Re: Gold linker patch to split unlikely text into a separate segment

["Sriraman Tallam via binutils" <binutils at sourceware dot org>]

On Wed, Oct 4, 2017 at 5:12 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On 10/4/17, Sriraman Tallam <tmsriram@google.com> wrote:
>> Hi,
>>
>>      This patch adds an option to gold to create a new ELF segment for
>> code determined unlikely by the compiler.  Currently, this can be done
>> with a linker plugin but I was wondering if it is fine to have this
>> support in gold with an option for ease of use.
>>
>>      The advantages of doing this are:
>>
>> * We could only map the hot segment to huge pages to keep iTLB misses low
>> * We could munlock the unlikely segment
>>
>> Is this alright?
>>
>> ChangeLog entry:
>>
>> * options.h (text_unlikely_segment): New option.
>> * layout.cc (Layout::layout): Create new output section
>> for .text.unlikely sections with the new option.
>> (Layout::segment_precedes): Check for the new option
>> when segment flags match.
>>
>> Patch attached.
>>
>
> This is an interesting approach.  Do you have some performace
> numbers?

With function re-ordering of hot code, segment splitting and mapping
only hot code to huge pages, we see a reduction in iTLB misses
translating to performance improvements of 0.5 to 1% on some of our
benchmarks.

>
> 2 Comments:
>
> 1.  I'd prefer "-z text-unlikely-segment" with '-', instead of '_'.
> 2.  Need a testcase.

Done and patch attached.

Thanks
Sri

>
> --
> H.J.

Attachment: segment_split.txt
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