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RE: [RFC] Rationale for passing vectors by value in SIMD registers


Andrew Pinski <pinskia@gmail.com> writes:
> On Sat, Feb 15, 2014 at 12:16 AM, Matthew Fortune
> <Matthew.Fortune@imgtec.com> wrote:
> >> On Fri, Feb 14, 2014 at 2:17 AM, Matthew Fortune
> >> <Matthew.Fortune@imgtec.com> wrote:
> >> > MIPS is currently evaluating the benefit of using SIMD registers to pass
> >> vector data by value. It is currently unclear how important it is for vector data
> >> to be passed in SIMD registers. I.e. the need for passing vector data by value
> >> in real world code is not immediately obvious. The performance advantage is
> >> therefore also unclear.
> >> >
> >> > Can anyone offer insight in the rationale behind decision decisions made
> >> for other architectures ABIs? For example, the x86 and x86_64 calling
> >> convention for vector data types presumes that they will passed in SSE/AVX
> >> registers and raises warnings if passed when sse/avx support is not enabled.
> >> This is what MIPS is currently considering however there are two concerns:
> >> >
> >> > 1) What about the ability to create architecture/implementation
> >> independent APIs that may include vector types in the prototypes. Such APIs
> >> may be built for varying levels of hardware support to make the most of a
> >> specific architecture implementation but be called from otherwise
> >> implementation agnostic code. To support such a scenario we would need to
> >> use a common calling convention usable on all architecture variants.
> >> > 2) Although vector types are not specifically covered by existing ABI
> >> definitions for MIPS we have unfortunately got a defacto standard for how
> >> to pass these by value. Vector types are simply considered to be small
> >> structures and passed as such following normal ABI rules. This is still a
> >> concern even though it is generally accepted that there is some room for
> >> change when it comes to vector data types in an existing ABI.
> >> >
> >> > If anyone could offer a brief history the x86 ABI with respect to vector data
> >> types that may also be interesting. One question would be whether the use
> >> of vector registers in the calling convention was only enabled by default once
> >> there was a critical mass of implementations, and therefore the default ABI
> >> was changed to start making assumptions about the availability of features
> >> like SSE and AVX.
> >> >
> >> > Comments from any other architecture that has had to make such changes
> >> over time would also be welcome.
> >>
> >> PPC and arm and AARCH64 are common targets where vectors are
> >> passed/return via value.  The idea is simple, sometimes you have functions
> >> like vector float vsinf(vector float a) where you want to be faster and avoid a
> >> round trip to L1 (or even L2).  These kind of functions are common for vector
> >> programming.  That is extending the scalar versions to the vector versions.
> >
> > I suppose this cost (L1/L2) is mitigated to some extent if the base ABI were to pass a
> vector in multiple GP/FP register rather than via the stack. There would of course still
> be a cost to marshall the data between GP/FP and SIMD registers. For such a support
> routine like vsinf I would expect it also needs a reduced clobber set to ensure that the
> caller's live SIMD registers don't need saving/restoring, such registers would normally be
> caller-saved. If the routine were to clobber all SIMD registers anyway then the
> improvement in argument passing seems negligible.
> >
> > Do you/anyone know of any open source projects, which have started adopting generic
> vector types, and show the use of this kind of construct?
> 
> Yes glibc provides these functions on x86 now.

Wow, old thread you must have a good memory! I saw libmvec go in some time ago, I
guess you are referring to that or is there something else now (I'm out of date
with glibc development)?

I am hoping to steer MIPS towards supporting passing vectors by value via a an ABI
extension that is opt-in rather than default. The main reason is the range of
competing vector extensions whether defined as official ASEs or core specific. I
think we can still get vectors passed by value with the only extra requirement
being that a prototype would need a calling convention attribute.

Thanks,
Matthew

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