David Laight wrote:
> Separate renaming allows:
> 1) The value to tested without waiting for pending updates to complete.
> Useful for IE and DIR.
I don't quite follow. It allows the value to be tested without waiting
for pending updates *of other bits* to complete.
Obviusly, the update of the bit being tested has to complete!
> I can't see any obvious gain from separating out O or Z (even with
> adcx and adox). You'd need some other instructions that don't set O (or Z)
> but set some other useful flags.
> (A decrement that only set Z for instance.)
I tried to describe the advantages in the previous message.
The problems arise much less often than the INC/DEC pair, but there are
instructions whick write only the O and C flags, (ROL, ROR) and only
the Z flag (CMPXCHG).
The sign, aux carry, and parity flags are *always* updated as
a group, so they can be renamed as a group.
> While LOOP could be used on Bulldozer+ an equivalently fast loop
> can be done with inc/dec and jnz.
> So you only care about LOOP/JCXZ when ADOX is supported.
>
> I think the fastest loop is:
> 10: adc %rax,0(%rdi,%rcx,8)
> inc %rcx
> jnz 10b
> but check if any cpu add an extra clock for the 'scaled' offset
> (they might be faster if %rdi is incremented).
> That loop looks like it will have no overhead on recent cpu.
Well, it should execute at 1 instruction/cycle. (No, a scaled offset
doesn't take extra time.) To break that requires ADCX/ADOX:
10: adcxq 0(%rdi,%rcx),%rax
adoxq 8(%rdi,%rcx),%rdx
leaq 16(%rcx),%rcx
jrcxz 11f
j 10b
11:
