[llvm] [DebugInfo] Don't set prologue_end behind line-zero call insts (PR #156850)
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Thu Sep 4 03:41:41 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-debuginfo
Author: Jeremy Morse (jmorse)
<details>
<summary>Changes</summary>
In functions that have been seriously deformed during optimisation, there can be call instructions with line-zero immediately after frame setup (see C reproducer in the test added). Our previous algorithms for prologue_end ignored these, meaning someone entering a function at prologue_end would break-in after a function call had completed. Prefer instead to not emit prologue_end at all: there is no good place to put it.
---
Full diff: https://github.com/llvm/llvm-project/pull/156850.diff
3 Files Affected:
- (modified) llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (+9)
- (modified) llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir (+1-1)
- (added) llvm/test/DebugInfo/X86/no-prologue-end-after-line0-calls.mir (+129)
``````````diff
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index c27f100775625..0208167fac6f7 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2291,6 +2291,15 @@ findPrologueEndLoc(const MachineFunction *MF) {
return *FoundInst;
}
+ // We choose to ignore line-zero locations when setting the prologue as they
+ // can't be stepped on anyway; however in very rare scenarios function calls
+ // can have line zero, and we shouldn't step over those. In these
+ // extraordinary conditions, just bail out and refuse to set a prologue_end.
+ if (CurInst->isCall())
+ if (const DILocation *Loc = CurInst->getDebugLoc().get())
+ if (Loc->getLine() == 0)
+ return std::make_pair(nullptr, true);
+
// Try to continue searching, but use a backup-location if substantive
// computation is happening.
auto NextInst = std::next(CurInst);
diff --git a/llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir b/llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir
index 01862f5905f9c..71489d5a5e485 100644
--- a/llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir
+++ b/llvm/test/DebugInfo/MIR/X86/debug-loc-0.mir
@@ -5,7 +5,7 @@
# CHECK: Ltmp0:
# CHECK: .loc 1 0 0
# CHECK-NOT: .loc 1 0 0
-# CHECK: .loc 1 37 1 prologue_end
+# CHECK: .loc 1 37 1
--- |
; ModuleID = '<stdin>'
diff --git a/llvm/test/DebugInfo/X86/no-prologue-end-after-line0-calls.mir b/llvm/test/DebugInfo/X86/no-prologue-end-after-line0-calls.mir
new file mode 100644
index 0000000000000..604b9d1d2eafb
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/no-prologue-end-after-line0-calls.mir
@@ -0,0 +1,129 @@
+# RUN: llc %s -start-after=livedebugvalues -o - -filetype=obj | llvm-dwarfdump - --debug-line | FileCheck %s --implicit-check-not=prologue_end
+#
+## Original code, compiled clang -O2 -g -c
+##
+## void ext();
+## int main(int argc, char **argv) {
+## if (argc == 1)
+## ext();
+## else
+## ext();
+## return 0;
+## }
+##
+## In the code sequence above, the call to ext is given line zero during
+## optimisation, because the code is duplicated down all function paths thus
+## gets merged. We get something like this as the output:
+##
+## 0: 50 push %rax
+## 1: 31 c0 xor %eax,%eax
+## 3: e8 00 00 00 00 call 8 <main+0x8>
+## 4: R_X86_64_PLT32 ext-0x4
+## 8: 31 c0 xor %eax,%eax
+## a: 59 pop %rcx
+## b: c3 ret
+##
+## Where prologue_end is placed on address 8, the clearing of the return
+## register, because it's the first "real" instruction that isn't line zero.
+## This then causes debuggers to skip over the call instruction when entering
+## the function, which is catastrophic.
+##
+## Instead: we shouldn't put a prologue_end on this function at all. It's too
+## deformed from the original code to truly have a position (with a line number)
+## that is both true, and after frame setup. This gives comsumers the
+## opportunity to recognise "this is a crazy function" and act accordingly.
+##
+## Check lines ensure that there's something meaningful in the line table
+## involving line 2, the implicit-check-not is making sure there isn't a
+## prologue_end flag on any line entry.
