From: Ivan Matek (libbooze_at_[hidden])
Date: 2025-01-21 16:04:25
On Tue, Jan 21, 2025 at 4:20 PM Peter Dimov <pdimov_at_[hidden]> wrote:
> Only decimal128_fast doesn't fit in two registers.
>
> > Also not all functions take 1 argument, in PDF I
> > explicitly used copysign as example.
>
> x86-64 uses up to 6 registers for parameter passing (RDI, RSI, RDX, RCX,
> R8, R9), which means that up to three 128 bit trivially copyable types
> can be passed in registers when pass by value is used.
>
> I got a bit confused to be honest with all this I should have not gone and
wrote reply before trying this out on godbolt, my apologies for noise.
But here are 2 things I believe are not correct in your response.
Passing up to three 128 bit trivial types in registers is not property of
X86-64, but of Linux ABI, Windows ABI is different.
please see this slide:
https://youtu.be/PCP3ckEqYK8?feature=shared&t=1183
On my machine decimal64_fast is not 128 bit because
typedef unsigned long int uint_fast16_t;
static_assert(sizeof(decimal64_fast) == 24);
if you do not believe me here is godbolt:
https://godbolt.org/z/xPzYTP6cM
It is kind of ironic that Windows has it as 128bit, but it can not pass in
register types larger than 64bits.