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
