Date: Thu, 16 Jan 2020 11:14:27 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Considering x86-64 fenv.s to C On Thu, Jan 16, 2020 at 03:30:03PM +1100, Damian McGuckin wrote: > > I was just looking at fenv.s in x86-64 with a view to doing this as > C-code with some __asm__. I have limited experience with playing > with this stuff in an SSE-only environment, i.e. no 80-bit floats. > Before I attempt anything, I need to expand my knowledge. > > My observations on > > musl-1.1.24/src/fenv/x86_64/fenv.s > > * feclearexcept > > considers both the X87 status word and the SSE MXCSR > > * feraiseexcept > > ignores the X87 status word and considers only the SSE MXCSR > > * fetesteexcept > > considers both the X87 status word and the SSE MXCSR > > * fesetround > > considers both the X87 control word and the SSE MXCSR > > * fegetround > > ignores the X87 control word and considers only the SSE MXCSR > > Why is the X87 component of the FPU control/status sometimes ignored > and sometimes not? > > Does the X87 control or status bits automatically flow through to the > equivalent stuff in the SSE or vica-versa? > > I assume that 'fwait' is irrelevant and unnecessary, even if one is > using the x87 FPU? As an aside, rather than implementing x86-specific C versions of these, I'd like to end up with a general framework where the arch just exposes a header (fenv_arch.h?) defining some primitives, and the actual fenv function logic is all shared C. I'm not sure what the right generalization is though. It looks like all archs have a get/set exception-flags (status word) operation, but the rest varies a bit. Would you be interested in assessing what kind of abstraction makes sense here? Rich
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