Re: My review of Decimal

From: Peter Dimov (pdimov_at_[hidden])
Date: 2025-01-21 16:41:46

Ivan Matek wrote:
> On Tue, Jan 21, 2025 at 4:20 PM Peter Dimov <pdimov_at_[hidden]
> <mailto: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.

Yes, that's why I said "non-Windows x86-64". :-)

> 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

That's true, and it's actually not passed in registers for this reason.

https://godbolt.org/z/qs5z6Th9e

Interesting. Looks like this is caused by the use of uint_fast16_t, which is
actually uint64_t. Maybe not the best choice.

https://godbolt.org/z/8fczrzbEY is better.

Although as I said in my other e-mail, maybe the exponent and the
significand should just be packed into a single uint64_t.