[Bug tree-optimization/82188] New: Missed optimization opportunity for constant folding

[ishiura-compiler at ml dot kwansei.ac.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82188

            Bug ID: 82188
           Summary: Missed optimization opportunity for constant folding
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ishiura-compiler at ml dot kwansei.ac.jp
  Target Milestone: ---

We compiled two programs (A1.c, A2.c) by GCC-8.0.0 with -O3 option.
Though the two programs are equivalent, GCC outputs different assembly codes.


+-------------------------------------+---------------------------------+
| A1.c                                | A2.c                            |
+-------------------------------------+---------------------------------+
|int main (void)                      |int main (void)                  |
|{                                    |{                                |
|  volatile int x = 1;                |  volatile int x = 1;            |
|                                     |                                 |
|  long t1 = 1/(2U-x);                |  long t1 = 1/(2U-x);            |
|  int t2 = 0!=(((int)(2%(2+t1)))/2); |  int t2 = 0!=(int)t1;           |
|                                     |                                 |
|  if (t1 != 1) __builtin_abort();    |  if (t1 != 1) __builtin_abort();|
|  if (t2 != 1) __builtin_abort();    |  if (t2 != 1) __builtin_abort();|
|                                     |                                 |
|  return 0;                          |  return 0;                      |
|}                                    |}                                |
+-------------------------------------+---------------------------------+

+-------------------------------------+-------------------------------------+
|    A1.s (gcc-8.0.0 A1.c -O3 -S)     |    A2.s (gcc-8.0.0 A2.c -O3 -S)     |
+-------------------------------------+-------------------------------------+
|main:                                |main:                                |
|.LFB0:                               |.LFB0:                               |
|   .cfi_startproc                    |   .cfi_startproc                    |
|   subq   $24, %rsp                  |   subq   $24, %rsp                  |
|   .cfi_def_cfa_offset 32            |   .cfi_def_cfa_offset 32            |
|   movl   $2, %esi                   |   movl   $2, %ecx                   |
|   xorl   %edx, %edx                 |   xorl   %edx, %edx                 |
|   movl   $1, 12(%rsp)               |   movl   $1, 12(%rsp)               |
|   movl   12(%rsp), %eax             |   movl   12(%rsp), %eax             |
|   subl   %eax, %esi                 |   subl   %eax, %ecx                 |
|   movl   $1, %eax                   |   movl   $1, %eax                   |
|   divl   %esi                       |   divl   %ecx                       |
|   movl   %eax, %esi                 |   testl  %eax, %eax                 |
|   movl   $2, %eax                   |   je     .L3                        |
|   movq   %rsi, %rcx                 |                                     |
|   cqto                              |                                     |
|   addq   $2, %rsi                   |                                     |
|   idivq  %rsi                       |                                     |
|   cmpl   $1, %ecx                   |                                     |
|   jne    .L3                        |                                     |
|   cmpq   $2, %rdx                   |                                     |
|   jne    .L3                        |                                     |
|   xorl   %eax, %eax                 |   xorl   %eax, %eax                 |
|   addq   $24, %rsp                  |   addq   $24, %rsp                  |
|   .cfi_def_cfa_offset 8             |   .cfi_def_cfa_offset 8             |
|   ret                               |   ret                               |
|   .cfi_endproc                      |   .cfi_endproc                      |
|   .section   .text.unlikely         |   .section   .text.unlikely         |
|   .cfi_startproc                    |   .cfi_startproc                    |
|   .type   main.cold.0, @function    |   .type   main.cold.0, @function    |
|main.cold.0:                         |main.cold.0:                         |
|.L3:                                 |.L3:                                 |
|   .cfi_def_cfa_offset 32            |   .cfi_def_cfa_offset 32            |
|   call   abort                      |   call   abort                      |
|   .cfi_endproc                      |   .cfi_endproc                      |
|.LFE0:                               |.LFE0:                               |
|   .section   .text.startup          |   .section   .text.startup          |
|   .size   main, .-main              |   .size   main, .-main              |
|   .section   .text.unlikely         |   .section   .text.unlikely         |
|   .size  main.cold.0, .-main.cold.0 |   .size  main.cold.0, .-main.cold.0 |
+-------------------------------------+-------------------------------------+


FYI, LLVM/clang generates the same same assembly codes for A1.c and A2.c.

+---------------------------------+---------------------------------+
|  A1.s (clang-6.0.0 A1.c -O3 -S) |  A2.s (clang-6.0.0 A2.c -O3 -S) |
+---------------------------------+---------------------------------+
|main:              # @main       |main:              # @main       |
|   .cfi_startproc                |   .cfi_startproc                |
|# BB#0:                          |# BB#0:                          |
|   pushq   %rax                  |   pushq   %rax                  |
|.Lcfi0:                          |.Lcfi0:                          |
|   .cfi_def_cfa_offset 16        |   .cfi_def_cfa_offset 16        |
|   movl   $1, 4(%rsp)            |   movl   $1, 4(%rsp)            |
|   movl   4(%rsp), %eax          |   movl   4(%rsp), %eax          |
|   cmpl   $1, %eax               |   cmpl   $1, %eax               |
|   jne   .LBB0_2                 |   jne   .LBB0_2                 |
|# BB#1:                          |   # BB#1:                       |
|   xorl   %eax, %eax             |   xorl   %eax, %eax             |
|   popq   %rcx                   |   popq   %rcx                   |
|   retq                          |   retq                          |
|.LBB0_2:                         |.LBB0_2:                         |
|   callq   abort                 |   callq   abort                 |
|.Lfunc_end0:                     |.Lfunc_end0:                     |
|   .size   main, .Lfunc_end0-main|   .size   main, .Lfunc_end0-main|
+---------------------------------+---------------------------------+

gcc-8.0 (GCC) 8.0.0 20170828 (experimental)
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

clang version 6.0.0 (trunk 308582) (llvm/trunk 308581)
Target: x86_64-unknown-linux-gnu
Thread model: posix