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

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.

---------

Co-authored-by: Paul T Robinson <[email protected]>

Author: ostannard

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());
+  }
 }

llvm/test/DebugInfo/ARM/align-func-start.ll

@@ -0,0 +1,30 @@
+; 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 that, when an aligned loop is the first thing in a function, we do not
+; emit an invalid .loc directive, which is rejected by the assembly parser.
+
+; CHECK-NOT: .loc    0
+; CHECK: .loc    1 2 3 prologue_end
+; CHECK-NOT: .loc 0
+
+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 ([email protected]: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)