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Date: Thu, 14 May 2015 12:25:39 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v3] Build process uses script to add CFI
 directives to x86 asm

* Rich Felker <dalias@...c.org> [2015-05-13 22:57:20 -0400]:
> 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.
> 

yes, i think the line numbers would be wrong then

the ; idea fixes it, but then .cfi should be in prefix position
(because of comments)

> > > 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?
> 

i guess it could be

	tools/aswrap.sh $< $@ $(ARCH) -- $(CC) $(CFLAGS_ALL_STATIC)

but it will be harder to follow what's going on in case of a failure
than currently

> > 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.
> 

ok

> > 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.
> 

ok, they are the same

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