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

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

Ping. Cary, is this alright to commit?

* options.h (-z,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.
* testsuite/text_unlikely_segment.cc: New test source.
* testsuite/text_unlikely_segment.sh: New test script.
* testsuite/Makefile.am (text_unlikely_segment): New test.




On Thu, Oct 5, 2017 at 4:41 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Thu, Oct 5, 2017 at 4:41 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On 10/5/17, Sriraman Tallam <tmsriram@google.com> wrote:
>>> 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.
>>
>> Please include this info in your commit log.
>>
>>>>
>>>> 2 Comments:
>>>>
>>>> 1.  I'd prefer "-z text-unlikely-segment" with '-', instead of '_'.
>>>> 2.  Need a testcase.
>>>
>>> Done and patch attached.
>>>
>>
>> LGTM.  But I can't approve it.
>
> Np, thanks!
>
> Sri
>
>>
>> Thanks.
>>
>> --
>> H.J.

Attachment: segment_split.txt
Description: Text document