Re: [PATCH 1/2] x86: improve .debug_line contents for assembly sources

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Apr 14, 2022 at 04:15:22PM +0200, Jan Beulich wrote:
> On 14.04.2022 15:31, Roger Pau Monné wrote:
> > On Thu, Apr 14, 2022 at 02:52:47PM +0200, Jan Beulich wrote:
> >> On 14.04.2022 14:40, Roger Pau Monné wrote:
> >>> On Tue, Apr 12, 2022 at 12:27:34PM +0200, Jan Beulich wrote:
> >>>> While future gas versions will allow line number information to be
> >>>> generated for all instances of .irp and alike [1][2], the same isn't
> >>>> true (nor immediately intended) for .macro [3]. Hence macros, when they
> >>>> do more than just invoke another macro or issue an individual insn, want
> >>>> to have .line directives (in header files also .file ones) in place.
> >>>>
> >>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>>>
> >>>> [1] 
> >>>> https://sourceware.org/git?p=binutils-gdb.git;a=commitdiff;h=7992631e8c0b0e711fbaba991348ef6f6e583725
> >>>> [2] 
> >>>> https://sourceware.org/git?p=binutils-gdb.git;a=commitdiff;h=2ee1792bec225ea19c71095cee5a3a9ae6df7c59
> >>>> [3] 
> >>>> https://sourceware.org/git?p=binutils-gdb.git;a=commitdiff;h=6d1ace6861e999361b30d1bc27459ab8094e0d4a
> >>>> ---
> >>>> Using .file has the perhaps undesirable side effect of generating a fair
> >>>> amount of (all identical) STT_FILE entries in the symbol table. We also
> >>>> can't use the supposedly assembler-internal (and hence undocumented)
> >>>> .appfile anymore, as it was removed [4]. Note that .linefile (also
> >>>> internal/undocumented) as well as the "# <line> <file>" constructs the
> >>>> compiler emits, leading to .linefile insertion by the assembler, aren't
> >>>> of use anyway as these are processed and purged when processing .macro
> >>>> [3].
> >>>>
> >>>> [4] 
> >>>> https://sourceware.org/git?p=binutils-gdb.git;a=commitdiff;h=c39e89c3aaa3a6790f85e80f2da5022bc4bce38b
> >>>>
> >>>> --- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
> >>>> +++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
> >>>> @@ -24,6 +24,8 @@
> >>>>  #include <asm/msr-index.h>
> >>>>  #include <asm/spec_ctrl.h>
> >>>>  
> >>>> +#define FILE_AND_LINE .file __FILE__; .line __LINE__
> >>>
> >>> Seeing as this seems to get added to all macros below, I guess you did
> >>> consider (and discarded) introducing a preprocessor macro do to the
> >>> asm macro definitons:
> >>>
> >>> #define DECLARE_MACRO(n, ...) \
> >>> .macro n __VA_ARGS__ \
> >>>     .file __FILE__; .line __LINE__
> >>
> >> No, I didn't even consider that. I view such as too obfuscating - there's
> >> then e.g. no visual match with the .endm. Furthermore, as outlined in the
> >> description, I don't think this wants applying uniformly. There are
> >> macros which better don't have this added. Yet I also would prefer to not
> >> end up with a mix of .macro and DECLARE_MACRO().
> > 
> > I think it's a dummy question, but why would we want to add this to
> > some macros?
> > 
> > Isn't it better to always have the file and line reference where the
> > macro gets used?
> 
> Like said in the description, a macro simply invoking another macro,
> or a macro simply wrapping a single insn, is likely better to have
> its generated code associated with the original line number. Complex
> macros, otoh, are imo often better to have line numbers associated
> with actual macro contents. IOW to some degree I support the cited
> workaround in binutils (which has been there for many years).

Seems a bit ad-hoc policy, but it's you and Andrew that mostly deal
with this stuff, so if you are fine with it.

Acked-by: roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.