Support of link time symbols reorder to improve performance

[Fangrui Song]

On 2021-05-07, Xi Ruoyao via Binutils wrote:
>On Fri, 2021-05-07 at 00:06 +0800, 陈云星 via Gcc-help wrote:
>> From llvm community:
>> https://reviews.llvm.org/rG1a33c0f2422c97dbfdbf11e561bb7823d71e3145 <
>> https://reviews.llvm.org/rG1a33c0f2422c97dbfdbf11e561bb7823d71e3145>
>>
>> LLD linker has an option :  --symbol-ordering-file
>>
>> Which support direct linker to sort symbol in result object by a user
>> supplied file;
>>
>> This improve performance by reduce instruction cache miss.
>>
>> The hottest function will cluster and code cache hit rate will be high.
>>
>> Does gnu linker can support this ?
>
>I think with -ffunction-sections and -fdata-sections, and a ld script it
>can be achieved.  But it may be very painful.
>
>Cc binutils, as it's a more proper location to discuss this.
>-- 
>Xi Ruoyao <xry111@mengyan1223.wang>
>School of Aerospace Science and Technology, Xidian University

gold has a --section-ordering-file, sorted by section name. It is less
useful than --symbol-ordering-file. In practice, text/data sections are
required to have different names (you cannot use clang
-funique-section-names; GCC feature request
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95095). For sorting based
on symbol names, you can use -funique-section-names.