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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.04.2022 18:02, Roger Pau Monné wrote:
> 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.

Actually I think I'll withdraw this patch. After quite a bit of further
consideration, it should really be the assembler to get this right, and
once properly working there the directives added here may actually get
in the way.

Jan