[Bug regression/44281] Global Register variable pessimisation and regression

[adam at consulting dot net dot nz]

------- Comment #1 from adam at consulting dot net dot nz  2010-06-07 05:35 -------
Example-specific workaround discovered for global register variable
pessimisation with recent versions of GCC:

void push_flag_into_global_reg_var(uint64_t a, uint64_t b) {
  uint64_t flag = (a==b);
  global_flag_stack <<= 8;
  __asm__ __volatile__("" : : : "memory"); /* ??? */
  global_flag_stack  |= flag;
}

Every version of GCC tested (including gcc (Debian 20100530-1) 4.6.0 20100530
(experimental) [trunk revision 160047]) produces similarly compact code:

0000000000400494 <push_flag_into_global_reg_var>:
  400494:       48 c1 e3 08             shl    rbx,0x8
  400498:       31 c0                   xor    eax,eax
  40049a:       48 39 f7                cmp    rdi,rsi
  40049d:       0f 94 c0                sete   al
  4004a0:       48 09 c3                or     rbx,rax
  4004a3:       c3                      ret

Telling the compiler that memory may have changed between global register
variable assignments seems to have coaxed the compiler into treating the global
register variable as volatile.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44281