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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/107675] New: [13 Regression] GCC-13 is significantly
 slower to startup on C++ programs
Date: Mon, 14 Nov 2022 13:03:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107675

            Bug ID: 107675
           Summary: [13 Regression] GCC-13 is significantly slower to
                    startup on C++ programs
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*

The following simple testcase:

#include <cstdio>

int main(int argc, char** argv) {
    printf("Hello world\n");
    return 0;
}

is on average twice as slow as GCC 12 during startup:

> perf diff perf.data.gcc.12 perf.data.gcc.13

# Event 'cycles'
#
# Baseline  Delta Abs  Shared Object     Symbol
# ........  .........  ................  ...............................
#
   100.00%    +42.66%  gcc-13.exe        [.] fde_single_encoding_compare

Looking at perf the entirety of of the difference is spent in __libc_start_=
main
which on GCC 13 calls __libc_csu_init which calls init_object.constprop.0.

This constprop function spends half the time in frame_heapsort and the other
half in fde_single_encoding_compare.  Together these two calls are responsi=
ble
for the 2x slower startup.

Is it possible to mitigate this? At startup it's currently executing more t=
han
6.5x the number of instructions as GCC-12 which is quite noticeable on small
devices.=