negative memcpy

[Kevin Cernekee]

On a MIPS 74K I would expect it to end up here:

>>   400338:     28c80008        slti    t0,a2,8
>>   40033c:     15000035        bnez    t0,400414 <__BMIPS3300_memcpy_last8>
>>   400340:     00801021        move    v0,a0

...

>> 00400414 <__BMIPS3300_memcpy_last8>:
>>   400414:     18c00006        blez    a2,400430 <__BMIPS3300_memcpy_lst8e>
>>   400418:     00c53821        addu    a3,a2,a1

...

>> 00400430 <__BMIPS3300_memcpy_lst8e>:
>>   400430:     03e00008        jr      ra
>>   400434:     00000000        nop

blez = branch if less than or equal to zero.

It would be a good idea to read through the whole function and figure
out whether $a2 can ever become negative in normal operation, before
changing the signed comparisons to unsigned.

Another possible hack is to AND $a2 with 0x7fffffff on entry.  This
would probably not catch cases like passing in a length of 0x80000010,
but it would catch lengths like 0xffffffff.