[DWARF] Emit line 0 source locations for BB padding nops (#99496)

This patch makes LLVM emit line 0 source locations for nops emitted as
basic block padding.

---------

Co-authored-by: Orlando Cazalet-Hyams <[email protected]>

Author: omern1

llvm/include/llvm/CodeGen/DebugHandlerBase.h

@@ -137,6 +137,8 @@ class DebugHandlerBase {
   void beginBasicBlockSection(const MachineBasicBlock &MBB);
   void endBasicBlockSection(const MachineBasicBlock &MBB);
 
+  virtual void beginCodeAlignment(const MachineBasicBlock &MBB) {}
+
   /// Return Label preceding the instruction.
   MCSymbol *getLabelBeforeInsn(const MachineInstr *MI);
 

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -3967,6 +3967,9 @@ void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) {
     CurrentSectionBeginSym = MBB.getSymbol();
   }
 
+  for (auto &Handler : DebugHandlers)
+    Handler->beginCodeAlignment(MBB);
+
   // Emit an alignment directive for this block, if needed.
   const Align Alignment = MBB.getAlignment();
   if (Alignment != Align(1))

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -3674,3 +3674,21 @@ bool DwarfDebug::alwaysUseRanges(const DwarfCompileUnit &CU) const {
     return true;
   return false;
 }
+
+void DwarfDebug::beginCodeAlignment(const MachineBasicBlock &MBB) {
+  if (MBB.getAlignment() == Align(1))
+    return;
+
+  auto *SP = MBB.getParent()->getFunction().getSubprogram();
+  bool NoDebug =
+      !SP || SP->getUnit()->getEmissionKind() == DICompileUnit::NoDebug;
+
+  if (NoDebug)
+    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());
+}

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h

@@ -730,6 +730,9 @@ class DwarfDebug : public DebugHandlerBase {
   /// Process beginning of an instruction.
   void beginInstruction(const MachineInstr *MI) override;
 
+  /// Process beginning of code alignment.
+  void beginCodeAlignment(const MachineBasicBlock &MBB) override;
+
   /// Perform an MD5 checksum of \p Identifier and return the lower 64 bits.
   static uint64_t makeTypeSignature(StringRef Identifier);
 

llvm/test/CodeGen/X86/fsafdo_test1.ll

@@ -4,9 +4,9 @@
 ; Check that fs-afdo discriminators are generated.
 ; V01: .loc    1 7 3 is_stmt 0 discriminator 2 # foo.c:7:3
 ; V01: .loc    1 9 5 is_stmt 1 discriminator 2 # foo.c:9:5
-; V0: .loc    1 9 5 is_stmt 0 discriminator 11266 # foo.c:9:5
+; V0: .loc    1 9 5 discriminator 11266 # foo.c:9:5
 ; V0: .loc    1 7 3 is_stmt 1 discriminator 11266 # foo.c:7:3
-; V1: .loc    1 9 5 is_stmt 0 discriminator 514 # foo.c:9:5
+; V1: .loc    1 9 5 discriminator 514 # foo.c:9:5
 ; V1: .loc    1 7 3 is_stmt 1 discriminator 258 # foo.c:7:3
 ; Check that variable __llvm_fs_discriminator__ is generated.
 ; V01: .type   __llvm_fs_discriminator__,@object # @__llvm_fs_discriminator__

llvm/test/CodeGen/X86/fsafdo_test4.ll

@@ -1,11 +1,16 @@
-; RUN: llc -enable-fs-discriminator -improved-fs-discriminator=false < %s | FileCheck %s
-; RUN: llc -enable-fs-discriminator -improved-fs-discriminator=true < %s | FileCheck %s
+; RUN: llc -enable-fs-discriminator -improved-fs-discriminator=false < %s | FileCheck --implicit-check-not=.loc %s
+; RUN: llc -enable-fs-discriminator -improved-fs-discriminator=true < %s | FileCheck --implicit-check-not=.loc %s
 ;
 ; Check that fs-afdo discriminators are NOT generated, as debugInfoForProfiling is false (not set).
+; CHECK: .loc    1 7 15 prologue_end discriminator 2 # foo.c:7:15
 ; CHECK: .loc    1 7 3 is_stmt 0 discriminator 2 # foo.c:7:3
+; CHECK: .loc    1 0 3 # foo.c:0:3
 ; CHECK: .loc    1 9 5 is_stmt 1 discriminator 2 # foo.c:9:5
-; CHECK-NOT: .loc    1 9 5 is_stmt 0 discriminator
-; CHECK-NOT: .loc    1 7 3 is_stmt 1 discriminator
+; CHECK: .loc    1 0 5 is_stmt 0 # :0:5
+; CHECK: .loc    1 9 5 discriminator 2 # foo.c:9:5
+; CHECK: .loc    1 0 5 # :0:5
+; CHECK: .loc	   1 7 3 is_stmt 1 discriminator 2 # foo.c:7:3
+; CHECK: .loc    1 14 3 # foo.c:14:3
 ; Check that variable __llvm_fs_discriminator__ is NOT generated.
 ; CHECK-NOT: __llvm_fs_discriminator__:
 

llvm/test/DebugInfo/X86/loop-align-debug.ll

@@ -0,0 +1,55 @@
+; RUN: llc %s --filetype=obj -o %t
+; RUN: llvm-objdump -d %t | FileCheck %s --check-prefixes=OBJ
+; RUN: llvm-dwarfdump --debug-line %t | FileCheck %s --check-prefixes=DBG
+; RUN: llc %s -o - | FileCheck %s --check-prefixes=ASM
+
+; OBJ: 1:{{.*}}nop
+
+;;     Address            Line   Column File   ISA Discriminator OpIndex Flags
+; DBG: 0x0000000000000000      3      0      0   0             0       0  is_stmt
+; DBG: 0x0000000000000001      0      0      0   0             0       0
+; DBG: 0x0000000000000010      5      0      0   0             0       0  is_stmt prologue_end
+; DBG: 0x0000000000000017      5      0      0   0             0       0  is_stmt end_sequence
+
+; ASM:      .loc    0 0 0 is_stmt 0
+; ASM-NEXT: .L{{.*}}:
+; ASM-NEXT: .p2align        4, 0x90
+
+;; $ cat test.cpp
+;; void g();
+;; void f() {
+;;   [[clang::code_align(16)]]
+;;   while (1) { g(); }
+;; }
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define dso_local void @f() local_unnamed_addr !dbg !9 {
+entry:
+  br label %while.body, !dbg !12
+
+while.body:                                       ; preds = %entry, %while.body
+  tail call void @g(), !dbg !12
+  br label %while.body, !dbg !12, !llvm.loop !13
+}
+
+declare !dbg !16 void @g() local_unnamed_addr
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 19.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{!"clang version 19.0.0git"}
+!9 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 3, type: !10, scopeLine: 3, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!10 = !DISubroutineType(types: !11)
+!11 = !{}
+!12 = !DILocation(line: 5, scope: !9)
+!13 = distinct !{!13, !12, !12, !14, !15}
+!14 = !{!"llvm.loop.mustprogress"}
+!15 = !{!"llvm.loop.align", i32 16}
+!16 = !DISubprogram(name: "g", scope: !1, file: !1, line: 2, type: !10, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)