[llvm] 9142f67 - [SimplifyCFG] Don't widen cond br if false branch has successors

[Dmitry Makogon via llvm-commits]

Author: Dmitry Makogon
Date: 2022-08-26T15:23:37+07:00
New Revision: 9142f67ef282570ebc4999045081e8c530f2054d

URL: https://github.com/llvm/llvm-project/commit/9142f67ef282570ebc4999045081e8c530f2054d
DIFF: https://github.com/llvm/llvm-project/commit/9142f67ef282570ebc4999045081e8c530f2054d.diff

LOG: [SimplifyCFG] Don't widen cond br if false branch has successors

Fixes https://github.com/llvm/llvm-project/issues/57221.

This limits the tryWidenCondBranchToCondBranch transform making it
work only if the false block of widenable condition branch
has no successors.

If that block has successors, then SimplifyCondBranchToCondBranch
may undo the transform done by tryWidenCondBranchToCondBranch, which
would lead to infinite cycle of transformation and eventually
an assert failing.

Differential Revision: https://reviews.llvm.org/D132356

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/test/Transforms/SimplifyCFG/pr52290.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 21f8d54703de2..7f2fb10183ca7 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3999,6 +3999,11 @@ static bool tryWidenCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
     return false;
   if (!IfFalseBB->phis().empty())
     return false; // TODO
+  // This helps avoid infinite loop with SimplifyCondBranchToCondBranch which
+  // may undo the transform done here.
+  // TODO: There might be a more fine-grained solution to this.
+  if (!llvm::succ_empty(IfFalseBB))
+    return false;
   // Use lambda to lazily compute expensive condition after cheap ones.
   auto NoSideEffects = [](BasicBlock &BB) {
     return llvm::none_of(BB, [](const Instruction &I) {

diff  --git a/llvm/test/Transforms/SimplifyCFG/pr52290.ll b/llvm/test/Transforms/SimplifyCFG/pr52290.ll
index 0f2f7a7393b39..b0de24663915a 100644
--- a/llvm/test/Transforms/SimplifyCFG/pr52290.ll
+++ b/llvm/test/Transforms/SimplifyCFG/pr52290.ll
@@ -1,15 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -simplifycfg -S | FileCheck %s
 ; RUN: opt < %s -passes=simplifycfg -S | FileCheck %s
 
-; XFAIL: *
-; REQUIRES: asserts
-; FIXME: Fails due to infinite loop in iterativelySimplifyCFG.
-
 ; ModuleID = 'test/Transforms/SimplifyCFG/pr-new.ll'
 source_filename = "test/Transforms/SimplifyCFG/pr-new.ll"
 
 define i32 @test(float %arg, i1 %c) gc "statepoint-example" personality i32* ()* @blam {
-; CHECK-LABEL: @test
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[TMP:%.*]] = call i1 @llvm.experimental.widenable.condition()
+; CHECK-NEXT:    br i1 [[TMP]], label [[BB2:%.*]], label [[BB1:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[BB7:%.*]], label [[BB5:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr inbounds i8, i8 addrspace(1)* undef, i64 16
+; CHECK-NEXT:    br i1 [[C]], label [[BB7]], label [[BB4:%.*]]
+; CHECK:       bb4:
+; CHECK-NEXT:    call void @snork() [ "deopt"() ]
+; CHECK-NEXT:    unreachable
+; CHECK:       bb5:
+; CHECK-NEXT:    ret i32 0
+; CHECK:       bb7:
+; CHECK-NEXT:    [[TMP8:%.*]] = call i32 (...) @llvm.experimental.deoptimize.i32(i32 10) [ "deopt"() ]
+; CHECK-NEXT:    ret i32 [[TMP8]]
+;
 bb:
   %tmp = call i1 @llvm.experimental.widenable.condition()
   br i1 %tmp, label %bb2, label %bb1