Re: gcc300 benchmarks slower than gcc295.3?

[Richard Henderson <rth at redhat dot com>]

On Wed, Jul 18, 2001 at 09:09:14AM -0700, Linus Torvalds wrote:
> It's not a win at all.  It's a loss on newer CPU's, and even on older
> CPU's the added register pressure it implies probably forces it to be a
> wash. 

The memory/register splitting that is done at present is done after
normal register allocation, and so does not impact allocation.

It is done when possible, given available registers, in order to
help P1 scheduling, and P2 dispatch.  (Even read-only memory operand
is at least 2 uop, which means decoder 0; separate read operate
instructions are one 1 uop each, which can use decoder 1 or 2.)

It did make a measurable difference at one point -- it's entirely
possible that this wants disabling for Athlon and P4.



r~