[llvm-dev] Asm Parser vars extraction

[Pietro D'Ettole via llvm-dev]

Hi,
I'll try to give you more context through an example.
What I need is to extract the value of a variable (aka a label) inside an
assembly file; for example given the following assembly (compiled with
armv7-a clang 10.0.0):

---------------------------------------------
main:
        sub     sp, sp, #12
        mov     r2, #0
        str     r2, [sp, #8]
        str     r0, [sp, #4]
        str     r1, [sp]
        ldr     r0, .LCPI0_0
        ldr     r1, [r0, #4]
        ldr     r0, [r0, #8]
        mul     r2, r1, r0
        mov     r0, r2
        add     sp, sp, #12
        bx      lr
.LCPI0_0:
        .long   myvar
myvar:
        .long   0                       @ 0x0
        .long   1                       @ 0x1
        .long   2                       @ 0x2
        .long   3                       @ 0x3
---------------------------------------------

As you can see there's a label, namely "myvar", which is a variable in the
C program. What I'm trying to achieve is, while parsing with the Asm
Parser, get the parsed value of "myvar" out of the asm file (i.e. get the
values of 0x0, 0x1, 0x2, 0x3). Is this possible?

Thanks a lot!

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Il giorno mar 9 feb 2021 alle ore 03:01 Brian Cain <brian.cain at gmail.com>
ha scritto:

> I don't quite know exactly, but I suppose one way would be to modify
> ELFWriter::writeSymbol() to emit something when a symbol appears that
> matches your criteria.  I'm taking some liberty here assuming you can use
> an ELF object file.  I imagine there's something similar for macho/coff.
>
> Then again, that information is present in the object file too.  You could
> use llvm-readelf or obj2yaml to extract what you want.
>
> Maybe you could give a little more context about how you plan to use the
> info and the community could offer a better answer.
>
>
>
>
>
> On Mon, Feb 8, 2021 at 5:03 PM Pietro D'Ettole <
> progettoiotpolimi2019 at gmail.com> wrote:
>
>> Hi Brian, thanks for your reply.
>>
>> My goal is to be able to extract from an assembly file (i.e. source
>> compiled to assembly, directly) the static global vars declared in the
>> source. So far I haven't found any API in the llvm asm parser to serve my
>> purpose. Do you know if/ how I can accomplish that?
>>
>> Thank you.
>>
>>
>> Il giorno sabato 6 febbraio 2021, Brian Cain <brian.cain at gmail.com> ha
>> scritto:
>>
>>> You need to extract it from the source?  Is it possible to use the
>>> resulting object file instead?
>>>
>>> Note that there's no way to get an 'address' but you can get a section
>>> offset. The section offset for both code and data are available in the
>>> object file and from the assembler as it writes the object file.
>>>
>>> The contents of the assembly file - instructions and directives -
>>> contribute to the resulting layout.  The AsmParser can find tokens and
>>> build instructions but shouldn't know how it will get layed out.
>>>
>>> On Sat, Feb 6, 2021, 3:16 AM Pietro D'Ettole via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I'm a little bit stuck in code reading right now. Maybe some of you can
>>>> help me to understand fast if what I need is feasible or not.
>>>> I was trying to understand if currently asm parser in llvm allows to
>>>> easily extract from an asm file variable names alongside with their values
>>>> and their addresses.
>>>>
>>>> Thanks for the help!
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>
> --
> -Brian
>
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