Skip to content

[MC][DebugInfo] Emit linetable entries with known offsets immediately #134677

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 7 commits into from
May 20, 2025
Merged

[MC][DebugInfo] Emit linetable entries with known offsets immediately #134677

Show file tree
Hide file tree
Changes from 6 commits
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
2bf5eb6
[MC][DebugInfo] Emit linetable entries with known offsets immediately
jmorse Apr 7, 2025
efd0368
Disable this performance opt on RISCV
jmorse Apr 7, 2025
174d5b7
Use absoluteSymbolDiff, add target hook for whether to prefer using r…
jmorse May 7, 2025
e39cb12
Test that simple .texts can have .debug_line emitted in one data frag…
jmorse May 7, 2025
2e5c839
Remove requiresDiffExpressionRelocations target hook
jmorse May 8, 2025
ba944d4
Update RISVC test for new expected output
jmorse May 8, 2025
c36881d
Review feedback (improved comments, mandatory REQUIRES)
jmorse May 12, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
13 changes: 13 additions & 0 deletions llvm/lib/MC/MCObjectStreamer.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -464,6 +464,19 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
Label, PointerSize);
return;
}

// If the two labels are within the same fragment and it's a plain data
Copy link
Member

@MaskRay MaskRay May 9, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

If the two labels are within the same data fragment, then the address-offset is ...

(a data fragment is a MCDataFragment. There is no special or plain data fragment)

// fragment, then the address-offset is already a fixed constant and is not
// relaxable. Emit the advance-line-addr data immediately to save time and
// memory.
if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
SmallString<16> Tmp;
MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
LineDelta, *OptAddrDelta, Tmp);
emitBytes(Tmp);
return;
}

const MCExpr *AddrDelta = buildSymbolDiff(*this, Label, LastLabel, SMLoc());
insert(getContext().allocFragment<MCDwarfLineAddrFragment>(LineDelta,
*AddrDelta));
Expand Down
10 changes: 4 additions & 6 deletions llvm/test/DebugInfo/RISCV/dwarf-riscv-relocs.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -38,7 +38,7 @@
; LINE-DUMP: .debug_line contents:
; LINE-DUMP-NEXT: debug_line[0x00000000]
; LINE-DUMP-NEXT: Line table prologue:
; LINE-DUMP-NEXT: total_length: 0x00000067
; LINE-DUMP-NEXT: total_length: 0x00000061
; LINE-DUMP-NEXT: format: DWARF32
; LINE-DUMP-NEXT: version: 5
; LINE-DUMP-NEXT: address_size: 4
Expand Down Expand Up @@ -83,12 +83,10 @@
; LINE-DUMP-NEXT: 0x000000000000001c 3 5 0 0 0 0 is_stmt prologue_end
; LINE-DUMP-NEXT:0x0000005d: 06 DW_LNS_negate_stmt
; LINE-DUMP-NEXT:0x0000005e: 0b DW_LNS_set_epilogue_begin
; LINE-DUMP-NEXT:0x0000005f: 03 DW_LNS_advance_line (3)
; LINE-DUMP-NEXT:0x00000061: 09 DW_LNS_fixed_advance_pc (addr += 0x0004, op-index = 0)
; LINE-DUMP-NEXT:0x00000064: 01 DW_LNS_copy
; LINE-DUMP-NEXT:0x0000005f: 4a address += 4, line += 0, op-index += 0
; LINE-DUMP-NEXT: 0x0000000000000020 3 5 0 0 0 0 epilogue_begin
; LINE-DUMP-NEXT:0x00000065: 09 DW_LNS_fixed_advance_pc (addr += 0x0010, op-index = 0)
; LINE-DUMP-NEXT:0x00000068: 00 DW_LNE_end_sequence
; LINE-DUMP-NEXT:0x00000060: 02 DW_LNS_advance_pc (addr += 16, op-index += 0)
; LINE-DUMP-NEXT:0x00000062: 00 DW_LNE_end_sequence
; LINE-DUMP-NEXT: 0x0000000000000030 3 5 0 0 0 0 end_sequence

; ModuleID = 'dwarf-riscv-relocs.c'
Expand Down
94 changes: 94 additions & 0 deletions llvm/test/DebugInfo/X86/debug-line-in-one-fragment.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,94 @@
; RUN: llc %s -o %t.o -filetype=obj
; RUN: llvm-dwarfdump --debug-line %t.o | FileCheck %s --check-prefix=LINES
; RUN: llc %s -o %t.o -filetype=obj -debug-only=mc-dump 2>&1 | FileCheck %s --check-prefix=FRAGMENTS
Copy link
Member

@MaskRay MaskRay May 9, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Adding -debug-only tests requires caution. They need REQUIRES: asserts


;; Test (using mc-dump debug output) that .debug_line can be arranged in memory
;; using a single data fragment for a simple function, instead of using multiple
;; MCDwarfFragment fragments in un-necessary cirucmstances. Some targets want
;; multiple fragments so that they can linker-relax the linetable, but x86
;; doesn't.
;;
;; First, sanity check that the linetable output is as expected,

