Re: [PATCH v2] i386: Allow -mlarge-data-threshold with -mcmodel=large

[Fangrui Song <maskray@google.com>]

On 2023-05-25, Jan Beulich wrote:
>On 25.05.2023 17:16, Fangrui Song wrote:
>> --- a/gcc/doc/invoke.texi
>> +++ b/gcc/doc/invoke.texi
>> @@ -32942,9 +32942,10 @@ the cache line size.  @samp{compat} is the default.
>>
>>  @opindex mlarge-data-threshold
>>  @item -mlarge-data-threshold=@var{threshold}
>> -When @option{-mcmodel=medium} is specified, data objects larger than
>> -@var{threshold} are placed in the large data section.  This value must be the
>> -same across all objects linked into the binary, and defaults to 65535.
>> +When @option{-mcmodel=medium} or @option{-mcmodel=large} is specified, data
>> +objects larger than @var{threshold} are placed in large data sections.  This
>> +value must be the same across all objects linked into the binary, and defaults
>> +to 65535.
>
>Where's the "must be the same" requirement coming from?

It's an existing requirement.  I think it may be related to discouraging
different COMDAT sections names due to different -mlarge-data-threshold=.
I don't think it makes sense but did not feel strongly dropping it.

Happy to drop the requirement if I revise this patch.

>As to the default - to remain compatible with earlier versions, shouldn't
>large model code default to "infinity"?
>
>Jan

I have thought about this compatibility need and feel that it is very
unlikly to be needed.  GNU ld has supported large data sections since
2005
(https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=3b22753a67cf616514de804ef6d5ed5e90a7d883).
Users' programs with the internal linker scripts will still be working
and -fdata-sections sections will be combined.

First, -mcmodel=large use cases are rare enough.  Rare perhaps
-mcmodel=largel was considered theoretic excercise  in
trying to reach feature completion
(https://groups.google.com/g/x86-64-abi/c/jnQdJeabxiU/m/NNuA0P7pAQAJ),
without this patch -mcmodel=large object files don't interract well with
existing -mcmodel=small object files.
Moreover, if a user expects a specific section prefix with
-mcmodel=large, that's a brittle assumption. I think it's fair to say
that the fault is on the user side and GCC doesn't need to work around
their issues.