Re: [PATCH] Make musl math depend less on libgcc builtins

[Sergey Dmitrouk <sdmitrouk@accesssoftek.com>]

On Thu, Sep 11, 2014 at 07:11:23AM -0700, Szabolcs Nagy wrote:
> musl checks for nan by if(y!=y) and my guess is that this is
> incorrectly done by a signaling comparision

I see

    vcmpe.f64      d0, d0

which seems to be part of "!isfinite(y)" expression, so your guess is
correct ("vcmpe" instruction raises exceptions, "vcmp" doesn't).

I also checked >, <, <= and >=, which Clang implements as "vcmpe" too.
Linaro GCC 4.9 seems to do the correct thing for ARMhf.  So different
targets behave differently, maybe there is no flag to control this...

> 
> i checked on x86_64 and both clang and gcc get comparisions
> wrong in the other direction: they use quite comparisions
> when signaling is needed, eg
> 
> http://goo.gl/GsdpZA
> 
> (on a correct implementation ==, != are quiet, but <,>,<=,>=
> raise invalid if any of the operands are nan, on x86_64 the
> quiet instruction is ucomis* and the signaling one is comis*
> and both gcc-4.9 and clang-3.4 seem to use ucomis* for all
> relational operations, may be there is some compiler flag to
> make them behave..)