Am 16.11.2010 18:09, schrieb Sean Kelly:
cas() contains an asm block. Though I guess in this case the compiler
isn't actually optimizing across it. Does atomic!"+="(&cnt, 1) work
correctly? I know the issue with shared would still have to be fixed,
but that code uses asm for the load as well, so it probably won't be
optimized the same way.
Thanks for looking into the issue around here. Just three comments from
my side, Sean.
Disclaimer: based on a couple of hours chasing a bug and not much D
experience (but some optimizing C++ compiler experience - so the issue
looked familiar :-) )
1) atomicOp is not concerned. You only read memory once in the function
call. Whether from a local variable that was loaded from something
global or directly from a global, doesn't really matter (except for
timing, maybe).
2) You are right, the compiler seems to not optimize across asm
statements. So, the example can be fixed with the following hack:
void atomicInc ( ) {
uint o;
while ( !cas( &cnt, o, o + 1 ) ) {
asm { nop; } o = cnt;
}
}
This is however more brittle than it looks, because it is not always
clear what "optimizing across an asm block". This version has the issue
again:
void atomicInc ( ) {
uint o = cnt;
do {
asm { nop; } o = cnt;
} while ( !cas( &cnt, o, o + 1 ) )
}
While this case might look somewhat obvious, I encountered some problems
in more complex code, and finally went for the all-inline-assembler
solution to be on the safe side.
3) During my debugging, I believe that I saw the optimizer not only
re-ordering reads of shared variables, but also writes to shared
variables. IIRC, my Dekker example on SO (which fails for the missing
s/l/mfence instructions), also sports a re-ordering of the lines
cnt++;
turn2 = true; flag1 = false;
into
turn2 = true;
cnt++;
flag1 = false;
which in this case is not really important, but might introduce another
bug if I was prepared to live with the risk of starvation (and remove
turn2). If the compiler would still re-order (haven't tested), cnt++
would be outside of the critical section.
Hope this helps & cheers,
Stephan
