[SimplifyCFG] Mark missed Changed to true.

[asbirlea]

Summary

DominatorTree is invalid after SimplifyCFG because of a missed Changed = true when simplifying a branch condition and removing an edge.
Resolves PR42272.

[leonardchan]

*ping* We would like to bump up the priority of this since this allows us to enable the new PM when building fuchsia.

[sanjoy]

lgtm

lib/Transforms/Utils/SimplifyCFG.cpp

4181

Is this check necessary? If not, might be nice to remove it (in a separate change).

4192

You could sink down the Changed = true; outside the if/else.

test/Transforms/SimplifyCFG/invalidate-dom.ll

16

You can use opt -metarenamer to give names to all these instructions. That makes the test a bit more resilient to changes.

[asbirlea]

Address comments.

lib/Transforms/Utils/SimplifyCFG.cpp

4192

Right, but this is non-obvious because of the else if below (instead of else). Unless one notices that this iterates over BB's predecessors, then they may think BB may not be either the 0 or 1 successor.
Let me send the clean-up patch for the above and for this separately, since we can also sink the call to EraseTerminatorAndDCECond(BI); here.

test/Transforms/SimplifyCFG/invalidate-dom.ll

16

SG, but I renamed just the instructions manually. The metarenamer renames everything and I for one get more confused when everything is named foo, bar etc :).

lib/Transforms/Utils/SimplifyCFG.cpp

if (&BB->front() != UI)
     return Changed;
 
   SmallVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
   for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
     Instruction *TI = Preds[i]->getTerminator();
     IRBuilder<> Builder(TI);
     if (auto *BI = dyn_cast<BranchInst>(TI)) {
       if (BI->isUnconditional()) {
         if (BI->getSuccessor(0) == BB) {
           new UnreachableInst(TI->getContext(), TI);
           TI->eraseFromParent();
           Changed = true;
         }
       } else {
         Value* Cond = BI->getCondition();
         if (BI->getSuccessor(0) == BB) {
           Builder.CreateAssumption(Builder.CreateNot(Cond));
           Builder.CreateBr(BI->getSuccessor(1));
           EraseTerminatorAndDCECond(BI);
+          Changed = true;
         } else if (BI->getSuccessor(1) == BB) {
           Builder.CreateAssumption(Cond);
           Builder.CreateBr(BI->getSuccessor(0));
           EraseTerminatorAndDCECond(BI);
           Changed = true;
         }
       }
     } else if (auto *SI = dyn_cast<SwitchInst>(TI)) {

test/Transforms/SimplifyCFG/invalidate-dom.ll

+; RUN: opt -aa-pipeline=default -passes="default<O2>" -disable-output %s
+; REQUIRES: asserts
+; PR42272
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@b = external global i32, align 4
+@f = external global i32, align 4
+@g = external global i32, align 4
+
+define i32* @a(i32 %h) #0 {
+entry:
+  %h.addr = alloca i32, align 4
+  store i32 %h, i32* %h.addr, align 4
+  %0 = load i32, i32* %h.addr, align 4
+  switch i32 %0, label %sw.default [
+    i32 4, label %sw.bb
+    i32 3, label %sw.bb1
+    i32 2, label %sw.bb3
+  ]
+
+sw.bb:                                            ; preds = %entry
+  %call = call i32 (...) @c()
+  unreachable
+
+sw.bb1:                                           ; preds = %entry
+  %call2 = call i32 (...) @c()
+  unreachable
+
+sw.bb3:                                           ; preds = %entry
+  %call4 = call i32 (...) @c()
+  %conv = sext i32 %call4 to i64
+  %1 = inttoptr i64 %conv to i32*
+  ret i32* %1
+
+sw.default:                                       ; preds = %entry
+  unreachable
+}
+
+define i32 @m() #1 {
+entry:
+  %call = call i32 @j()
+  %call1 = call i32 @j()
+  ret i32 undef
+}
+
+define internal i32 @j() #0 {
+entry:
+  %i = alloca i32, align 4
+  store i32 0, i32* %i, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %entry
+  %0 = load i32, i32* %i, align 4
+  %1 = load i32, i32* @f, align 4
+  %cmp = icmp ult i32 %0, %1
+  br i1 %cmp, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  %2 = load i32, i32* @f, align 4
+  %call3 = call i32* @a(i32 %2)
+  ret i32 undef
+
+for.body:                                         ; preds = %for.cond
+  %call = call i32 (...) @c()
+  %call1 = call i32 (...) @c()
+  %call2 = call i32 (...) @c()
+  %3 = load i32, i32* @b, align 4
+  %4 = load i32, i32* @g, align 4
+  %sub = sub nsw i32 %4, %3
+  store i32 %sub, i32* @g, align 4
+  %5 = load i32, i32* %i, align 4
+  %inc = add i32 %5, 1
+  store i32 %inc, i32* %i, align 4
+  br label %for.cond
+}
+
+declare i32 @c(...) #0
+
+; Function Attrs: argmemonly nounwind
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #2
+
+; Function Attrs: argmemonly nounwind
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #2
+
+attributes #0 = { "use-soft-float"="false" }
+attributes #1 = { "target-cpu"="x86-64" }
+attributes #2 = { argmemonly nounwind }
+