From 903cf140d0118cf0d3f0f6f8967c6a20d9c5be6b Mon Sep 17 00:00:00 2001
From: Vedant Kumar <vsk@apple.com>
Date: Wed, 24 Jun 2020 11:33:28 -0700
Subject: [PATCH] [InstCombine] Drop debug loc in TryToSinkInstruction

Summary:
The advice in HowToUpdateDebugInfo.rst is to "... preserve the debug
location of an instruction if the instruction either remains in its
basic block, or if its basic block is folded into a predecessor that
branches unconditionally".

TryToSinkInstruction doesn't seem to satisfy the criteria as it's
sinking an instruction to some successor block. Preserving the debug loc
can make single-stepping appear to go backwards, or make a breakpoint
hit on that location happen "too late" (since single-stepping from that
breakpoint can cause the function to return unexpectedly).

So, drop the debug location.

Reviewers: aprantl, davide

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82487
---
 .../InstCombine/InstructionCombining.cpp           |  4 +++
 .../InstCombine/sink_to_unreachable_dbg.ll         | 42 ++++++++++++++++++++++
 2 files changed, 46 insertions(+)
 create mode 100644 llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 1f97f0c..3bdf052 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3355,6 +3355,10 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock) {
   I->moveBefore(&*InsertPos);
   ++NumSunkInst;
 
+  // Drop the debug loc. This prevents single-stepping from going backwards.
+  // See HowToUpdateDebugInfo.rst for the full rationale.
+  I->setDebugLoc(DebugLoc());
+
   // Also sink all related debug uses from the source basic block. Otherwise we
   // get debug use before the def. Attempt to salvage debug uses first, to
   // maximise the range variables have location for. If we cannot salvage, then
diff --git a/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
new file mode 100644
index 0000000..2b7aa75
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
@@ -0,0 +1,42 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test(
+; CHECK: [[phi:%.*]] = phi i32
+; CHECK-NEXT: [[add:%.*]] = add i32 %x, 1{{$}}
+; CHECK-NEXT: add i32 [[phi]], [[add]], !dbg
+define i32 @test(i32 %x, i1 %c) !dbg !6 {
+bb0:
+  %a = add i32 %x, 1, !dbg !8
+  br i1 %c, label %bb1, label %bb2, !dbg !9
+
+bb1:                                              ; preds = %bb0
+  br label %bb3, !dbg !10
+
+bb2:                                              ; preds = %bb0
+  br label %bb3, !dbg !11
+
+bb3:                                              ; preds = %bb2, %bb1
+  %p = phi i32 [ 0, %bb1 ], [ 1, %bb2 ], !dbg !12
+  %r = add i32 %p, %a, !dbg !13
+  ret i32 %r, !dbg !14
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "sink_to_unreachable_dbg.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 7}
+!4 = !{i32 0}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "test", linkageName: "test", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 1, column: 1, scope: !6)
+!9 = !DILocation(line: 2, column: 1, scope: !6)
+!10 = !DILocation(line: 3, column: 2, scope: !6)
+!11 = !DILocation(line: 4, column: 3, scope: !6)
+!12 = !DILocation(line: 5, column: 4, scope: !6)
+!13 = !DILocation(line: 6, column: 4, scope: !6)
+!14 = !DILocation(line: 7, column: 4, scope: !6)
-- 
2.7.4