Linus Torvalds wrote:
> Sure, that's *one* thing that "volatile" guarantees (it guarantees that
> gcc won't optimize away things where the end result isn't actually visibly
> used).
>
> But gcc docs also talk about the other things volatile means, including
> "not significantly moved".
>
Actually, it doesn't. In fact it goes out of its way to say that "asm
volatile" statements can be moved quite a bit, with respect to other
asms, other code, jumps, basic blocks, etc. The only reliable way to
control the placement of an asm is have the right dependencies.
The `volatile' keyword indicates that the instruction has important
side-effects. GCC will not delete a volatile `asm' if it is reachable.
(The instruction can still be deleted if GCC can prove that
control-flow will never reach the location of the instruction.) Note
that even a volatile `asm' instruction can be moved relative to other
code, including across jump instructions.
also:
An `asm' instruction without any output operands will be treated
identically to a volatile `asm' instruction.
So there isn't anything very special about "asm volatile". It's purely
to stop apparently useless code from being removed.
J
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