Re: How to force gcc to vectorize the loop with particular vectorization width
Hello Richard,
Thank you. I achieved vectorization with vf = 16, using
#pragma GCC optimize ("no-unroll-loops")
__attribute__ ((__target__ ("sse4.2")))
and options -march=core-avx2 -mprefer-avx-128
But now I have a question: Is it possible in gcc to have vectorization
with vf < 16?
On 20/10/2017, Richard Biener wrote:
> On Fri, Oct 20, 2017 at 12:13 PM, Denis Bakhvalov
> wrote:
>> Thank you for the reply!
>>
>> Regarding last part of your message, this is also what clang will do
>> when you are passing vf of 4 (with the pragma from my first message)
>> for the loop operating on chars plus using SSE2. It will do meaningful
>> work only for 4 chars per iteration (a[0], zero, zero, zero, a[1],
>> zero, zero, zero, etc.).
>>
>> Please see example here:
>> https://godbolt.org/g/3LAqZw
>>
>> Let's say that I know all possible trip counts for my inner loop. They
>> all do not exceed 32. In the example above vf for this loop is 32.
>> There is a runtime check, such that if trip count do not exceed 32 it
>> will fall back to scalar version.
>>
>> As long as trip count is always lower that 32 - it always chooses
>> scalar version at runtime.
>> But theoretically, using SSE2 for trip count = 8 it can use half of
>> xmm register (8 chars) to do meaningfull work.
>>
>> Is gcc vectorizer capable of doing this?
>> If yes, can I somehow achieve this in gcc by tweaking the code or
>> adding some pragma?
>
> The closest is to use -mprefer-avx128 so you get SSE rather than AVX
> vector sizes. Eventually this option is among the valid target attributes
> for #pragma GCC target
>
>> On 19/10/2017, Jakub Jelinek wrote:
>>> On Thu, Oct 19, 2017 at 10:38:28AM +0200, Richard Biener wrote:
On Thu, Oct 19, 2017 at 9:22 AM, Denis Bakhvalov
wrote:
> Hello!
>
> I have a hot inner loop which was vectorized by gcc, but I also want
> compiler to unroll this loop by some factor.
> It can be controled in clang with this pragma:
> #pragma clang loop vectorize(enable) vectorize_width(8)
> Please see example here:
> https://godbolt.org/g/UJoUJn
>
> So I want to tell gcc something like this:
> "I want you to vectorize the loop. After that I want you to unroll
> this vectorized loop by some defined factor."
>
> I was playing with #pragma omp simd with the safelen clause, and
> #pragma GCC optimize("unroll-loops") with no success. Compiler option
> -fmax-unroll-times is not suitable for me, because it will affect
> other parts of the code.
>
> Is it possible to achieve this somehow?
No.
>>>
>>> #pragma omp simd has simdlen clause which is a hint on the preferable
>>> vectorization factor, but the vectorizer doesn't use it so far;
>>> probably it wouldn't be that hard to at least use that as the starting
>>> factor if the target has multiple ones if it is one of those.
>>> The vectorizer has some support for using wider vectorization factors
>>> if there are mixed width types within the same loop, so perhaps
>>> supporting 2x/4x/8x etc. sizes of the normally chosen width might not be
>>> that hard.
>>> What we don't have right now is support for using smaller
>>> vectorization factors, which might be sometimes beneficial for -O2
>>> vectorization of mixed width type loops. We always use the vf derived
>>> from the smallest width type, say when using SSE2 and there is a char
>>> type,
>>> we try to use vf of 16 and if there is also int type, do operations on
>>> those
>>> in 4x as many instructions, while there is also an option to use
>>> vf of 4 and for operations on char just do something meaningful only in
>>> 1/4
>>> of vector elements. The various x86 vector ISAs have instructions to
>>> widen or narrow for conversions.
>>>
>>> In any case, no is the right answer right now, we don't have that
>>> implemented.
>>>
>>> Jakub
>>>
>>
>>
>> --
>> Best regards,
>> Denis.
>
--
Best regards,
Denis.
Re: atomic_thread_fence() semantics
On Fri, 2017-10-20 at 18:46 +0300, Alexander Monakov wrote: > On Fri, 20 Oct 2017, Torvald Riegel wrote: > > On Thu, 2017-10-19 at 15:31 +0300, Alexander Monakov wrote: > > > On Thu, 19 Oct 2017, Andrew Haley wrote: > > > > No, you did not. This looks like a bug. Please report it. > > > > > > This bug is fixed on trunk, so should work from gcc-8 onwards (PR 80640). > > > > The test case is invalid (I added some more detail as a comment on this > > bug). > > Sorry, I was imprecise. To be clear, the issue I referred to above as the > "bug [that was] fixed on trunk" is the issue Andrew Haley pointed out: when > GCC transitioned from GIMPLE to RTL IR, empty RTL was emitted for the fence > statement, losing its compile-time effect as a compiler memory barrier > entirely. What I tried to convey was that I think this can be a (part of a) valid implementation on certain hardware, and when considering C11/C++11 or more recent, which require programs to not have any data races (as defined by these standards).
