On 16/07/12 09:27, Jonathan Nieder wrote:
> Michael Cree wrote:
>> On 15/07/2012, at 8:50 AM, Jonathan Nieder wrote:
>>>                   gcc takes full advantage by converting the get_be32
>>> calls back to a load and bswap and producing a whole bunch of
>>> unaligned access traps.
>> Alpha does not have a bswap (or similar) instruction.
> [...]
>>                                    .  If the pointer is unaligned
>> then two neighbouring aligned 32bit loads are required to ensure
>> that all four bytes are loaded.  If the pointer is aligned then a
>> single 32bit load gets all the four bytes.  Having never looked at
>> the generated assembler code, I nevertheless suspect that is the
>> guts of the optimisation --- the compiler can eliminate an access to
>> memory if it knows the pointer is aligned.
> How about:
>       gcc takes full advantage by using a single 32-bit load,
>       resulting in a whole bunch of unaligned access traps.

Yes, that's good.

I just checked the generated assembler and I am correct except that I
underestimated the number of memory accesses for the a priori known
unaligned data case.  It is actually doing four long-word memory access,
i.e. a memory access for each individual byte despite that it only needs
to do a maximum of two, so the optimisation for the a priori known
aligned data saves three memory accesses, not just one.  (And probably
also saves on a load-load replay trap on the EV6 and later CPUs, i.e., a
complete and very costly replay of the whole pipeline when the CPU
realises just in the nick of time that it has cocked up the
interrelationships between reordered load instructions.)

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