https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80080
--- Comment #11 from stli at linux dot ibm.com <stli at linux dot ibm.com> ---
Hi,
I've retested the samples with gcc 7, 8 and head from 2018-07-20, but there are
still issues:
The examples foo1 and foo2 are okay.
The issue in example foo3 is still present (see description of the bug-report):
00000000000000a0 <foo3>:
a0: a7 18 00 05 lhi %r1,5
a4: c4 2d 00 00 00 00 lrl %r2,a4 <foo3+0x4>
a6: R_390_PC32DBL foo3_mem+0x2
aa: c0 30 00 00 00 00 larl %r3,aa <foo3+0xa>
ac: R_390_PC32DBL foo3_mem+0x2
b0: ba 21 30 00 cs %r2,%r1,0(%r3)
b4: a7 74 ff fb jne aa <foo3+0xa>
The address of the global variable is still reloaded within the loop. If the
value was not swapped with cs, the jne can jump directly to the cs instruction
instead of the larl-instruction.
b8: b9 14 00 22 lgfr %r2,%r2
bc: 07 fe br %r14
be: 07 07 nopr %r7
I've found a further issue which is observable with the following two examples.
See the questions in the disassembly:
void foo4(int *mem)
{
int oldval = 0;
if (!__atomic_compare_exchange_n (mem, (void *) &oldval, 1,
1, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
{
bar (mem);
}
/*
0000000000000000 <foo4>:
0: e3 10 20 00 00 12 lt %r1,0(%r2)
6: a7 74 00 06 jne 12 <foo4+0x12>
Why do we need to jump to 0x12 first instead of directly jumping to 0x18?
a: a7 38 00 01 lhi %r3,1
e: ba 13 20 00 cs %r1,%r3,0(%r2)
12: a7 74 00 03 jne 18 <foo4+0x18>
16: 07 fe br %r14
18: c0 f4 00 00 00 00 jg 18 <foo4+0x18>
1a: R_390_PC32DBL bar+0x2
1e: 07 07 nopr %r7
*/
}
void foo5(int *mem)
{
int oldval = 0;
__atomic_compare_exchange_n (mem, (void *) &oldval, 1,
1, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
if (oldval != 0)
bar (mem);
/*
0000000000000040 <foo5>:
40: e3 10 20 00 00 12 lt %r1,0(%r2)
46: a7 74 00 06 jne 52 <foo5+0x12>
This is similar to foo4, but the variable oldval is compared against zero
instead of using the return value of __atomic_compare_exchange_n.
Can't we jump directly to 0x5a instead of 0x52?
4a: a7 38 00 01 lhi %r3,1
4e: ba 13 20 00 cs %r1,%r3,0(%r2)
52: 12 11 ltr %r1,%r1
54: a7 74 00 03 jne 5a <foo5+0x1a>
58: 07 fe br %r14
5a: c0 f4 00 00 00 00 jg 5a <foo5+0x1a>
5c: R_390_PC32DBL bar+0x2
*/
}
