amd64: partially depessimize cpu_fetch_syscall_args and cpu_set_syscall_retval

[mjg]

Summary

cpu_fetch_syscall_args becoms jmp-free for the common case (modulo sv_mask which is going away soon). Bumps getuid rate on Broadwell from 105mln to 107.5mln.

This pessimizes 6 or more arg syscalls, which are a complete minority.

cpu_set_syscall_retval change removes 3 branches in favor of 1 and prevents reloads of ->td_frame.

Note the entire mechanism should be reworked - as it is there is a func call to amd64_syscall, then 3 indirect calls (fetch, syscall itself, set). The initial call can be indirect to a routine which does the fetch/set work and includes inlined amd64_syscall work.

[kib]

sys/amd64/amd64/trap.c

973–974

Does it need to be non-static ?

983–984

May be pass frame and sa as the arguments. You already calculated them in non-fallback version.

1002–1003

Change to nitems() ?

1081

Can you replace all three if()s with single or-ed condition and change goto to direct return(fallback()); statement ?

1085

Change to nitems() ?

1092

Again, can you replace this with return () ?

1095

Why 6 and not regcnt ? Or, do you need reg and regcnt in this function at all ?

1101

Then, remove the label.

[mjg]

• also depessimize cpu_set_syscall_retval

[kib]

Commit _retval() change now, I agree with it.

sys/amd64/amd64/trap.c

985

There regcnt is used as a constant.

I suggest you to add a define to machine/proc.h e.g. NARGREGS 6, and use it consistently instead of the literal '6' or regcnt, in both functions.

1073

This and previous lines, () are excessive.

1087

argp is not needed, inline its initializator there.

[mjg]

I did not commit set retval due to the freeze.