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Date: Wed, 13 May 2015 22:57:20 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v3] Build process uses script to add CFI
 directives to x86 asm

On Wed, May 13, 2015 at 09:22:52PM +0200, Szabolcs Nagy wrote:
> * Alex Dowad <alexinbeijing@...il.com> [2015-05-13 19:54:39 +0200]:
> > 
> > I've noticed that using tempfiles for the augmented asm has a drawback:
> > In the source file/line debugging info generated by the assembler, it records
> > the source file as "/tmp/<random-garbage>". Then, when you try to debug a program
> > which is linked against the resulting musl, GDB tries to open "/tmp/<random-garbage>"
> > to show in the source window.
> > 
> > Suggestions?? Perhaps generate .cfi.s files as Szabolcs suggested??
> > 
> 
> you can use
> 
>  .file "foo.s"

One question -- will the fact that the line numbers don't match up
interfere with debugging? If so, the CFI generation should add the
directives to existing lines separated by ;'s rather than inserting
lines.

> > diff --git a/Makefile b/Makefile
> > index 6559295..9aefd62 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -118,7 +118,7 @@ $(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval $(call mkasmdep,$(s))))
> >  	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $(dir $<)$(shell cat $<)
> >  
> >  %.o: $(ARCH)/%.s
> > -	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
> > +	tools/aswrap.sh $< $@ $(ARCH) "$(CC) $(CFLAGS_ALL_STATIC)"
> >  
> 
> i think passing down the build command that way is not ok

The quoting is probably off, but otherwise it doesn't look so bad.

> " may be used inside CFLAGS (and there are other shell quote issues)
> it hides the build command in the make output
> it's not clear if the build correctly handles if CC fails

Do you have a better design in mind?

> > +function get_const1() {
> > +  # for instructions with 2 operands, get 1st operand (assuming it is constant)
> > +  match($0, /-?(0x[0-9a-fA-F]+|[0-9]+),/)
> > +  return parse_const(substr($0, RSTART, RLENGTH-1))
> > +}
> 
> it only matches with immediate ,
> 
> i'd just clean the whitespaces up so you dont have to add \s* or \s+
> to every regex (see below)
> 
> awk converts strings to numbers, but hex is unfortunately not guaranteed
> to be supported (otherwise strtod conversion rules apply)
> 
> but you can implement parse_const(s) as
> 
> 	sign = sub(/^-/,"",s)
> 	hex = sub(/^0x/,"",s)
> 	if (hex)
> 		n = hex2int(s)
> 	else
> 		n = s+0
> 	return sign ? -n : n
> 
> this does not handle binary (0b11) and octal (0123) asm consts
> (i think you should check for those and emit a warning).

Is 0b something that could even be relied upon? We generally use a
minimal asm dialect without extensions that real or hypothetical
alternate assemblers might not have. I'm happy with a YAGNI approach
to the CFI generation.

> > +/^.global\s+\w+/ {
> > +  globals[$2] = 1
> 
> may be spelt as .globl too

Yes, that might be used somewhere.

> > +/pushl?/ {
> > +  if (match($0, /\s+%(ax|bx|cx|dx|di|si|bp|sp)/))
> > +    adjust_sp_offset(2)
> > +  else
> > +    adjust_sp_offset(4)
> > +}
> 
> i think
> 
>  pushl $123
>  push $123
> 
> are different

How so? Likewise I don't think the stuff for 2-byte push/pop is
terribly useful. It's not a meaningful operation to be performing in
32-bit code.

Rich

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