performance impact of branch prediction?

[Hubert Feyrer]

Branch prediction is used in many parts throught the kernel, e.g.

subr_pool.c: if (__predict_false((pc->pc_pool->pr_flags & PR_WANTED) != 0)) {
subr_pool.c- goto destruct;
subr_pool.c- }

Does anyone know the performance impact of this prediction?
Were any kind of measurements done? What were the results?


Curious,

Hubert

[Nathan J. Williams]

Post by Hubert Feyrer
Branch prediction is used in many parts throught the kernel, e.g.
subr_pool.c: if (__predict_false((pc->pc_pool->pr_flags & PR_WANTED) != 0)) {
subr_pool.c- goto destruct;
subr_pool.c- }
Does anyone know the performance impact of this prediction?
Were any kind of measurements done? What were the results?

Note that __predict_false() isn't really about the microarchitectural
concept of branch prediction; it's more about moving code out-of-line
to reduce the memory footprint of the common path. That is, the
following code:

A
if (B)
C
D

would typically assemble to something like:

A
evaluate B
if false, goto 1f:
C
1f:
D

changing (B) to __predict_false(B) makes it likely to be rearranged
as:

A
evaluate B
if true, goto 2f:
1f:
D
...
return
2f:
C
goto 1f

I doubt it helps much when the wrapped code is a single goto.

At the microarchitectural level, dynamic branch prediction is
considered more useful these days than static compiler-generated
branch prediction anyway.

- Nathan