[JumpThreading] Retain debug info when replacing branch instructions

Fixes bug 37966: https://bugs.llvm.org/show_bug.cgi?id=37966

The Jump Threading pass will replace certain conditional branch
instructions with unconditional branches when it can prove that only one
branch can occur. Prior to this patch, it would not carry the debug
info from the old instruction to the new one.

This patch fixes the bug described by copying the debug info from the
conditional branch instruction to the new unconditional branch
instruction, and adds a regression test for the Jump Threading pass that
covers this case.

Patch by Stephen Tozer!

Differential Revision: https://reviews.llvm.org/D58963

llvm-svn: 355822

2 files changed, 46 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 7d6ffa0e0e4..264ea3aa22a 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1142,7 +1142,9 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
         BasicBlock *ToRemoveSucc = CondBr->getSuccessor(ToRemove);
         ToRemoveSucc->removePredecessor(BB, true);
-        BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
+        BranchInst *UncondBr =
+          BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
+        UncondBr->setDebugLoc(CondBr->getDebugLoc());
         CondBr->eraseFromParent();
         if (CondCmp->use_empty())
           CondCmp->eraseFromParent();
@@ -1245,7 +1247,8 @@ bool JumpThreadingPass::ProcessImpliedCondition(BasicBlock *BB) {
       BasicBlock *KeepSucc = BI->getSuccessor(*Implication ? 0 : 1);
       BasicBlock *RemoveSucc = BI->getSuccessor(*Implication ? 1 : 0);
       RemoveSucc->removePredecessor(BB);
-      BranchInst::Create(KeepSucc, BI);
+      BranchInst *UncondBI = BranchInst::Create(KeepSucc, BI);
+      UncondBI->setDebugLoc(BI->getDebugLoc());
       BI->eraseFromParent();
       DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}});
       return true;
diff --git a/llvm/test/Transforms/JumpThreading/branch-debug-info.ll b/llvm/test/Transforms/JumpThreading/branch-debug-info.ll
new file mode 100644
index 00000000000..773ed0f71de
--- /dev/null
+++ b/llvm/test/Transforms/JumpThreading/branch-debug-info.ll
@@ -0,0 +1,41 @@
+; RUN: opt %s -debugify -jump-threading -S | FileCheck %s
+; Tests Bug 37966
+
+define void @test0(i32 %i) {
+; CHECK-LABEL: @test0(
+; CHECK: left:
+; CHECK: br label %left, !dbg ![[DBG0:[0-9]+]]
+ entry:
+  %c0 = icmp ult i32 %i, 5
+  br i1 %c0, label %left, label %right
+
+ left:
+  br i1 %c0, label %left, label %right
+
+ right:
+  ret void
+}
+
+define void @test1(i32 %i, i32 %len) {
+; CHECK-LABEL: @test1(
+; CHECK: left:
+; CHECK: br label %right, !dbg ![[DBG1:[0-9]+]]
+ entry:
+  %i.inc = add nuw i32 %i, 1
+  %c0 = icmp ult i32 %i.inc, %len
+  br i1 %c0, label %left, label %right
+
+ left:
+  %c1 = icmp ult i32 %i, %len
+  br i1 %c1, label %right, label %left0
+
+ left0:
+  ret void
+
+ right:
+  ret void
+}
+
+; CHECK-DAG: ![[DBG0]] = !DILocation(
+; CHECK-DAG: ![[DBG1]] = !DILocation(
+