Re: RFC: x86: Enable -z separate-code by default

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Feb 22, 2018 at 5:22 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Thu, 22 Feb 2018, H.J. Lu wrote:
>
>> >> Meh :-( Have you done any performance measurements on, say, SPECcpu?
>> >
>> > We will.
>>
>> We compared SPEC CPU 2017 performance before and after this change on
>> Skylake server.  There are no any significant performance changes.
>> Everything is mostly below +/-1%.
>
> Thanks, seems fine.
>
>> >>> Should I add an ld configure option, --enable-separate-code, instead?
>> >>
>> >> Yes, definitely.  For old distros at least I definitely want to retain old
>> >> behaviour by default and a configure switch would make that easier.
>> >
>> > Will do.
>> >
>>
>> Here is the patch to add --enable-separate-code to ld configure and
>> enable it by default for Linux/x86.  But the default maximum page size
>> is changed to 4KB for x86-64.
>>
>> Any comments?
>
> Should the max page size setting (without override on cmdline) depend on
> separate-code, or not?  I'm split minds here, I'm worried that the 2mb

I will give it a try.

> page size optimization we had for x86-64 isn't effective by default, but
> OTOH it already isn't since relro, so can't be that bad either, plus the
> kernel is of course free to use 2mb TLB entries when in the end the
> moons^Waddresses happen to be aligned right.

Gold defaults to 4KB page for x86-64.


-- 
H.J.