Float128 calling convention and alignment

[simonbyrne]

At the moment we fake the calling convention on Mac and Linux by reinterpreting as NTuple{2,VecElement{Float64}}. Unfortunately this doesn't seem to work on Windows.

Upstream:

• [RFC/WIP] Specify alignment of primitve types JuliaLang/julia#22649
• C compatible Int128 layout on x64 JuliaLang/julia#20415
• Allocate objects in pools with correct alignment JuliaLang/julia#21959

Also mentioned in

• reinterpret segfault JuliaLang/julia#21216

[simonbyrne]

cc: @vchuravy @vjtnash @yuyichao

[yuyichao]

Just a thought, have you try to add a x86_vectorcallcc on the ccall? Tht seems to produce the correct calling convention assuming you only have fp128.

[simonbyrne]

how do I do that? is that an undocumented calling convention?

[simonbyrne]

Ah, we don't seem to support it. Should I open an issue?

[yuyichao]

I don't think our ccall supports it and I'm not sure where it's documented in LLVM but you can use this trick

[yuyichao]

Note that I think you won't be able to use it in ccall directly even if we support it since supporting it in ccall would probably need to come with automatic name mangling support...

[simonbyrne]

I see, thanks.

[RalphAS]

I tried some experiments following @yuyichao 's suggestion, for example

function baz(x::Float128, y::Float128)
    r = Base.llvmcall("""%f = inttoptr i64 %2 to <2 x double> (<2 x double>, <2 x double>)*
                    %vv = call x86_vectorcallcc <2 x double> %f(<2 x double> %0, <2 x double> %1)
                    ret <2 x double> %vv""",
                 Cfloat128, Tuple{Cfloat128,Cfloat128,Ptr{Cvoid}},
                 x.data, y.data, cglobal((:__addtf3,quadoplib)))
    return Float128(r)
end

Replacing methods in Quadmath with patterns like this works fine for Linux and OSX. It runs on Windows (no segfaults) but produces incorrect answers, apparently because the Windows version of libgcc_s doesn't return Float128 results in xmm0 as expected from the ABI. (AFAICT the value of xmm0 is preserved across the call.)

[yuyichao]

Come to think about it, depending on if LLVM and GCC agrees on the calling convention, if you are already using llvmcall, you can probably directly use fp128 in it ;-p....

Other than that, I have no idea what calling convention gcc uses..... It is worth noting that GCC does NOT use the same vector calling convention as clang or msvc. (I didn't realize you are calling gcc compiled library....) I just finished dealing with a similar issue with vector calls on windows so I'm pretty positive on that....... You might have to check the assembly code to figure out what calling convention gcc is using..... =(

[simonbyrne]

(I didn't realize you are calling gcc compiled library....)

we're calling into libquadmath, which is bundled as part of the gcc runtime (which we ship with Julia apparently)

[simonbyrne]

Replacing methods in Quadmath with patterns like this works fine for Linux and OSX. It runs on Windows (no segfaults) but produces incorrect answers, apparently because the Windows version of libgcc_s doesn't return Float128 results in xmm0 as expected from the ABI. (AFAICT the value of xmm0 is preserved across the call.)

Not sure if it makes a difference, but did you change how Cfloat128 is defined on Windows to be the same as linux and mac?