Date: Sun, 19 Jan 2020 20:07:56 +1100 (AEDT) From: Damian McGuckin <damianm@....com.au> To: musl@...ts.openwall.com 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 modes. Regards - Damian Pacific Engineering Systems International, 277-279 Broadway, Glebe NSW 2037 Ph:+61-2-8571-0847 .. Fx:+61-2-9692-9623 | unsolicited email not wanted here Views & opinions here are mine and not those of any past or present employer
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