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

[Jan Beulich <jbeulich@xxxxxxxx>]

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().

Jan