From: Alex Dowad <alexinbeijing@gmail.com>
To: musl@lists.openwall.com
Subject: Re: [PATCH v3] Build process uses script to add CFI directives to x86 asm
Date: Fri, 15 May 2015 19:31:32 +0200 [thread overview]
Message-ID: <20150515173132.GA18390@alex-ThinkPad-L530> (raw)
Dear Szabolcs Nagy (and other interested parties),
> you can use
>
> .file "foo.s"
Thanks for the idea! Unfortunately, implementing it has proved very troublesome.
The GAS documentation does refer to ".file <source file>". There's just a tiny little
problem with it -- it doesn't actually work for the desired purpose. It does
register <source file> as a dependency, so it will be included in the dependency
file which is written out if you invoke GAS with the --MD option. But that's about it.
If you look at asm generated by GCC, it includes a ".file <source file>" line at the
top, but it *also* includes a ".file <number> <source file>" line. Which actually
sets the "source file" in the debugging info! (Yay!) Subsequent ".loc" directives
use the source file number when identifying source lines.
So we just use ".file <number> <source file>" and everybody is happy, right? Yes?
Good? Right?
Wrong.
Allow me to quote gas/dwarf2dbg.c:596-598 from the binutils repo:
/* A .file directive implies compiler generated debug information is
being supplied. Turn off gas generated debug info. */
debug_type = DEBUG_NONE;
Snap.
Normally, GAS automatically generates debug info on source line numbers, and a few
other basic things. As soon as you use a ".file <number> <source file>" directive,
all that automatic debugging output is shut off, and you have to use explicit
assembler directives for *everything*.
I guess this makes sense, because if the "source file" is a completely
different file from the input asm file, the automatically generated line number
info will be completely wrong.
What a pain! Well, I guess I'll just have to use my own, explicit ".loc" directives.
> i think passing down the build command that way is not ok
It does seem like a hack -- but I'm not sure what a better way to do it is.
(I'm not a "real" shell programmer, if you hadn't noticed yet. I just fake it using
some combination of Stack Overflow and manpages.)
> i think
>
> pushl $123
> push $123
>
> are different
'push %eax' and 'pushl %eax' assemble to exactly the same machine code. Likewise,
'push $1' and 'pushl $1' assemble just the same.
Interestingly, 'pushl %ax' assembles to 'push %eax', 'push %ax' is just 'push %ax'.
> set LC_ALL=C because you depend on collation order
> in the awk script
Please see if I did this right in the v4.
> add new lines at the end
Done.
Thanks for other enhancements to the awk script (I will credit you in the commit log
message).
Kind regards, AD
next reply other threads:[~2015-05-15 17:31 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-15 17:31 Alex Dowad [this message]
-- strict thread matches above, loose matches on Subject: below --
2015-05-13 17:54 Alex Dowad
2015-05-13 19:22 ` Szabolcs Nagy
2015-05-14 2:57 ` Rich Felker
2015-05-14 10:25 ` Szabolcs Nagy
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