:       join:
+; CHECK-NEXT:    br i1 [[C2:%.*]], label [[LOOP2]], label [[LOOP_LATCH:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i32 0
+;
+  br i1 %c1, label %exit, label %loop.pre.preheader
+
+loop.pre.preheader:
+  %cmp = icmp sgt i32 %y, -1
+  br i1 %cmp, label %loop.preheader, label %exit
+
+loop.preheader:
+  %cmp2 = icmp eq i32 %y, 0
+  br label %loop
+
+loop:
+  br i1 %c1, label %loop2, label %loop.latch
+
+loop.latch:
+  br i1 %cmp, label %loop, label %exit
+
+loop2:
+  br i1 %cmp2, label %join, label %if
+
+if:
+  call void @dummy()
+  br label %join
+
+join:
+  br i1 %c2, label %loop2, label %loop.latch
+
+exit:
+  ret i32 0
+
+; uselistorder directives
+  uselistorder label %loop2, { 1, 0 }
+}