musl - Re: [PATCH v3] Build process uses script to add CFI directives to x86 asm

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Date: Fri, 15 May 2015 19:31:32 +0200
From: Alex Dowad <alexinbeijing@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v3] Build process uses script to add CFI directives to
 x86 asm

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

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