On Thu, 26 Nov 2015, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> writes:
>
> > On Thu, Nov 26, 2015 at 12:50:08AM +0000, Måns Rullgård wrote:
> >> If not calling the function saves an I-cache miss, the benefit can be
> >> substantial. No, I have no proof of this being a problem, but it's
> >> something that could happen.
> >
> > That's a simplistic view of modern CPUs.
> >
> > As I've already said, modern CPUs which have branch prediction, but
> > they also have speculative instruction fetching and speculative data
> > prefetching - which the CPUs which have idiv support will have.
> >
> > With such features, the branch predictor is able to learn that the
> > branch will be taken, and because of the speculative instruction
> > fetching, it can bring the cache line in so that it has the
> > instructions it needs with minimal or, if working correctly,
> > without stalling the CPU pipeline.
>
> It doesn't matter how many fancy features the CPU has. Executing more
> branches and using more cache lines puts additional pressure on those
> resources, reducing overall performance. Besides, the performance
> counters readily show that the prediction is nothing near as perfect as
> you seem to believe.
OK... Let's try to come up with actual numbers.
We know that letting gcc emit idiv by itself is the ultimate solution.
And it is free of maintenance on our side besides passing the
appropriate argument to gcc of course. So this is worth doing.
For the case where you have a set of target machines in your kernel that
may or may not have idiv, then the first step should be to patch
__aeabi_uidiv and __aeabi_idiv. This is a pretty small and simple
change that might turn out to be more than good enough. It is necessary
anyway as the full patching solution does not cover all cases.
Then, IMHO, it would be a good idea to get performance numbers to
compare that first step and the full patching solution. Of course the
full patching will yield better performance. It has to. But if the
difference is not significant enough, then it might not be worth
introducing the implied complexity into mainline. And it is not because
the approach is bad. In fact I think this is a very cool hack. But it
comes with a cost in maintenance and that cost has to be justified.
Just to have an idea, I produced the attached micro benchmark. I tested
on a TC2 forced to a single Cortex-A15 core and I got those results:
Testing INLINE_DIV ...
real 0m7.182s
user 0m7.170s
sys 0m0.000s
Testing PATCHED_DIV ...
real 0m7.181s
user 0m7.170s
sys 0m0.000s
Testing OUTOFLINE_DIV ...
real 0m7.181s
user 0m7.170s
sys 0m0.005s
Testing LIBGCC_DIV ...
real 0m18.659s
user 0m18.635s
sys 0m0.000s
As you can see, whether the div is inline or out-of-line, whether
arguments are moved into r0-r1 or not, makes no difference at all on a
Cortex-A15.
Now forcing it onto a Cortex-A7 core:
Testing INLINE_DIV ...
real 0m8.917s
user 0m8.895s
sys 0m0.005s
Testing PATCHED_DIV ...
real 0m11.666s
user 0m11.645s
sys 0m0.000s
Testing OUTOFLINE_DIV ...
real 0m13.065s
user 0m13.025s
sys 0m0.000s
Testing LIBGCC_DIV ...
real 0m51.815s
user 0m51.750s
sys 0m0.005s
So on A cortex-A7 the various overheads become visible. How significant
is it in practice with normal kernel usage? I don't know.
Nicolas
#!/bin/sh
set -e
for test in INLINE_DIV PATCHED_DIV OUTOFLINE_DIV LIBGCC_DIV; do
gcc -o divtest_$test divtest.S -D$test
echo "Testing $test ..."
time ./divtest_$test
echo
rm -f divtest_$test
done
.arm
.arch_extension idiv
.globl main
main:
stmfd sp!, {r4, r5, lr}
mov r4, #17
1: mov r5, #1
2:
#if defined(INLINE_DIV)
udiv r0, r4, r5
#elif defined(OUTOFLINE_DIV)
mov r0, r4
mov r1, r5
bl my_div
#elif defined(PATCHED_DIV)
mov r0, r4
mov r1, r5
udiv r0, r0, r1
#elif defined(LIBGCC_DIV)
mov r0, r4
mov r1, r5
bl __aeabi_uidiv
#else
#error "define INLINE_DIV, OUTOFLINE_DIV or LIBGCC_DIV"
#endif
add r5, r5, #1
cmp r4, r5
bhs 2b
adds r4, r4, r4, lsl #1
bpl 1b
mov r0, #0
ldmfd sp!, {r4, r5, pc}
.space 1024
my_div:
udiv r0, r0, r1
bx lr
