[SimplifyCFG] Preserve metadata when unconditionalizing branches (same target).

[Meinersbur]

Summary

When replacing a conditional branch by an unconditional one because the targets are identical, transfer the metadata to the new branch instruction.

[Meinersbur]

• Transfer only selected metadata kinds.

[Meinersbur]

• clang-format

[lebedev.ri]

This LGTM, thank you.

llvm/lib/Transforms/Utils/Local.cpp

155

I'm guessing you are intentionally not copying mustprogress?

[Meinersbur]

llvm/lib/Transforms/Utils/Local.cpp

155

As @jeroen.dobbelaere mentioned in D101141, mustprogress is a property inside the MD_loop metadata.

llvm/lib/Transforms/Utils/Local.cpp

if (Dest2 == Dest1) {       // Conditional branch to same location?
       //     br bool %cond, label %Dest, label %Dest
       // and changes it into:  br label %Dest
 
       // Let the basic block know that we are letting go of one copy of it.
       assert(BI->getParent() && "Terminator not inserted in block!");
       Dest1->removePredecessor(BI->getParent());
 
       // Replace the conditional branch with an unconditional one.
-      Builder.CreateBr(Dest1);
+      BranchInst *NewBI = Builder.CreateBr(Dest1);
+
+      // Transfer the metadata to the new branch instruction.
+      NewBI->copyMetadata(*BI, {LLVMContext::MD_loop, LLVMContext::MD_dbg,
+                                LLVMContext::MD_annotation});
+
       Value *Cond = BI->getCondition();
       BI->eraseFromParent();
       if (DeleteDeadConditions)
         RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
       return true;
     }
 
     if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {

llvm/test/Transforms/SimplifyCFG/commondest-loopid.ll

+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+;
+; Ensure that the loop metadata is preserved when converting the
+; conditional branch to an unconditional.
+
+define void @commondest_loopid(i1 %T) {
+; CHECK-LABEL: @commondest_loopid(
+; CHECK: !llvm.loop !0
+; CHECK: !0 = distinct !{!0, !1}
+; CHECK: !1 = !{!"loopprop"}
+entry:
+        br label %loop
+
+loop:
+        br i1 %T, label %loop, label %loop, !llvm.loop !0
+}
+
+!0 = distinct !{!0, !1}
+!1 = !{!"loopprop"}