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[MC][DebugInfo] Emit linetable entries with known offsets immediately (#134677)
DWARF linetable entries are usually emitted as a sequence of MCDwarfLineAddrFragment fragments containing the line-number difference and an MCExpr describing the instruction-range the linetable entry covers. These then get relaxed during assembly emission. However, a large number of these instruction-range expressions are ranges within a fixed MCDataFragment, i.e. a range over fixed-size instructions that are not subject to relaxation at a later stage. Thus, we can compute the address-delta immediately, and not spend time and memory describing that computation so it can be deferred.
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‎llvm/lib/MC/MCObjectStreamer.cpp

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@@ -464,6 +464,18 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
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Label, PointerSize);
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return;
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}
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// If the two labels are within the same fragment, then the address-offset is
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// already a fixed constant and is not relaxable. Emit the advance-line-addr
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// data immediately to save time and memory.
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if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
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SmallString<16> Tmp;
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MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
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LineDelta, *OptAddrDelta, Tmp);
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emitBytes(Tmp);
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return;
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}
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const MCExpr *AddrDelta = buildSymbolDiff(*this, Label, LastLabel, SMLoc());
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insert(getContext().allocFragment<MCDwarfLineAddrFragment>(LineDelta,
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*AddrDelta));

‎llvm/test/DebugInfo/RISCV/dwarf-riscv-relocs.ll

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@@ -38,7 +38,7 @@
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; LINE-DUMP: .debug_line contents:
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; LINE-DUMP-NEXT: debug_line[0x00000000]
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; LINE-DUMP-NEXT: Line table prologue:
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; LINE-DUMP-NEXT: total_length: 0x00000067
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; LINE-DUMP-NEXT: total_length: 0x00000061
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; LINE-DUMP-NEXT: format: DWARF32
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; LINE-DUMP-NEXT: version: 5
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; LINE-DUMP-NEXT: address_size: 4
@@ -83,12 +83,10 @@
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; LINE-DUMP-NEXT: 0x000000000000001c 3 5 0 0 0 0 is_stmt prologue_end
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; LINE-DUMP-NEXT:0x0000005d: 06 DW_LNS_negate_stmt
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; LINE-DUMP-NEXT:0x0000005e: 0b DW_LNS_set_epilogue_begin
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; LINE-DUMP-NEXT:0x0000005f: 03 DW_LNS_advance_line (3)
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; LINE-DUMP-NEXT:0x00000061: 09 DW_LNS_fixed_advance_pc (addr += 0x0004, op-index = 0)
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; LINE-DUMP-NEXT:0x00000064: 01 DW_LNS_copy
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; LINE-DUMP-NEXT:0x0000005f: 4a address += 4, line += 0, op-index += 0
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; LINE-DUMP-NEXT: 0x0000000000000020 3 5 0 0 0 0 epilogue_begin
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; LINE-DUMP-NEXT:0x00000065: 09 DW_LNS_fixed_advance_pc (addr += 0x0010, op-index = 0)
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; LINE-DUMP-NEXT:0x00000068: 00 DW_LNE_end_sequence
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; LINE-DUMP-NEXT:0x00000060: 02 DW_LNS_advance_pc (addr += 16, op-index += 0)
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; LINE-DUMP-NEXT:0x00000062: 00 DW_LNE_end_sequence
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; LINE-DUMP-NEXT: 0x0000000000000030 3 5 0 0 0 0 end_sequence
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; ModuleID = 'dwarf-riscv-relocs.c'

