On Fri, Feb 14, 2014 at 2:17 AM, Matthew Fortune <matthew.fort...@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. Thanks, Andrew Pinski > > Thanks in advance, > Matthew >
