https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80004
Bug ID: 80004
Summary: non-atomic load moved to before atomic load with
std::memory_order_acquire
Product: gcc
Version: 6.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: fboranek at atlas dot cz
Target Milestone: ---
Folowing code
(https://gist.github.com/fboranek/5c99f96a99f902f5cf9dc5d597ff8fe3) for 4
thread doesn't compute the correct result if it is compiled by gcc (Debian
6.3.0-6) 6.3.0 20170205. The code is corect compiled by gcc 4.9.2 and 5.4.1. It
is also correct compiled if the variable counter is not static bat global.
This issue can be related to bug 78778, but bug 78778 is appear in 5.1.0 while
this issue is since version 6.
static int counter {0};
static std::atomic<bool> flag {false};
void increment1(int cycles)
{
for (int i=0; i < cycles; ++i)
{
while (flag.exchange(true, std::memory_order_acquire));
++counter;
flag.store(false, std::memory_order_release);
}
}
// asm for gcc 5.4.1
00000000004010f0 <_Z10increment1i>:
4010f0: 31 c9 xor %ecx,%ecx
4010f2: 85 ff test %edi,%edi
4010f4: ba 01 00 00 00 mov $0x1,%edx
4010f9: 7e 26 jle 401121 <_Z10increment1i+0x31>
4010fb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
401100: 89 d0 mov %edx,%eax
401102: 86 05 ec 20 20 00 xchg %al,0x2020ec(%rip) #
6031f4 <_ZL4flag>
401108: 84 c0 test %al,%al
40110a: 75 f4 jne 401100 <_Z10increment1i+0x10>
40110c: 83 c1 01 add $0x1,%ecx
40110f: 83 05 e2 20 20 00 01 addl $0x1,0x2020e2(%rip) #
6031f8 <_ZL7counter>
401116: 39 cf cmp %ecx,%edi
401118: c6 05 d5 20 20 00 00 movb $0x0,0x2020d5(%rip) #
6031f4 <_ZL4flag>
40111f: 75 df jne 401100 <_Z10increment1i+0x10>
401121: f3 c3 repz retq
401123: 0f 1f 00 nopl (%rax)
401126: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
40112d: 00 00 00
// asm for gcc 6.3.0
0000000000001420 <_Z10increment1i>:
1420: 8b 0d d2 1d 20 00 mov 0x201dd2(%rip),%ecx #
2031f8 <_ZL7counter>
1426: 85 ff test %edi,%edi
1428: ba 01 00 00 00 mov $0x1,%edx
142d: 8d 34 0f lea (%rdi,%rcx,1),%esi
1430: 7e 26 jle 1458 <_Z10increment1i+0x38>
1432: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
1438: 89 d0 mov %edx,%eax
143a: 86 05 b4 1d 20 00 xchg %al,0x201db4(%rip) #
2031f4 <_ZL4flag>
1440: 84 c0 test %al,%al
1442: 75 f4 jne 1438 <_Z10increment1i+0x18>
1444: 83 c1 01 add $0x1,%ecx
1447: 39 ce cmp %ecx,%esi
1449: 89 0d a9 1d 20 00 mov %ecx,0x201da9(%rip) #
2031f8 <_ZL7counter>
144f: c6 05 9e 1d 20 00 00 movb $0x0,0x201d9e(%rip) #
2031f4 <_ZL4flag>
1456: 75 e0 jne 1438 <_Z10increment1i+0x18>
1458: f3 c3 repz retq
145a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