‎llvm/test/DebugInfo/X86/debug-line-in-one-fragment.ll

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,96 @@
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; RUN: llc %s -o %t.o -filetype=obj
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; RUN: llvm-dwarfdump --debug-line %t.o | FileCheck %s --check-prefix=LINES
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; RUN: llc %s -o %t.o -filetype=obj -debug-only=mc-dump 2>&1 | FileCheck %s --check-prefix=FRAGMENTS
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;
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; REQUIRES: asserts
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;; Test (using mc-dump debug output) that .debug_line can be arranged in memory
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;; using a single data fragment for a simple function, instead of using multiple
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;; MCDwarfFragment fragments in un-necessary cirucmstances. Some targets want
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;; multiple fragments so that they can linker-relax the linetable, but x86
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;; doesn't.
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;;
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;; First, sanity check that the linetable output is as expected,
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; LINES: Address Line Column File ISA Discriminator OpIndex Flags
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; LINES-NEXT: ------------------ ------ ------ ------ --- ------------- ------- -------------
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; LINES-NEXT: 0x0000000000000000 3 5 0 0 0 0 is_stmt prologue_end
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; LINES-NEXT: 0x0000000000000003 4 12 0 0 0 0 is_stmt
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; LINES-NEXT: 0x0000000000000007 4 3 0 0 0 0
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; LINES-NEXT: 0x0000000000000008 4 3 0 0 0 0 end_sequence
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;; Here's a typical example of .debug_line in a suboptimal arrangement: for each
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;; address-delta there's an MCDwarfFragment computing the delta during
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;; relaxation.
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;;
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;; <MCSection Name:.debug_line Fragments:[
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;; <MCDataFragment<MCFragment
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;; Contents:[...
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;; Fixups:[...
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;; <MCDwarfFragment<MCFragment 0x5624fe435a80 LayoutOrder:1 Offset:86 HasInstructions:0 BundlePadding:0>
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;; AddrDelta:- LineDelta:1>,
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;; <MCDataFragment<MCFragment 0x5624fe435b00 LayoutOrder:2 Offset:87 HasInstructions:0 BundlePadding:0>
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;; Contents:[05,03,06] (3 bytes)>,
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;; <MCDwarfFragment<MCFragment 0x5624fe435bd0 LayoutOrder:3 Offset:90 HasInstructions:0 BundlePadding:0>
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;; AddrDelta:- LineDelta:0>,
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;; <MCDwarfFragment<MCFragment 0x5624fe435c50 LayoutOrder:4 Offset:91 HasInstructions:0 BundlePadding:0>
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;; AddrDelta:- LineDelta:9223372036854775807>,
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;; <MCDataFragment<MCFragment 0x5624fe435cd0 LayoutOrder:5 Offset:96 HasInstructions:0 BundlePadding:0>
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;; Contents:[] (0 bytes)>]>,
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;;
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;; The function in question is made of a single data fragment where the address
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;; deltas are known at assembly time. We can (and should) emit .debug_line as a
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;; single data fragment. (Check that we see one data fragment, then no more
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;; fragments until the next section).
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;
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; FRAGMENTS: <MCSection Name:.debug_line Fragments:[
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; FRAGMENTS-NEXT: <MCDataFragment<MCFragment
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; FRAGMENTS-NEXT: Contents:[
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; FRAGMENTS-NEXT: Fixups:[
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;
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; FRAGMENTS-NOT: MCDataFragment
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; FRAGMENTS-NOT: MCFragment
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;
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; FRAGMENTS: <MCSection Name:.debug_line_str Fragments:[
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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"
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target triple = "x86_64-unknown-linux-gnu"
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define dso_local i32 @foo(i32 noundef %a, i32 noundef %b) local_unnamed_addr #0 !dbg !10 {
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entry:
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#dbg_value(i32 %a, !15, !DIExpression(), !17)
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#dbg_value(i32 %b, !16, !DIExpression(), !17)
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%add = add nsw i32 %a, 1, !dbg !18
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#dbg_value(i32 %add, !15, !DIExpression(), !17)
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%mul = mul nsw i32 %b, 5, !dbg !19
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#dbg_value(i32 %mul, !16, !DIExpression(), !17)
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%add1 = add nsw i32 %add, %mul, !dbg !20
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ret i32 %add1, !dbg !21
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}
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!llvm.dbg.cu = !{!0}
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!llvm.module.flags = !{!2, !3, !4, !5, !6, !7, !8}
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!llvm.ident = !{!9}
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!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)
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!1 = !DIFile(filename: "text.c", directory: "/fast/fs/llvm-main", checksumkind: CSK_MD5, checksum: "e81ed96ea393640bf1b965103b190e09")
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!2 = !{i32 7, !"Dwarf Version", i32 5}
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!3 = !{i32 2, !"Debug Info Version", i32 3}
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!4 = !{i32 1, !"wchar_size", i32 4}
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!5 = !{i32 8, !"PIC Level", i32 2}
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!6 = !{i32 7, !"PIE Level", i32 2}
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!7 = !{i32 7, !"uwtable", i32 2}
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!8 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
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!9 = !{!"clang version 21.0.0git (/fast/fs/llvm4 8bd196e0eaf4310ad2d9598512d13220b28b9aee)"}
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!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)
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!11 = !DISubroutineType(types: !12)
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!12 = !{!13, !13, !13}
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!13 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!14 = !{!15, !16}
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!15 = !DILocalVariable(name: "a", arg: 1, scope: !10, file: !1, line: 1, type: !13)
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!16 = !DILocalVariable(name: "b", arg: 2, scope: !10, file: !1, line: 1, type: !13)
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!17 = !DILocation(line: 0, scope: !10)
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!18 = !DILocation(line: 2, column: 5, scope: !10)
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!19 = !DILocation(line: 3, column: 5, scope: !10)
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!20 = !DILocation(line: 4, column: 12, scope: !10)
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!21 = !DILocation(line: 4, column: 3, scope: !10)

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