From: James Bottomley
> Sent: 28 September 2015 16:12
> > > > The x86 cpus will also do 32bit wide rmw cycles for the 'bit' operations.
> > >
> > > That's different: it's an atomic RMW operation. The problem with the
> > > alpha was that the operation wasn't atomic (meaning that it can't be
> > > interrupted and no intermediate output states are visible).
> >
> > It is only atomic if prefixed by the 'lock' prefix.
> > Normally the read and write are separate bus cycles.
>
> The essential point is that x86 has atomic bit ops and byte writes.
> Early alpha did not.
Early alpha didn't have any byte accesses.
On x86 if you have the following:
struct {
char a;
volatile char b;
} *foo;
foo->a |= 4;
The compiler is likely to generate a 'bis #4, 0(rbx)' (or similar)
and the cpu will do two 32bit memory cycles that read and write
the 'volatile' field 'b'.
(gcc definitely used to do this...)
A lot of fields were made 32bit (and probably not bitfields) in the linux
kernel tree a year or two ago to avoid this very problem.
> > > > You still have to ensure the compiler doesn't do wider rmw cycles.
> > > > I believe the recent versions of gcc won't do wider accesses for volatile data.
> > >
> > > I don't understand this comment. You seem to be implying gcc would do a
> > > 64 bit RMW for a 32 bit store ... that would be daft when a single
> > > instruction exists to perform the operation on all architectures.
> >
> > Read the object code and weep...
> > It is most likely to happen for operations that are rmw (eg bit set).
> > For instance the arm cpu has limited offsets for 16bit accesses, for
> > normal structures the compiler is likely to use a 32bit rmw sequence
> > for a 16bit field that has a large offset.
> > The C language allows the compiler to do it for any access (IIRC including
> > volatiles).
>
> I think you might be confusing different things. Most RISC CPUs can't
> do 32 bit store immediates because there aren't enough bits in their
> arsenal, so they tend to split 32 bit loads into a left and right part
> (first the top then the offset). This (and other things) are mostly
> what you see in code. However, 32 bit register stores are still atomic,
> which is all we require. It's not really the compiler's fault, it's
> mostly an architectural limitation.
No, I'm not talking about how 32bit constants are generated.
I'm talking about structure offsets.
David
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