Re: How to force gcc to vectorize the loop with particular vectorization width
On October 21, 2017 9:50:13 PM GMT+02:00, Denis Bakhvalov
wrote:
>Hello Richard,
>Thank you. I achieved vectorization with vf = 16, using
>#pragma GCC optimize ("no-unroll-loops")
>__attribute__ ((__target__ ("sse4.2")))
>and options -march=core-avx2 -mprefer-avx-128
>
>But now I have a question: Is it possible in gcc to have vectorization
>with vf < 16?
No, not at the moment.
Richard.
>On 20/10/2017, Richard Biener wrote:
>> On Fri, Oct 20, 2017 at 12:13 PM, Denis Bakhvalov
>
>> wrote:
>>> Thank you for the reply!
>>>
>>> Regarding last part of your message, this is also what clang will do
>>> when you are passing vf of 4 (with the pragma from my first message)
>>> for the loop operating on chars plus using SSE2. It will do
>meaningful
>>> work only for 4 chars per iteration (a[0], zero, zero, zero, a[1],
>>> zero, zero, zero, etc.).
>>>
>>> Please see example here:
>>> https://godbolt.org/g/3LAqZw
>>>
>>> Let's say that I know all possible trip counts for my inner loop.
>They
>>> all do not exceed 32. In the example above vf for this loop is 32.
>>> There is a runtime check, such that if trip count do not exceed 32
>it
>>> will fall back to scalar version.
>>>
>>> As long as trip count is always lower that 32 - it always chooses
>>> scalar version at runtime.
>>> But theoretically, using SSE2 for trip count = 8 it can use half of
>>> xmm register (8 chars) to do meaningfull work.
>>>
>>> Is gcc vectorizer capable of doing this?
>>> If yes, can I somehow achieve this in gcc by tweaking the code or
>>> adding some pragma?
>>
>> The closest is to use -mprefer-avx128 so you get SSE rather than AVX
>> vector sizes. Eventually this option is among the valid target
>attributes
>> for #pragma GCC target
>>
>>> On 19/10/2017, Jakub Jelinek wrote:
On Thu, Oct 19, 2017 at 10:38:28AM +0200, Richard Biener wrote:
> On Thu, Oct 19, 2017 at 9:22 AM, Denis Bakhvalov
>
> wrote:
> > Hello!
> >
> > I have a hot inner loop which was vectorized by gcc, but I also
>want
> > compiler to unroll this loop by some factor.
> > It can be controled in clang with this pragma:
> > #pragma clang loop vectorize(enable) vectorize_width(8)
> > Please see example here:
> > https://godbolt.org/g/UJoUJn
> >
> > So I want to tell gcc something like this:
> > "I want you to vectorize the loop. After that I want you to
>unroll
> > this vectorized loop by some defined factor."
> >
> > I was playing with #pragma omp simd with the safelen clause, and
> > #pragma GCC optimize("unroll-loops") with no success. Compiler
>option
> > -fmax-unroll-times is not suitable for me, because it will
>affect
> > other parts of the code.
> >
> > Is it possible to achieve this somehow?
>
> No.
#pragma omp simd has simdlen clause which is a hint on the
>preferable
vectorization factor, but the vectorizer doesn't use it so far;
probably it wouldn't be that hard to at least use that as the
>starting
factor if the target has multiple ones if it is one of those.
The vectorizer has some support for using wider vectorization
>factors
if there are mixed width types within the same loop, so perhaps
supporting 2x/4x/8x etc. sizes of the normally chosen width might
>not be
that hard.
What we don't have right now is support for using smaller
vectorization factors, which might be sometimes beneficial for -O2
vectorization of mixed width type loops. We always use the vf
>derived
from the smallest width type, say when using SSE2 and there is a
>char
type,
we try to use vf of 16 and if there is also int type, do operations
>on
those
in 4x as many instructions, while there is also an option to use
vf of 4 and for operations on char just do something meaningful
>only in
1/4
of vector elements. The various x86 vector ISAs have instructions
>to
widen or narrow for conversions.
In any case, no is the right answer right now, we don't have that
implemented.
Jakub
>>>
>>>
>>> --
>>> Best regards,
>>> Denis.
>>
