[llvm] [DebugInfo][SimplifyCFGPass] Fix the missing debug location update for the new br instruction (PR #97389)

[Shan Huang via llvm-commits]

https://github.com/Apochens created https://github.com/llvm/llvm-project/pull/97389

Fix #97388 .

>From de773f30a0f83b152000d702090fffbd2a73b510 Mon Sep 17 00:00:00 2001
From: Apochens <52285902006 at stu.ecnu.edu.cn>
Date: Tue, 2 Jul 2024 07:11:55 +0000
Subject: [PATCH] Fix the missing debug location update

---
 .../lib/Transforms/Scalar/SimplifyCFGPass.cpp |  4 +-
 .../SimplifyCFG/preserving-debugloc-br.ll     | 72 +++++++++++++++++++
 2 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/SimplifyCFG/preserving-debugloc-br.ll

diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 7017f6adf3a2b..b7baf34f27c21 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -142,8 +142,10 @@ performBlockTailMerging(Function &F, ArrayRef<BasicBlock *> BBs,
 
     // And turn BB into a block that just unconditionally branches
     // to the canonical block.
+    Instruction *BI = BranchInst::Create(CanonicalBB, BB);
+    BI->setDebugLoc(Term->getDebugLoc());
     Term->eraseFromParent();
-    BranchInst::Create(CanonicalBB, BB);
+
     if (Updates)
       Updates->push_back({DominatorTree::Insert, BB, CanonicalBB});
   }
diff --git a/llvm/test/Transforms/SimplifyCFG/preserving-debugloc-br.ll b/llvm/test/Transforms/SimplifyCFG/preserving-debugloc-br.ll
new file mode 100644
index 0000000000000..298701ff61b49
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/preserving-debugloc-br.ll
@@ -0,0 +1,72 @@
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+; Check that SimplifyCFGPass's performBlockTailMerging() propagates
+; the debug location of the old block terminator to the new branch
+; instruction.
+
+define i32 @foo(i64 %x, i64 %y) !dbg !5 {
+; CHECK-LABEL: define i32 @foo(
+; CHECK:       a:
+; CHECK:         br label %[[COMMON_RET:.*]], !dbg [[DBG14:![0-9]+]]
+; CHECK:       b:
+; CHECK:         br label %[[COMMON_RET]], !dbg [[DBG17:![0-9]+]]
+;
+entry:
+  %eq = icmp eq i64 %x, %y, !dbg !8
+  br i1 %eq, label %b, label %switch, !dbg !9
+
+switch:                                           ; preds = %entry
+  %lt = icmp slt i64 %x, %y, !dbg !10
+  %qux = select i1 %lt, i32 0, i32 2, !dbg !11
+  switch i32 %qux, label %bees [
+  i32 0, label %a
+  i32 1, label %b
+  i32 2, label %b
+  ], !dbg !12
+
+a:                                                ; preds = %switch
+  tail call void @bees.a(), !dbg !13
+  ret i32 1, !dbg !14
+
+b:                                                ; preds = %switch, %switch, %entry
+  %retval = phi i32 [ 0, %switch ], [ 0, %switch ], [ 2, %entry ], !dbg !15
+  tail call void @bees.b(), !dbg !16
+  ret i32 %retval, !dbg !17
+
+bees:                                             ; preds = %switch
+  tail call void @llvm.trap(), !dbg !18
+  unreachable, !dbg !19
+}
+
+
+declare void @llvm.trap()
+declare void @bees.a()
+declare void @bees.b()
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+; CHECK: [[DBG14]] = !DILocation(line: 7,
+; CHECK: [[DBG17]] = !DILocation(line: 10,
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "main.ll", directory: "/")
+!2 = !{i32 12}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !DILocation(line: 1, column: 1, scope: !5)
+!9 = !DILocation(line: 2, column: 1, scope: !5)
+!10 = !DILocation(line: 3, column: 1, scope: !5)
+!11 = !DILocation(line: 4, column: 1, scope: !5)
+!12 = !DILocation(line: 5, column: 1, scope: !5)
+!13 = !DILocation(line: 6, column: 1, scope: !5)
+!14 = !DILocation(line: 7, column: 1, scope: !5)
+!15 = !DILocation(line: 8, column: 1, scope: !5)
+!16 = !DILocation(line: 9, column: 1, scope: !5)
+!17 = !DILocation(line: 10, column: 1, scope: !5)
+!18 = !DILocation(line: 11, column: 1, scope: !5)
+!19 = !DILocation(line: 12, column: 1, scope: !5)