On Mar 07, 2007, at 20:25:14, Michael K. Edwards wrote:
On 3/7/07, Linus Torvalds <[EMAIL PROTECTED]> wrote
In general, using software prefetching is just a stupid idea, unless
- the prefetch really is very strict (ie for a linked list you do
exactly the above kinds of things to make sure that you don't try
to prefetch the non-existent end entry)
AND
- the CPU is stupid (in-order in particular).
I think Intel even suggests in their optimization manuals to *not*
do software prefetching, because hw can usually simply do better
without it.
Not the XScale -- it performs quite poorly without prefetch, as
people who have run ARMv5-optimized binaries on it can testify.
The Intel XScale(r) core prefetch load instruction is a true
prefetch instruction because the load destination is the data or
mini-data cache and not a register. Compilers for processors which
have data caches, but do not support prefetch, sometimes use a load
instruction to preload the data cache. This technique has the
disadvantages of using a register to load data and requiring
additional registers for
subsequent preloads and thus increasing register pressure. By
contrast, the prefetch can be used to reduce register pressure
instead of increasing it.
The prefetch load is a hint instruction and does not guarantee that
the data will be loaded. Whenever the load would cause a fault or a
table walk, then the processor will ignore the prefetch
instruction, the fault or table walk, and continue processing the
next instruction. This is particularly advantageous in the case
where a linked list or recursive data structure is terminated by a
NULL pointer. Prefetching the NULL pointer will not fault program
flow.
Prefetching is also fairly critical on a Power4 or G5 PowerPC system
as they have a long memory latency; an L2-cache miss can cost 200+
cycles. On such systems the "dcbt" prefetch instruction brings in a
single 128-byte cacheline and has no serializing effects whatsoever,
making it ideal for use in a linked-list-traversal inner loop.
Cheers,
Kyle Moffett
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