[llvm] [DebugInfo] Don't emit .loc directive with all values zero (PR #109978)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-debuginfo

Author: Oliver Stannard (ostannard)

<details>
<summary>Changes</summary>

When emitting debug info for code alignment, it was possible to emit a .loc directive with a file number of zero, which is invalid for DWARF 4 and earlier. This happened because getCurrentDwarfLoc() returned a zero-initialised value when there hadn't been a previous .loc directive emitted.

---
Full diff: https://github.com/llvm/llvm-project/pull/109978.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (+6-4) 
- (added) llvm/test/DebugInfo/ARM/align-func-start.ll (+26) 


``````````diff
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index e9649f9ff81658..f94240e6d2224b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -3682,8 +3682,10 @@ void DwarfDebug::beginCodeAlignment(const MachineBasicBlock &MBB) {
     return;
 
   auto PrevLoc = Asm->OutStreamer->getContext().getCurrentDwarfLoc();
-  Asm->OutStreamer->emitDwarfLocDirective(
-      PrevLoc.getFileNum(), 0, PrevLoc.getColumn(), 0, 0, 0, StringRef());
-  MCDwarfLineEntry::make(Asm->OutStreamer.get(),
-                         Asm->OutStreamer->getCurrentSectionOnly());
+  if (PrevLoc.getLine()) {
+    Asm->OutStreamer->emitDwarfLocDirective(
+        PrevLoc.getFileNum(), 0, PrevLoc.getColumn(), 0, 0, 0, StringRef());
+    MCDwarfLineEntry::make(Asm->OutStreamer.get(),
+                           Asm->OutStreamer->getCurrentSectionOnly());
+  }
 }
diff --git a/llvm/test/DebugInfo/ARM/align-func-start.ll b/llvm/test/DebugInfo/ARM/align-func-start.ll
new file mode 100644
index 00000000000000..60900ae30351cc
--- /dev/null
+++ b/llvm/test/DebugInfo/ARM/align-func-start.ll
@@ -0,0 +1,26 @@
+; RUN: llc -mtriple=arm-none-eabi < %s | FileCheck %s
+; RUN: llc -mtriple=arm-none-eabi < %s | llvm-mc --triple=arm-none-eabi -mcpu=cortex-m3
+
+; CHECK-NOT: .loc    0 0 0
+; CHECK: .loc    1 2 3 prologue_end
+
+define dso_local void @foo() "target-cpu"="cortex-m3" !dbg !8 {
+entry:
+  br label %while.body, !dbg !11
+
+while.body:
+  br label %while.body, !dbg !11
+}
+
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 20.0.0git (git at github.com:llvm/llvm-project.git 1c984b86b389bbc71c8c2988d1d707e2f32878bd)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/work/scratch")
+!2 = !{i32 7, !"Dwarf Version", i32 4}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!9 = !DISubroutineType(types: !10)
+!10 = !{null}
+!11 = !DILocation(line: 2, column: 3, scope: !8)

``````````

</details>


https://github.com/llvm/llvm-project/pull/109978