[llvm] 29ca7d5 - [SimplifyCFG] simplifyUnreachable(): fix handling of degenerate same-destination conditional branch

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2021-01-06T01:52:36+03:00
New Revision: 29ca7d5a1ad968c371124b8d82edd8d91eee7b4f

URL: https://github.com/llvm/llvm-project/commit/29ca7d5a1ad968c371124b8d82edd8d91eee7b4f
DIFF: https://github.com/llvm/llvm-project/commit/29ca7d5a1ad968c371124b8d82edd8d91eee7b4f.diff

LOG: [SimplifyCFG] simplifyUnreachable(): fix handling of degenerate same-destination conditional branch

One would hope that it would have been already canonicalized into an
unconditional branch, but that isn't really guaranteed to happen
with SimplifyCFG's visitation order.

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/test/Transforms/SimplifyCFG/simplifyUnreachable-degenerate-conditional-branch-with-matching-destinations.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index cf4854d27f4a..7921a7462e2d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4683,16 +4683,18 @@ bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
     Instruction *TI = Predecessor->getTerminator();
     IRBuilder<> Builder(TI);
     if (auto *BI = dyn_cast<BranchInst>(TI)) {
-      if (BI->isUnconditional()) {
-        assert(BI->getSuccessor(0) == BB && "Incorrect CFG");
+      // We could either have a proper unconditional branch,
+      // or a degenerate conditional branch with matching destinations.
+      if (all_of(BI->successors(),
+                 [BB](auto *Successor) { return Successor == BB; })) {
         new UnreachableInst(TI->getContext(), TI);
         TI->eraseFromParent();
         Changed = true;
       } else {
+        assert(BI->isConditional() && "Can't get here with an uncond branch.");
         Value* Cond = BI->getCondition();
-        //        assert(BI->getSuccessor(0) != BI->getSuccessor(1) &&
-        //               "Same-destination conditional branch instruction was "
-        //               "already canonicalized into an unconditional branch.");
+        assert(BI->getSuccessor(0) != BI->getSuccessor(1) &&
+               "The destinations are guaranteed to be 
diff erent here.");
         if (BI->getSuccessor(0) == BB) {
           Builder.CreateAssumption(Builder.CreateNot(Cond));
           Builder.CreateBr(BI->getSuccessor(1));

diff  --git a/llvm/test/Transforms/SimplifyCFG/simplifyUnreachable-degenerate-conditional-branch-with-matching-destinations.ll b/llvm/test/Transforms/SimplifyCFG/simplifyUnreachable-degenerate-conditional-branch-with-matching-destinations.ll
index b56d9d435925..66218a10521b 100644
--- a/llvm/test/Transforms/SimplifyCFG/simplifyUnreachable-degenerate-conditional-branch-with-matching-destinations.ll
+++ b/llvm/test/Transforms/SimplifyCFG/simplifyUnreachable-degenerate-conditional-branch-with-matching-destinations.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -simplifycfg -simplifycfg-require-and-preserve-domtree=0 < %s | FileCheck %s
+; RUN: opt -S -simplifycfg -simplifycfg-require-and-preserve-domtree=1 < %s | FileCheck %s
 
 @global = external global i16, align 1
 @global.1 = external global i16, align 1
@@ -11,14 +11,6 @@ define void @widget() {
 ; CHECK-NEXT:    [[I:%.*]] = load i16, i16* @global, align 1
 ; CHECK-NEXT:    [[I13:%.*]] = icmp ne i16 [[I]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[I13]])
-; CHECK-NEXT:    [[I17:%.*]] = load i16, i16* @global, align 1
-; CHECK-NEXT:    [[I18:%.*]] = sdiv i16 2, [[I17]]
-; CHECK-NEXT:    [[I19:%.*]] = icmp ne i16 [[I18]], 0
-; CHECK-NEXT:    [[I20:%.*]] = zext i1 [[I19]] to i16
-; CHECK-NEXT:    [[I21:%.*]] = load i16, i16* @global.1, align 1
-; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[I19]], i16 [[I20]], i16 [[I21]]
-; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[I19]], true
-; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
 ; CHECK-NEXT:    unreachable
 ;
 bb: