Just remember adding a system wide vector calling convention has wide compatible issues we need to worry about, like jump buf (for setjmp/longjmp) will need to keep vector status, it doesn't need to keep before since all vectors are call-clobber by default.
Also that may cause performance issue for vector, that will increase the init cost for vector register - because part of vector reg become callee save register now, so most case in current vector code gen don't need backup/restore at prologue/epilogue, but it will change once we change the default to vector calling convention by default. So I would suggest system wilde should still keep using lp64d even though the vector is available as one of the proposers for the vector calling convention, but I am fine if the intention is having an option to do some exercise or experiment. On Thu, Sep 5, 2024 at 6:56 AM Jeff Law <jeffreya...@gmail.com> wrote: > > > > On 9/4/24 2:26 PM, Palmer Dabbelt wrote: > > Now that we've got the riscv_vector_cc attribute it's pretty much free > > to add a system-wide ABI -- at least in terms of implementation. So > > this just adds a new ABI command-line value that defaults to enabling > > the vector calling convention, essentially the same as scattering the > > attribute on every function. > > > > gcc/ChangeLog: > > > > * config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins): Add LP64DV. > > * config/riscv/riscv-d.cc (riscv_d_handle_target_float_abi): > > Likewise. > > * config/riscv/riscv-opts.h (enum riscv_abi_type): Likewise. > > * config/riscv/riscv.cc (riscv_vector_cc_function_p): Use > > LP64DV. > > (riscv_option_override): Likewise. > > * config/riscv/riscv.opt: Add LP64DV. > > > > gcc/testsuite/ChangeLog: > > > > * gcc.target/riscv/rvv/base/lp64dv.c: New test. > > --- > > So this is very much an RFC, again. As such it's basically not tested, > > I just manually inspected the test case and it looks sane. > > > > This concept of a yes-V-by-default ABI has come up a bunch of times. > > There's some marginal performance benefit here (the added test saves a > > stack spill, for example). I have no idea how exciting this would be in > > real code, but I don't think having autovectorized values with lifetimes > > that cross function calls is super esoteric or anything. The > > implementation is basically free, though, and it seems kind of odd to > > just leave some performance on the floor for the sake of compatibility > > with the pre-official distro ABIs. > Well, that's really the question, isn't it. Will the distros pick it up > or not? If they don't, then it's just an academic exercise. I don't > think we've ever managed to get any kind of distro level buy-in on a > baseline architecture. > > So I don't object to the idea, I just don't know if it's going to end up > being a dead end of effort or not. > > jeff >
