[llvm-bugs] [Bug 26585] New: DBG_VALUE is accidentally promoted to location for the entire function

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26585

            Bug ID: 26585
           Summary: DBG_VALUE is accidentally promoted to location for the
                    entire function
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: aprantl at apple.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

This condition in DwarfDebug::collectVariableInfo() is wrong:

    // Check if the first DBG_VALUE is valid for the rest of the function.
    if (Ranges.size() == 1 && Ranges.front().second == nullptr) {
      RegVar->initializeDbgValue(MInsn);
      continue;
    }

If a variable is described by a single DBG_VALUE and that location isn't
clobbered until the end of the function this will cause the range to be
extended to cover the entire function (and thus before the first write to that
location).

Example
-------

Compile this with -O.
void h(int);
int g();
void f() {
  h(0);
  int a = g();
  h(a);
}


output of -stop-after=ssa:

--- |
  ; ModuleID = 'test.ll'
  target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
  target triple = "x86_64-apple-macosx"

  ; Function Attrs: nounwind ssp uwtable
  define void @f() #0 !dbg !4 {
  entry:
    tail call void @h(i32 0) #3, !dbg !14
    %call = tail call i32 (...) @g() #3, !dbg !15
    tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !8,
metadata !16), !dbg !17
    tail call void @h(i32 %call) #3, !dbg !18
    ret void, !dbg !19
  }

  declare void @h(i32) #1

  declare i32 @g(...) #1

  ; Function Attrs: nounwind readnone
  declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #2

  attributes #0 = { nounwind ssp uwtable }
  attributes #1 = { }
  attributes #2 = { nounwind readnone }
  attributes #3 = { nounwind }

  !llvm.dbg.cu = !{!0}
  !llvm.module.flags = !{!10, !11, !12}
  !llvm.ident = !{!13}

  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer:
"clang version 3.9.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: 1,
enums: !2, subprograms: !3)
  !1 = !DIFile(filename: "test.c", directory: "/Volumes/Data/llvm")
  !2 = !{}
  !3 = !{!4}
  !4 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 3, type:
!5, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true,
variables: !7)
  !5 = !DISubroutineType(types: !6)
  !6 = !{null}
  !7 = !{!8}
  !8 = !DILocalVariable(name: "a", scope: !4, file: !1, line: 5, type: !9)
  !9 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
  !10 = !{i32 2, !"Dwarf Version", i32 2}
  !11 = !{i32 2, !"Debug Info Version", i32 3}
  !12 = !{i32 1, !"PIC Level", i32 2}
  !13 = !{!"clang version 3.9.0 "}
  !14 = !DILocation(line: 4, column: 3, scope: !4)
  !15 = !DILocation(line: 5, column: 11, scope: !4)
  !16 = !DIExpression()
  !17 = !DILocation(line: 5, column: 7, scope: !4)
  !18 = !DILocation(line: 6, column: 3, scope: !4)
  !19 = !DILocation(line: 7, column: 1, scope: !4)

...
---
name:            f
alignment:       4
exposesReturnsTwice: false
hasInlineAsm:    false
isSSA:           false
tracksRegLiveness: true
tracksSubRegLiveness: false
calleeSavedRegisters: [ '%bh', '%bl', '%bp', '%bpl', '%bx', '%ebp', '%ebx', 
                        '%rbp', '%rbx', '%r12', '%r13', '%r14', '%r15', 
                        '%r12b', '%r13b', '%r14b', '%r15b', '%r12d', '%r13d', 
                        '%r14d', '%r15d', '%r12w', '%r13w', '%r14w', '%r15w' ]
frameInfo:       
  isFrameAddressTaken: false
  isReturnAddressTaken: false
  hasStackMap:     false
  hasPatchPoint:   false
  stackSize:       8
  offsetAdjustment: 0
  maxAlignment:    0
  adjustsStack:    true
  hasCalls:        true
  maxCallFrameSize: 0
  hasOpaqueSPAdjustment: false
  hasVAStart:      false
  hasMustTailInVarArgFunc: false
fixedStack:      
  - { id: 0, type: spill-slot, offset: -16, size: 8, alignment: 16 }
body:             |
  bb.0.entry:
    liveins: %rbp

    frame-setup PUSH64r killed %rbp, implicit-def %rsp, implicit %rsp
    CFI_INSTRUCTION .cfi_def_cfa_offset 16
    CFI_INSTRUCTION .cfi_offset %rbp, -16
    %rbp = frame-setup MOV64rr %rsp
    CFI_INSTRUCTION .cfi_def_cfa_register %rbp
    %edi = XOR32rr undef %edi, undef %edi, implicit-def dead %eflags,
debug-location !14
    CALL64pcrel32 @h, csr_64, implicit %rsp, implicit killed %edi, implicit-def
%rsp, debug-location !14
    dead %eax = XOR32rr undef %eax, undef %eax, implicit-def dead %eflags,
implicit-def %al, debug-location !15
    CALL64pcrel32 @g, csr_64, implicit %rsp, implicit %al, implicit-def %rsp,
implicit-def %eax, debug-location !15
    DBG_VALUE debug-use %eax, debug-use _, !8, !16, debug-location !17
    %edi = MOV32rr killed %eax, debug-location !18
    %rbp = POP64r implicit-def %rsp, implicit %rsp, debug-location !18
    TAILJMPd64 @h, csr_64, implicit %rsp, implicit %rsp, implicit killed %edi,
debug-location !18

...


Dwarfdump:


0x0000004a:         TAG_variable [3]  
                     AT_location( rax, piece 0x00000004 )
                                  ^^^^^ this should be a location list instead!
                     AT_name( "a" )
                     AT_decl_file( "/Volumes/Data/llvm/test.c" )
                     AT_decl_line( 5 )
                     AT_type( {0x0000005a} ( int ) )

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