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: