Re: Will GCC eventually support SSE2 or SSE4.1?

[LIU Hao <lh_mouse@126.com>]

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在 2023-05-26 23:40, Stefan Kanthak 写道:
> Feel free to propose this alternative here (better elsewhere, where you'll
> earn less laughter).
> But don't forget that this 23-bit mantissa will be all zeroes for quite some
> 64-bit (and even 32-bit) integers which are no power of 2, for example
> 0x8000003fffffffff, and that both FILD and CVT2SI2SS only work on SIGNED
> integers.

The precision loss can be detected by examining the PF bit (6th bit i.e. `0x20`) of the x87 status 
register. It doesn't matter whether the number is interpreted as signed or unsigned: 
`-0x80000000'00000000` still only has one bit in its mantissa. Another option is to store the number 
in the 80-bit extended precision format, with a 64-bit mantissa which includes the otherwise hidden 
bit (so if the number is a power of two, the mantissa will be `0x80000000'00000000`).

But anyway, traditional x86 has very few GPRs and GCC doesn't optimize multi-word arithmetic very 
well. Performance may or may not vary depending on cache locality and number of μops; not to mention 
`movq` and `movd` which have relative high latencies. I would like to see some benchmarking results 
first.



-- 
Best regards,
LIU Hao


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