Re: [PATCH v3] Use __executable_start as the lowest address for profiling [BZ #28153]

[Fangrui Song via Libc-alpha <libc-alpha@sourceware.org>]

On 2021-08-05, Fangrui Song wrote:
>
>On 2021-08-05, H.J. Lu via Libc-alpha wrote:
>>Glibc assumes that ENTRY_POINT is the lowest address for which we need
>>to keep profiling records and BFD linker uses a linker script to place
>>the input sections.
>>
>>Starting from GCC 4.6, the main function is placed in .text.startup
>>section and starting from binutils 2.22, BFD linker with
>>
>>commit add44f8d5c5c05e08b11e033127a744d61c26aee
>>Author: Alan Modra <amodra@gmail.com>
>>Date:   Thu Nov 25 03:03:02 2010 +0000
>>
>>           * scripttempl/elf.sc: Group .text.exit, text.startup and .text.hot
>>           sections.
>>
>>places .text.startup section before .text section, which leave the main
>>function out of profiling records.
>>
>>Starting from binutils 2.15, linker provides __executable_start to mark
>>the lowest address of the executable.  Use __executable_start as the
>>lowest address to keep the main function in profiling records. This fixes
>>[BZ #28153].
>>
>>Tested on Linux/x86-64, Linux/x32 and Linux/i686 as well as with
>>build-many-glibcs.py.
>>---
>>csu/gmon-start.c              | 10 +++++++++-
>>gmon/tst-gmon-gprof.sh        |  2 ++
>>gmon/tst-gmon-static-gprof.sh |  2 ++
>>3 files changed, 13 insertions(+), 1 deletion(-)
>
>LGTM

[I had not subscribed the list until few days ago]

Comment to v2: gold/ld.lld don't necessarily place .text.unlikely
before other text sections.
GNU ld's fixed section ordering may make certain optimization difficult.

RISC architectures typically create range extension thunks to overcome the
limitation of short range branches. Hot code can usually be moved to the
middle to increase the chance that one range extension thunk can be
shared by more code.

8MiB cold
2MiB hot
8MiB cold

(
.text.sorted.* is a recent addition emulation link-time layout for
improving instruction cache and TLB. An optimized case may require the
sections to be interleaved with others.)