linux-kernel - Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Date:	Fri, 24 Jul 2015 12:16:03 +0800
From:	He Kuang <hekuang@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	"Wangnan (F)" <wangnan0@...wei.com>, pi3orama <pi3orama@....com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce
 function for outputing data to perf event

Hi, Alexei

On 2015/7/24 11:20, Alexei Starovoitov wrote:
> On 7/23/15 1:49 PM, Alexei Starovoitov wrote:
>> On 7/23/15 4:54 AM, He Kuang wrote:
>>
>> trimmed cc-list, since it's not related to kernel.
>>
>>> Thank you for your guidence, and by referencing your last mail
>>> and other llvm backends, I found setting
>>> BPFMCAsmInfo::SupportsDebugInformation = true in BPFMCAsmInfo.h
>>
>> thanks! yes. it was missing.
>>
>>> and fix some unhandeled switch can make llc output debug_info,
>>
>> what do you mean ?
>>
>>> but important information is missing in the result:
>>
>> hmm. I see slightly different picture.
>> With 'clang -O2 -target bpf -g -S a.c'
>> I see all the right info inside .s file.
>> with '-c a.c' for some reasons it produces bogus offset:
>>     Abbrev Offset: 0xffff0000
>>     Pointer Size:  8
>> /usr/local/bin/objdump: Warning: Debug info is corrupted, abbrev offset
>> (ffff0000) is larger than abbrev section size (4b)
>>
>> and objdump fails to parse .o
>> I'm using llvm trunk 3.8. Do you see this as well?
>
> there were few issues related to relocations.
> Fixed it up and pushed to llvm trunk r243087.
> Please pull and give it a try.
> AT_location should be correct, but AT_name still looks buggy.

I've pulled the lastest version "[bpf] initial support for
debug_info" and tested it. This version can output debug_info but
still not generate correct AT_location, I tested as following:

$ cat > main.c <<EOF
int testprog(int myvar_a, int myvar_b)
{
         int myvar_c;
         myvar_c = myvar_a + myvar_b;
         return myvar_c;
}
EOF

$ clang -g  -O2 -target bpf -c main.c -o test.obj.bpf
$ clang -g  -O2             -c main.c -o test.obj.x86

$ objdump --dwarf=info test.obj.x86   > test.obj.x86.dump
$ objdump --dwarf=info test.obj.bpf   > test.obj.bpf.dump

Compare those two dump files:

test.obj.x86.dump:

<2><43>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <44>   DW_AT_location    : 3 byte block: 55 93 4    (DW_OP_reg5 (rdi);
                                                         DW_OP_piece: 4)
    <48>   DW_AT_name        : (indirect string, offset: 0xdc): myvar_a
    <4c>   DW_AT_decl_file   : 1
    <4d>   DW_AT_decl_line   : 1
    <4e>   DW_AT_type        : <0x71>

test.obj.bpf.dump:

<2><43>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <44> DW_AT_name          : (indirect string, offset: 0x0): clang
                                version 3.8.0 (http://llvm.org/git/clang.git
                                3a7c733b80f156a547f3f1517e6fbce9c0a33026)
                                (http://llvm.org/git/llvm.git
                                 90908cb34d73460d3 a83e2194a58d82c6d1f199)
    <48>   DW_AT_decl_file   : 1
    <49>   DW_AT_decl_line   : 1
    <4a>   DW_AT_type        : <0x65>


No DW_AT_location info for formal parameters, but if we change
the function 'testprog' to 'main', DW_AT_location of formal
parameters appear but that of local variables are still missed,
don't know why..

$ cat > main.c <<EOF
#include <stdio.h>

int main(int argc, char *argv[])
{
         int myvar_a, myvar_b;
         int myvar_c;
         myvar_c = myvar_a + myvar_b;
         return myvar_c;
}

test.obj.bpf.dump:
<2><43>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <44>   DW_AT_location    : 1 byte block: 51         (DW_OP_reg1 (r1))
    <46>   DW_AT_name        : (indirect string, offset: 0x0): clang version 3.8.
    ..
<2><5d>: Abbrev Number: 4 (DW_TAG_variable)
    <5e>   DW_AT_name        : (indirect string, offset: 0x0): clang version 3.8.

test.obj.x86.dump:

<2><43>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <44>   DW_AT_location    : 3 byte block: 55 93 4    (DW_OP_reg5 (rdi);
                                                         DW_OP_piece: 4)
    <48>   DW_AT_name        : (indirect string, offset: 0xd8): argc
    ..
<2><5f>: Abbrev Number: 4 (DW_TAG_variable)
    <60>   DW_AT_name        : (indirect string, offset: 0xe7): myvar_a
    ..

Thank you.


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