[Bug c/110069] New: [Perf] -finstrument-functions causes program size to double

[Bug c/110069] New: [Perf] -finstrument-functions causes program size to double

[chipweinberger at jamcorder dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110069

            Bug ID: 110069
           Summary: [Perf] -finstrument-functions causes program size to
                    double
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chipweinberger at jamcorder dot com
  Target Milestone: ---

Created attachment 55228
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55228&action=edit
Assembly comparison with and without finstrument functions

Hello,

First time issue filer here. 

First some context: I am using -finstrument-functions to implement a "stack
mirror" feature on the ESP32 microcontroller from Espressif. This mirror is
logged after detecting stack corruption, and it is incrediblly useful for
debugging.

Unfortunately, -finstrument-functions causes the program size to double, using
100KB extra of internal SRAM (over 20% of the entire MCUs memory), meaning most
people cannot even enable this feature because spare ram is usually very
scarce.

The main problem is the function signature of __cyg_profile_func_enter. ~35
bytes of instructions are needed to set up the 2 function arguments, and these
35 bytes need to be inserted into *every* function. This is a major cost for
both performance and memory.

void __cyg_profile_func_enter(void *func, void *callsite)

These arguments are not needed by us. We can traverse the stack ourself.

I am hoping we can consider a new flag, -function-entry-exit-hooks, with a much
simpler function signature:

void __func_hook_entry(void)
void __func_hook_exit(void)

Without the arguments, each function only needs a simple 'call' instruction.

This would be incredibly useful for us, and I imagine a lot of other people as
well.

Thanks,
Chip

[Bug middle-end/110069] [Perf] -finstrument-functions causes program size to double

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110069

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
In your backend you could implement all of this I think. 

Are you talking about -O0 code generation or -O2?

[Bug middle-end/110069] [Perf] -finstrument-functions causes program size to double

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110069

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug middle-end/110069] [Perf] -finstrument-functions causes program size to double

[chipweinberger at jamcorder dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110069

--- Comment #2 from Chip Weinberger <chipweinberger at jamcorder dot com> ---
The 35 bytes figure is from -O0. This is a debug feature.

[Bug middle-end/110069] [Perf] -finstrument-functions causes program size to double

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110069

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amonakov at gcc dot gnu.org

--- Comment #3 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
For such space-constrained environments I'd suggest using '-pg' instead. It
emits just one call to the _mcount function on entry without overhead. If you
need to hook the exit, you can replace the return address in your
implementation of _mcount (this function usually has a non-standard ABI, so
it's always necessary to implement it in assembly).