Re: Number of registers on x86

[dewar at gnat dot com]

<<Register renaming only makes the processor more tolerant to
suboptimal instruction scheduling. It doesn't give the compiler
any more registers to use per se.
>>

Of course, but the question is how to stop the register allocator
THINKING there is a conflict when there isn't, and thus using
more registers than it should.

Take your example

       mov     (%esi),eax
       movl    eax,(%edi)
       movl    4(%esi),eax
       mov.l   eax,4(%edi)

A register allocator not realizing that the two eax's are different here
may try to use two registers.

The suggested technique, which is to pretend there are 40 registers,
and then when you have a schedule with 40 registers, collapse it
with knowledge of how the renaming works, is an interesting approach,
which might work better than ignoring renaming completely.