In a reasonable world, gcc should just make that be (on x86)
addl $1,i(%rip)
on x86-64, which is indeed what it does without the volatile. But with
the
volatile, the compiler gets really nervous, and doesn't dare do it in
one
instruction, and thus generates crap like
movl i(%rip), %eax
addl $1, %eax
movl %eax, i(%rip)
instead. For no good reason, except that "volatile" just doesn't have
any
good/clear semantics for the compiler, so most compilers will just
make it
be "I will not touch this access in any way, shape, or form". Including
even trivially correct instruction optimization/combination.
It's just a (target-specific, perhaps) missed-optimisation kind
of bug in GCC. Care to file a bug report?
but is
(again) something that gcc doesn't dare do, since "i" is volatile.
Just nobody taught it it can do this; perhaps no one wanted to
add optimisations like that, maybe with a reasoning like "people
who hit the go-slow-in-unspecified-ways button should get what
they deserve" ;-)
Segher
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