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Date: Sun, 19 Jan 2020 20:07:56 +1100 (AEDT)
From: Damian McGuckin <>
Subject: Re: Considering x86-64 fenv.s to C

On Fri, 17 Jan 2020, Rich Felker wrote:

>> x86_64
>> *	In assembler
>> *	Why does 'feclearexcept' disrespect the flags by clearing ALL x86 bits?
> As you said above, updating x87 status register is expensive because
> the only way to write it is to read-modify-write the whole fenv. But
> since we know on x86_64 we have sse registers, we can just move all
> the flags to the sse register, then use fnclex to clear the x87 one
> inexpensively, and the effective set of raised flags remains the same.

I am worried about the lack of consistency.

What if something that is only using (say) 80-bit floating arithmetic 
comes back looking for a raised exception in the x87 status register &
MUSL has moved effectively moved it to the mxcsr.

Mind you, I think that BSD is guilty of almost the same error in rounding 

Regards - Damian

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