[Bug c/91852] New: Compile the code with -O0 is slower than with -O1/-O2/-O3

[hehaochen at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91852

            Bug ID: 91852
           Summary: Compile the code with -O0 is slower than with
                    -O1/-O2/-O3
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hehaochen at hotmail dot com
  Target Milestone: ---

Compile the following code with -O0 is slower than with -O1/-O2/-O3 :

-O1 applies some optimization on execution time and binary size, I expected
using -O1 takes more time than -O0. So is there something wrong with -O0 in
this case?

------------------------------------------------
 class c {
    public:
        c() {}
    };

    class a {
        c array[500000] {};
    };

    int main() {
        a t;
    }
------------------------------------------------

root@72c6552d0d56:/# gcc --version
gcc (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.

root@72c6552d0d56:/downloads# time gcc -std=c++14 -O0 -Wall a.cc

real    1m14.121s  #### SLOW ####
user    1m7.966s
sys     0m5.948s

root@72c6552d0d56:/downloads# time gcc -std=c++14 -O1 -Wall a.cc

real    0m26.268s
user    0m23.591s
sys     0m2.657s

root@72c6552d0d56:/downloads# time gcc -std=c++14 -O2 -Wall a.cc

real    0m26.413s
user    0m23.704s
sys     0m2.708s

root@72c6552d0d56:/downloads# time gcc -std=c++14 -O3 -Wall a.cc

real    0m26.553s
user    0m23.874s
sys     0m2.678s

The result is nearly same in
    gcc-9.2.0
    gcc-7.3.0
    gcc-6.5.0

The root cause lies in "phase opt and generate" in -ftime-report

The problem falls on '-O1' in older version: (gcc-4.8.5 gcc-5.4.0)
------------------------------------------------
 class c {
    public:
        c() {}
    };

    class a {
        // As described in bug 77443:
        // compile time grows super-linearly in (gcc-4.8.5 gcc-5.4.0)
        c array[30000] {};
    };

    int main() {
        a t;
    }
------------------------------------------------
# time gcc -O0 -std=c++11 -Wall a.cc
1.25s user 0.22s system 99% cpu 1.464 total

# time gcc -O1 -std=c++11 -Wall a.cc
7.93s user 0.16s system 99% cpu 8.089 total

# time gcc -O2 -std=c++11 -Wall a.cc
7.94s user 0.15s system 99% cpu 8.097 total

# time gcc -O3 -std=c++11 -Wall a.cc
7.92s user 0.18s system 99% cpu 8.095 total

In gcc-5.4.0
-O0: 0m1.834s
-O1: 0m5.467s
-O2: 0m5.407s