; LINES: Address Line Column File ISA Discriminator OpIndex Flags
; LINES-NEXT: ------------------ ------ ------ ------ --- ------------- ------- -------------
; LINES-NEXT: 0x0000000000000000 3 5 0 0 0 0 is_stmt prologue_end
; LINES-NEXT: 0x0000000000000003 4 12 0 0 0 0 is_stmt
; LINES-NEXT: 0x0000000000000007 4 3 0 0 0 0
; LINES-NEXT: 0x0000000000000008 4 3 0 0 0 0 end_sequence

;; Here's a typical example of .debug_line in a suboptimal arrangement: for each
;; address-delta there's an MCDwarfFragment computing the delta during
;; relaxation.
;;
;; <MCSection Name:.debug_line Fragments:[
;; <MCDataFragment<MCFragment
;; Contents:[...
;; Fixups:[...
;; <MCDwarfFragment<MCFragment 0x5624fe435a80 LayoutOrder:1 Offset:86 HasInstructions:0 BundlePadding:0>
;; AddrDelta:- LineDelta:1>,
;; <MCDataFragment<MCFragment 0x5624fe435b00 LayoutOrder:2 Offset:87 HasInstructions:0 BundlePadding:0>
;; Contents:[05,03,06] (3 bytes)>,
;; <MCDwarfFragment<MCFragment 0x5624fe435bd0 LayoutOrder:3 Offset:90 HasInstructions:0 BundlePadding:0>
;; AddrDelta:- LineDelta:0>,
;; <MCDwarfFragment<MCFragment 0x5624fe435c50 LayoutOrder:4 Offset:91 HasInstructions:0 BundlePadding:0>
;; AddrDelta:- LineDelta:9223372036854775807>,
;; <MCDataFragment<MCFragment 0x5624fe435cd0 LayoutOrder:5 Offset:96 HasInstructions:0 BundlePadding:0>
;; Contents:[] (0 bytes)>]>,
;;
;; The function in question is made of a single data fragment where the address
;; deltas are known at assembly time. We can (and should) emit .debug_line as a
;; single data fragment. (Check that we see one data fragment, then no more
;; fragments until the next section).
;
; FRAGMENTS: <MCSection Name:.debug_line Fragments:[
; FRAGMENTS-NEXT: <MCDataFragment<MCFragment
; FRAGMENTS-NEXT: Contents:[
; FRAGMENTS-NEXT: Fixups:[
;
; FRAGMENTS-NOT: MCDataFragment
; FRAGMENTS-NOT: MCFragment
;
; FRAGMENTS: <MCSection Name:.debug_line_str Fragments:[

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 i32 @foo(i32 noundef %a, i32 noundef %b) local_unnamed_addr #0 !dbg !10 {
entry:
#dbg_value(i32 %a, !15, !DIExpression(), !17)
#dbg_value(i32 %b, !16, !DIExpression(), !17)
%add = add nsw i32 %a, 1, !dbg !18
#dbg_value(i32 %add, !15, !DIExpression(), !17)
%mul = mul nsw i32 %b, 5, !dbg !19
#dbg_value(i32 %mul, !16, !DIExpression(), !17)
%add1 = add nsw i32 %add, %mul, !dbg !20
ret i32 %add1, !dbg !21
}

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!2, !3, !4, !5, !6, !7, !8}
!llvm.ident = !{!9}

!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 21.0.0git (/fast/fs/llvm4 8bd196e0eaf4310ad2d9598512d13220b28b9aee)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "text.c", directory: "/fast/fs/llvm-main", checksumkind: CSK_MD5, checksum: "e81ed96ea393640bf1b965103b190e09")
!2 = !{i32 7, !"Dwarf Version", i32 5}
!3 = !{i32 2, !"Debug Info Version", i32 3}
!4 = !{i32 1, !"wchar_size", i32 4}
!5 = !{i32 8, !"PIC Level", i32 2}
!6 = !{i32 7, !"PIE Level", i32 2}
!7 = !{i32 7, !"uwtable", i32 2}
!8 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
!9 = !{!"clang version 21.0.0git (/fast/fs/llvm4 8bd196e0eaf4310ad2d9598512d13220b28b9aee)"}
!10 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !11, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !14)
!11 = !DISubroutineType(types: !12)
!12 = !{!13, !13, !13}
!13 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!14 = !{!15, !16}
!15 = !DILocalVariable(name: "a", arg: 1, scope: !10, file: !1, line: 1, type: !13)
!16 = !DILocalVariable(name: "b", arg: 2, scope: !10, file: !1, line: 1, type: !13)
!17 = !DILocation(line: 0, scope: !10)
!18 = !DILocation(line: 2, column: 5, scope: !10)
!19 = !DILocation(line: 3, column: 5, scope: !10)
!20 = !DILocation(line: 4, column: 12, scope: !10)
!21 = !DILocation(line: 4, column: 3, scope: !10)
Loading