Hi,
over weekend I found that vectorizer is missing scale_loop_profile for
epilogues. It already adjusts loop_info to set max iteraitons, so
adding it was easy. However now predicts the first loop to iterate at
most once (which is too much, I suppose it forgets to divide by epilogue
unrolling factor) and second never.
>
> The -O2 cost model doesn't want to do epilogues:
>
> /* If using the "very cheap" model. reject cases in which we'd keep
> a copy of the scalar code (even if we might be able to vectorize it).
> */
> if (loop_cost_model (loop) == VECT_COST_MODEL_VERY_CHEAP
> && (LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo)
> || LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> || LOOP_VINFO_PEELING_FOR_NITER (loop_vinfo)))
> {
> if (dump_enabled_p ())
> dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> "some scalar iterations would need to be
> peeled\n");
> return 0;
> }
>
> it's because of the code size increase.
I know, however -O2 is not -Os and here the tradeoffs of
performance/code size seems a lot better than other code expanding
things we do at -O2 (such as the unrolling 3 times).
I think we set the very cheap cost model very conservatively in order to
get -ftree-vectorize enabled with -O2 and there is some room for finding
right balance.
I get:
jan@localhost:~> cat t.c
int a[99];
__attribute((noipa, weak))
void
test()
{
for (int i = 0 ; i < 99; i++)
a[i]++;
}
void
main()
{
for (int j = 0; j < 10000000; j++)
test();
}
jan@localhost:~> gcc -O2 t.c -fno-unroll-loops ; time ./a.out
real 0m0.529s
user 0m0.528s
sys 0m0.000s
jan@localhost:~> gcc -O2 t.c ; time ./a.out
real 0m0.427s
user 0m0.426s
sys 0m0.000s
jan@localhost:~> gcc -O3 t.c ; time ./a.out
real 0m0.136s
user 0m0.135s
sys 0m0.000s
jan@localhost:~> clang -O2 t.c ; time ./a.out
<warnings>
real 0m0.116s
user 0m0.116s
sys 0m0.000s
Code size (of function test):
gcc -O2 -fno-unroll-loops 17 bytes
gcc -O2 29 bytes
gcc -O3 50 bytes
clang -O2 510 bytes
So unroling 70% code size growth for 23% speedup.
Vectorizing is 294% code size growth for 388% speedup
Clang does 3000% codde size growth for 456% speedup
>
> That's clearly much larger code. On x86 we're also fighting with
> large instruction encodings here, in particular EVEX for AVX512 is
> "bad" here. We hardly get more than two instructions decoded per
> cycle due to their size.
Agreed, I found it surprising clang does that much of complette unrolling
at -O2. However vectorizing and not unrolling here seems like it may be
a better default for -O2 than what we do currently...
Honza
>
> Richard.
