[AArch64] DW_TAG_label prevents elf from loading in Trace32 and other tools

[Weiming Zhao]

Hi David,

Thanks for your response.
The tool complained that "Entry error near offset xxxx in file yyyy". That offset is in .debug_info, around a DIE about asm label.

For C files, clang -g is ok.
I just tried C files with labels and it's OK.

We do suspect it's an issue of the tool. 

Thanks,
Weiming


-----Original Message-----
From: David Blaikie [mailto:dblaikie at gmail.com] 
Sent: Friday, August 22, 2014 10:57 AM
To: Ana Pazos
Cc: Tim Northover; llvm-commits; weimingz at codeaurora.org
Subject: Re: [AArch64] DW_TAG_label prevents elf from loading in Trace32 and other tools

On Thu, Aug 21, 2014 at 6:44 PM, Ana Pazos <apazos at codeaurora.org> wrote:
> Hi Tim,
>
>
>
> Trace32 and some other internal tools are not loading AArch64 LLVM 
> executables built with -g flag and the integrated assembler.
>
>
>
> The tools complain about the entry DW_TAG_label in debug_info section 
> emitted for assembly files (code in lib/MC/MCDwarf.cpp).

You'd have to be more specific about what those complaints are.

So far as I know (and I've looked at a fair bit of debug info - but not much of the asm debug info specifically) the DWARF we emit there is correct. I could imagine we might have an incorrect relocation for a label on a particular architecture or somesuch, but there's not much I can do to help diagnose that with the information you've provided.

> Building the executables with LLVM with no-integrated-as flag works 
> because GAS does not emit this entry.

Have you tried compiling a piece of C code with an explicit label, using GCC and throwing that at your tools? (clang/LLVM don't produce label debug info for non-asm source, but GCC does - so if it's something to do with the label debug info, that could show it)

And/or do your tools work with clang -g output? If it's something generically related to relocations/addresses in debug info, I'd imagine it would show up with that just as well.

>
>
>
> At first I thought setting DW_AT_prototyped to 0 was the issue (it 
> seems it should be 1 for non-C code). I changed it, but it did not help.
>
>
>
> The only thing that worked was to not emit DW_TAG_label entries at all.
>
>
>
> Is this a tool issue or LLVM is not emitting the fields required by 
> DW_TAG_label correctly?
>
>
>
> Thanks for the help,
>
> Ana.
>
>
>
>
>
> Here is a reduced example:
>
>
>
>                 .text
>
>                 .file        "a.c"
>
>                 .globl     main
>
>                 .align     2
>
>                 .type     main, at function
>
> main:                                   // @main
>
> // BB#0:
>
>                 sub         sp, sp, #16             // =16
>
>                 movz     w8, #0
>
>                 str           w8, [sp, #12]
>
>                 str           w0, [sp, #8]
>
>                 str           x1, [sp]
>
>                 ldr           w0, [sp, #8]
>
>                 add        sp, sp, #16             // =16
>
>                 ret
>
> .Ltmp1:
>
>                 .size       main, .Ltmp1-main
>
>
>
>
>
>
>
> aarch64-linux-gnu-objdump -dwarf=info  a.exe
>
>
>
> a0.exe:     file format elf64-littleaarch64
>
>
>
> Contents of the .debug_info section:
>
>
>
>   Compilation Unit @ offset 0x0:
>
>    Length:        0xa8 (32-bit)
>
>    Version:       3
>
>    Abbrev Offset: 0x0
>
>    Pointer Size:  8
>
> <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
>
>     <c>   DW_AT_stmt_list   : 0x0
>
>     <10>   DW_AT_low_pc      : 0x400568
>
>     <18>   DW_AT_high_pc     : 0x400588
>
>     <20>   DW_AT_name        : a.s
>
>     <24>   DW_AT_comp_dir    : xxx
>
>     <6f>   DW_AT_producer    : LLVM ARM
>
>     <90>   DW_AT_language    : 32769         (MIPS assembler)
>
> <1><92>: Abbrev Number: 2 (DW_TAG_label)
>
>     <93>   DW_AT_name        : main
>
>     <98>   DW_AT_decl_file   : 0x1
>
>     <9c>   DW_AT_decl_line   : 0x6
>
>     <a0>   DW_AT_low_pc      : 0x400568
>
>     <a8>   DW_AT_prototyped  : 0
>
> <2><a9>: Abbrev Number: 3 (DW_TAG_unspecified_parameters)
>
> <2><aa>: Abbrev Number: 0
>
> <1><ab>: Abbrev Number: 0
>
>
>
>
>
> For the executable generated by gcc:
>
>
>
> aarch64-linux-gnu-objdump -dwarf=info g.exe
>
>
>
> g.exe:     file format elf64-littleaarch64
>
>
>
> Contents of the .debug_info section:
>
>
>
>   Compilation Unit @ offset 0x0:
>
>    Length:        0x7b (32-bit)
>
>    Version:       2
>
>    Abbrev Offset: 0x0
>
>    Pointer Size:  8
>
> <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
>
>     <c>   DW_AT_stmt_list   : 0x0
>
>     <10>   DW_AT_low_pc      : 0x400510
>
>     <18>   DW_AT_high_pc     : 0x400530
>
>     <20>   DW_AT_name        : a.s
>
>     <24>   DW_AT_comp_dir    : xxx
>
>     <6f>   DW_AT_producer    : GNU AS 2.24.0
>
>     <7d>   DW_AT_language    : 32769        (MIPS assembler)
>
>
>
>
>
>
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