Messages in this thread | | | Date | 5 Feb 2005 10:54:16 +0100 | Date | Sat, 5 Feb 2005 10:54:16 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH] relayfs redux, part 3 II - another comment |
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Another comment on relay_write fast pathing
> +static inline unsigned relay_write(struct rchan *chan, > + const void *data, > + unsigned length) > +{ > + unsigned long flags; > + struct rchan_buf *buf = relay_get_buf(chan, smp_processor_id()); > + > + local_irq_save(flags); > + if (unlikely(buf->offset + length > chan->subbuf_size)) > + length = relay_switch_subbuf(buf, length); > + memcpy(buf->data + buf->offset, data, length);
I said earlier gcc would optimize the memcpy, but thinking about this again I'm not so sure anymore. The problem is that with variable buf->offset gcc cannot prove the alignment of the destination, and with unknown alignment it tends to generate a out of line call to memcpy, which is quite typically slow.
You can take a look at the generated assembly if that's true or not, but I suspect it is.
To avoid this it may be needed to play
if (__builtin_constant_p(length)) switch (length) { case 1: /* optimized version for 1 byte */ break; case 2: ... case 4: ... case 8: ... } else memcpy(...);
games like e.g. uaccess.h does. Problem is that sometimes gcc seems to break __builtin_constant_p inside inlines, so it may be needed to move it into a macro (i would double check if that is really needed though for this case, the code is much nicer with a inline)
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