+#
+# CHECK: standard_opcode_lengths[DW_LNS_set_prologue_end] = 0
+#
+# CHECK: Address Line Column File
+# CHECK: 2 0 0
+# CHECK: end_sequence
+--- |
+ ; ModuleID = '/tmp/test.c'
+ source_filename = "/tmp/test.c"
+ 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"
+
+ ; Function Attrs: nounwind uwtable
+ define dso_local noundef i32 @main(i32 noundef %argc, ptr noundef readnone captures(none) %argv) local_unnamed_addr !dbg !10 {
+ entry:
+ #dbg_value(i32 %argc, !19, !DIExpression(), !21)
+ #dbg_value(ptr %argv, !20, !DIExpression(), !21)
+ tail call void (...) @ext(), !dbg !22
+ ret i32 0, !dbg !24
+ }
+
+ declare !dbg !25 void @ext(...) local_unnamed_addr
+
+ !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 22.0.0git (/fast/fs/llvm4 8989ec5439dc2df2aeb7e5ea3e6c255ce8e9634d)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+ !1 = !DIFile(filename: "/tmp/test.c", directory: "/fast/fs/llvm-stage/debug", checksumkind: CSK_MD5, checksum: "9862df54ae1fdd9354308eae69de364a")
+ !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 22.0.0git (/fast/fs/llvm4 8989ec5439dc2df2aeb7e5ea3e6c255ce8e9634d)"}
+ !10 = distinct !DISubprogram(name: "main", scope: !11, file: !11, line: 2, type: !12, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !18, keyInstructions: true)
+ !11 = !DIFile(filename: "/tmp/test.c", directory: "", checksumkind: CSK_MD5, checksum: "9862df54ae1fdd9354308eae69de364a")
+ !12 = !DISubroutineType(types: !13)
+ !13 = !{!14, !14, !15}
+ !14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+ !15 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !16, size: 64)
+ !16 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !17, size: 64)
+ !17 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+ !18 = !{!19, !20}
+ !19 = !DILocalVariable(name: "argc", arg: 1, scope: !10, file: !11, line: 2, type: !14)
+ !20 = !DILocalVariable(name: "argv", arg: 2, scope: !10, file: !11, line: 2, type: !15)
+ !21 = !DILocation(line: 0, scope: !10)
+ !22 = !DILocation(line: 0, scope: !23)
+ !23 = distinct !DILexicalBlock(scope: !10, file: !11, line: 3, column: 7)
+ !24 = !DILocation(line: 7, column: 4, scope: !10, atomGroup: 2, atomRank: 1)
+ !25 = !DISubprogram(name: "ext", scope: !11, file: !11, line: 1, type: !26, spFlags: DISPFlagOptimized)
+ !26 = !DISubroutineType(types: !27)
+ !27 = !{null}
+...
+---
+name: main
+alignment: 16
+tracksRegLiveness: true
+noPhis: true
+isSSA: false
+noVRegs: true
+hasFakeUses: false
+debugInstrRef: true
+tracksDebugUserValues: true
+frameInfo:
+ stackSize: 8
+ offsetAdjustment: -8
+ maxAlignment: 1
+ adjustsStack: true
+ hasCalls: true
+ maxCallFrameSize: 0
+ isCalleeSavedInfoValid: true
+machineFunctionInfo:
+ amxProgModel: None
+body: |
+ bb.0.entry:
+ DBG_VALUE $edi, $noreg, !19, !DIExpression(), debug-location !21
+ DBG_VALUE $rsi, $noreg, !20, !DIExpression(), debug-location !21
+ frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+ frame-setup CFI_INSTRUCTION def_cfa_offset 16
+ dead $eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags, implicit-def $al, debug-location !22
+ CALL64pcrel32 target-flags(x86-plt) @ext, csr_64, implicit $rsp, implicit $ssp, implicit killed $al, implicit-def $rsp, implicit-def $ssp, debug-location !22
+ DBG_VALUE $rsi, $noreg, !20, !DIExpression(DW_OP_LLVM_entry_value, 1), debug-location !21
+ DBG_VALUE $edi, $noreg, !19, !DIExpression(DW_OP_LLVM_entry_value, 1), debug-location !21
+ $eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags, debug-location !24
+ $rcx = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !24
+ frame-destroy CFI_INSTRUCTION def_cfa_offset 8, debug-location !24
+ RET64 $eax, debug-location !24
+...
``````````
</details>
https://github.com/llvm/llvm-project/pull/156850
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