number of registers (was: My 66000 and High word facility)

mitchalsup@aol.com (MitchAlsup1) writes:

On Mon, 19 Aug 2024 18:52:39 +0000, Brett wrote:
>

MitchAlsup1 <mitchalsup@aol.com> wrote:

The thing is that one you go down the GBOoO route, your lack of
registers
"namable in ASM" ceases to become a performance degrader. With renaming
one can have R7 in use 40 times in a 100 instruction deep execution
window.

>
If this was true we would have 16 or even 8 visible registers, and all
would be fine. x86 does mostly fine with 16

And yet Intel went to 32 SIMD registers with AVX-512 (which admittedly
was first developed for an in-order microarchitecture) and are now
going to 32 GPRs with APX (no in-order excuse here).  And IIRC the
announcement of APX says something about 10% fewer memory accesses or
somesuch.

Careful, here::
>
x86 has LD-OPs and LD-OP-STs which makes the 16 register file feel more
like it has 20-22 registers.

You feeling is strong (as shown by your repeatedly ignoring the
counterevidence), but wrong:

LD-OPs and LD-OP-STs as on AMD64 and PDP-11 make the 16 registers
equivalent to 17 registers on a load/store architecture:

Let's call the 17th register r16:

On a load-store architecture you replace "LD-OP dest,src" with:

ld r16=src
op dest,dest,r16

On a load-store architecture you replace "LD-OP-ST dest,src" with:

ld r16=dest
op r16,r16,src
st dest=r16

For a VAX-like three-memory-argument instruction you need two extra
registers, r16 and r17:

"mem1 = mem2 op mem3" becomes:

ld r16=mem2
ld r17=mem3
op r16,r16,r17
st mem1=r17

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>