Date: Wed, 18 Sep 2019 16:04:01 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Cc: info@...ile-stream.com Subject: Re: [PATCH] mips: add single-instruction math functions On Wed, Sep 18, 2019 at 08:18:04PM +0300, info@...ile-stream.com wrote: > R> Why is this dependent on __mips_abs2008? > > There is also __mips_nan2008 (always set for hard-float R6 and > -mnan=2008). > > Binaries built with this option (implicit or not) are unusable on > -mnan=legacy system, this is enforced by kernel (unless booted with > some debugging option). > > The fabs code could be changed to also depend on __mips_nan2008 > (since these ISA features are paired) to prevent -mabs=2008 musl on > -mabs=legacy system (rather unrealistic). > > Why is it wrong to depend on fine-grained ISA features after all? It's not. The presence of a new instruction for non-arithmetic abs would be a fine-grained ISA feature. An incompatible change in an existing instruction is a *different ISA*, which needs a different ldsoname per musl policy of always allowing different ISAs to coexist in the same filesystem and have their own library ecosystems. I'm guessing we've hit a situation where people have been building binaries for an incompatible MIPS-family ISA reusing the same ldsoname, which is a huge mess we probably need to figure out how to deal with... > Why is it wrong to explicitly depend e.g. on __mips_dsp in the > strchr code fearing improper usage on a system without DSP ASE? Because it's an ISA level, not an incompatible ISA. With libc built for the baseline ISA level (or any ISA level not assuming dsp; actually it probably doesn't matter even if it does since I can't imagine the compiler generates dsp insns for anything in libc) you can run *both baseline non-dsp mips binaries, and ones using dsp features*. Note that this is the same situation as i386; as long as libc is built for a baseline (like i486; i386 is actually a misnomer) you can run both baseline binaries and ones built for i686 or whatever more recent ISA level you like using the same libc (and library ecosystem and filesystem). > powerpc64, s390x have similar ifdefs in their math code and IIUC > nothing prevents running (until SIGILL) statically-linked > _ARCH_PWR5X binary on an _ARCH_PWR5 system. You're looking at it the other way around. > Or some powerpc64 code depends on __VSX__. Is it wrong to depend on > __mips_msa? > > What is different with mips here? Reversal of direction of the incompatibility. Rich
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