[codeview] Try to avoid emitting .cv_loc with line zero

[rnk]

Summary

Visual Studio doesn't like it while stepping. It kicks you out of the
source view of the file being stepped through and tries to fall back to
the disassembly view.

Fixes PR43530

The fix is incomplete, because it's possible to have a basic block with
no source locations at all. In this case, we don't emit a .cv_loc, but
that will result in wrong stepping behavior in the debugger if the
layout predecessor of the location-less BB has an unrelated source
location. We could try harder to find a valid location that dominates or
post-dominates the current BB, but in general it's a dataflow problem,
and one still might not exist. I left a FIXME about this.

As an alternative, we might want to consider having the middle-end check
if its emitting codeview and get it to stop using line zero.

[akhuang]

lgtm-

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

2863

typo

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

void CodeViewDebug::endFunctionImpl(const MachineFunction *MF) {
   CurFn->Annotations = MF->getCodeViewAnnotations();
   CurFn->HeapAllocSites = MF->getCodeViewHeapAllocSites();
 
   CurFn->End = Asm->getFunctionEnd();
 
   CurFn = nullptr;
 }
 
+// Usable locations are valid with non-zero line numbers. A line number of zero
+// corresponds to optimized code that doesn't have a distinct source location.
+// In this case, we try to use the previous or next source location depending on
+// the context.
+static bool isUsableDebugLoc(DebugLoc DL) {
+  return DL && DL.getLine() != 0;
+}
+
 void CodeViewDebug::beginInstruction(const MachineInstr *MI) {
   DebugHandlerBase::beginInstruction(MI);
 
   // Ignore DBG_VALUE and DBG_LABEL locations and function prologue.
   if (!Asm || !CurFn || MI->isDebugInstr() ||
       MI->getFlag(MachineInstr::FrameSetup))
     return;
 
   // If the first instruction of a new MBB has no location, find the first
   // instruction with a location and use that.
   DebugLoc DL = MI->getDebugLoc();
-  if (!DL && MI->getParent() != PrevInstBB) {
+  if (!isUsableDebugLoc(DL) && MI->getParent() != PrevInstBB) {
     for (const auto &NextMI : *MI->getParent()) {
       if (NextMI.isDebugInstr())
         continue;
       DL = NextMI.getDebugLoc();
-      if (DL)
+      if (isUsableDebugLoc(DL))
         break;
     }
+    // FIXME: Handle the case where the BB has no valid locations. This would
+    // probably require doing a real dataflow analysis.
   }
   PrevInstBB = MI->getParent();
 
   // If we still don't have a debug location, don't record a location.
-  if (!DL)
+  if (!isUsableDebugLoc(DL))
     return;
 
   maybeRecordLocation(DL, Asm->MF);
 }
 
 MCSymbol *CodeViewDebug::beginCVSubsection(DebugSubsectionKind Kind) {
   MCSymbol *BeginLabel = MMI->getContext().createTempSymbol(),
            *EndLabel = MMI->getContext().createTempSymbol();

llvm/test/DebugInfo/COFF/line-zero.ll

+; RUN: llc < %s | FileCheck %s
+
+; C++ source to regenerate:
+; int main() {
+;   volatile int x;
+;   x = 1;
+; #line 0
+;   x = 2;
+; #line 7
+;   x = 3;
+; }
+
+
+; CHECK-LABEL: main:                                   # @main
+; CHECK:         .cv_loc 0 1 1 0                 # t.cpp:1:0
+; CHECK:         .cv_loc 0 1 3 0                 # t.cpp:3:0
+; CHECK:         movl    $1, 4(%rsp)
+; CHECK-NOT: .cv_loc {{.*}} t.cpp:0:0
+; CHECK:         movl    $2, 4(%rsp)
+; CHECK:         .cv_loc 0 1 7 0                 # t.cpp:7:0
+; CHECK:         movl    $3, 4(%rsp)
+; CHECK:         .cv_loc 0 1 8 0                 # t.cpp:8:0
+; CHECK:         xorl    %eax, %eax
+; CHECK:         retq
+
+; ModuleID = 't.cpp'
+source_filename = "t.cpp"
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.22.27905"
+
+; Function Attrs: norecurse nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 !dbg !8 {
+entry:
+  %x = alloca i32, align 4
+  %x.0.x.0..sroa_cast = bitcast i32* %x to i8*, !dbg !15
+  call void @llvm.dbg.declare(metadata i32* %x, metadata !13, metadata !DIExpression()), !dbg !15
+  store volatile i32 1, i32* %x, align 4, !dbg !16, !tbaa !17
+  store volatile i32 2, i32* %x, align 4, !dbg !21, !tbaa !17
+  store volatile i32 3, i32* %x, align 4, !dbg !22, !tbaa !17
+  ret i32 0, !dbg !23
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #2
+
+attributes #0 = { norecurse nounwind uwtable }
+attributes #1 = { argmemonly nounwind willreturn }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "t.cpp", directory: "C:\5Csrc\5Cllvm-project\5Cbuild", checksumkind: CSK_MD5, checksum: "8b6d53b166e6fa660f115eff7beedf3b")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0"}
+!8 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!13}
+!13 = !DILocalVariable(name: "x", scope: !8, file: !1, line: 2, type: !14)
+!14 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !11)
+!15 = !DILocation(line: 2, scope: !8)
+!16 = !DILocation(line: 3, scope: !8)
+!17 = !{!18, !18, i64 0}
+!18 = !{!"int", !19, i64 0}
+!19 = !{!"omnipotent char", !20, i64 0}
+!20 = !{!"Simple C++ TBAA"}
+!21 = !DILocation(line: 0, scope: !8)
+!22 = !DILocation(line: 7, scope: !8)
+!23 = !DILocation(line: 8, scope: !8)

llvm/test/DebugInfo/COFF/local-variables.ll

 ; ASM:         .cv_loc 2 1 4 7                 # t.cpp:4:7
 ; ASM:         movl    $3, 48(%rsp)
 ; ASM:         leaq    48(%rsp), %rcx
 ; ASM:         .cv_loc 2 1 5 3                 # t.cpp:5:3
 ; ASM:         callq   capture
 ; ASM:         leaq    36(%rsp), %rcx
 ; ASM: [[else_end:\.Ltmp.*]]:
 ; ASM: .LBB0_3:                                # %if.end
-; ASM:         .cv_loc 0 1 0 0                # t.cpp:0:0
+; ASM:         .cv_loc 0 1 17 1                # t.cpp:17:1
 ; ASM:         callq   capture
 ; ASM:         nop
 ; ASM:         addq    $56, %rsp
 ; ASM:         retq
 ; ASM: [[param_end:\.Ltmp.*]]:
 
 ; ASM: .short  4414                    # Record kind: S_LOCAL
 ; ASM: .long   116                     # TypeIndex