[LLVMbugs] [Bug 14016] New: Clang doesn't emit enough debug info for some functions with -gline-tables-only and -O1

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=14016

             Bug #: 14016
           Summary: Clang doesn't emit enough debug info for some
                    functions with -gline-tables-only and -O1
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: samsonov at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


$ cat zbug.cc
#include <stdio.h>

namespace {
void zzz (void *foo) {
  fprintf(stderr, "zzz\n");
}
void bar () {
  void *foo;
  zzz(foo);
}
}
void run() { bar(); }
$ clang++ -gline-tables-only -O1 zbug.cc -c -o zbug.o

Now the function "zzz" is present in the object file:
$ objdump -d zbug.o | grep zzz
  10:    eb 0e                    jmp    20 <_ZN12_GLOBAL__N_13zzzEPv>
0000000000000020 <_ZN12_GLOBAL__N_13zzzEPv>:
  20:    48 8b 0d 00 00 00 00     mov    0x0(%rip),%rcx        # 27
<_ZN12_GLOBAL__N_13zzzEPv+0x7>
  36:    e9 00 00 00 00           jmpq   3b <_ZN12_GLOBAL__N_13zzzEPv+0x1b>

But the debug info for it doesn't have low_pc/high_pc attributes:
$ readelf -wi zbug.o
...
 <1><58>: Abbrev Number: 4 (DW_TAG_subprogram)
    <59>   DW_AT_name        : (indirect string, offset: 0x4d): zzz    
    <5d>   DW_AT_decl_file   : 1    
    <5e>   DW_AT_decl_line   : 4    

It seems that LLVM IR is misleading. Metadata for function "bar" has pointer to
LLVM function @_ZN12_GLOBAL__N_13barEv while metadata for "zzz" doesn't have
pointer to @_ZN12_GLOBAL__N_13zzzEPv

$ clang++ -gline-tables-only -O1 zbug.cc -c -S -emit-llvm -o zbug.ll
$ cat zbug.ll
...
!8 = metadata !{i32 786478, i32 0, metadata !6, metadata !"bar", metadata
!"bar", metadata !"", metadata !6, i32 7, metadata !7, i1 true, i1 true, i32 0,
i32 0, null, i32 256, i1 true, void ()* @_ZN12_GLOBAL__N_13barEv, null, null,
metadata !1, i32 7} ; [ DW_TAG_subprogram ] [line 7] [local] [def] [bar]
!9 = metadata !{i32 786478, i32 0, metadata !6, metadata !"zzz", metadata
!"zzz", metadata !"", metadata !6, i32 4, metadata !7, i1 true, i1 true, i32 0,
i32 0, null, i32 256, i1 true, null, null, null, metadata !1, i32 4} ; [
DW_TAG_subprogram ] [line 4] [local] [def] [zzz]

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