On Thu, Aug 09, 2007 at 12:36:17PM -0400, Chris Snook wrote:
> Paul E. McKenney wrote:
> >The compiler is within its rights to read a 32-bit quantity 16 bits at
> >at time, even on a 32-bit machine. I would be glad to help pummel any
> >compiler writer that pulls such a dirty trick, but the C standard really
> >does permit this.
>
> Yes, but we don't write code for these compilers. There are countless
> pieces of kernel code which would break in this condition, and there
> doesn't seem to be any interest in fixing this.
>
> >Use of volatile does in fact save you from the compiler pushing stores out
> >of loops regardless of whether you are also doing reads. The C standard
> >has the notion of sequence points, which occur at various places including
> >the ends of statements and the control expressions for "if" and "while"
> >statements. The compiler is not permitted to move volatile references
> >across a sequence point. Therefore, the compiler is not allowed to
> >push a volatile store out of a loop. Now the CPU might well do such a
> >reordering, but that is a separate issue to be dealt with via memory
> >barriers. Note that it is the CPU and I/O system, not the compiler,
> >that is forcing you to use reads to flush writes to MMIO registers.
>
> Sequence points enforce read-after-write ordering, not write-after-write.
> We flush writes with reads for MMIO because of this effect as well as the
> CPU/bus effects.
Neither volatile reads nor volatile writes may be moved across sequence
points.
> >And you would be amazed at what compiler writers will do in order to
> >get an additional fraction of a percent out of SpecCPU...
>
> Probably not :)
>
> >In short, please retain atomic_set()'s volatility, especially on those
> >architectures that declared the atomic_t's counter to be volatile.
>
> Like i386 and x86_64? These used to have volatile in the atomic_t
> declaration. We removed it, and the sky did not fall.
Interesting. You tested all possible configs on all possible hardware?
Thanx, Paul
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