Re: WHY? commit ac76bc1145dd7f4476e5d982ce8f355f71015713

On Fri, Nov 05, 2021 at 10:32:26PM +0100, Dimitry Andric wrote:
> On 5 Nov 2021, at 21:13, Steve Kargl <sgk@troutmask.apl.washington.edu> wrote:
> > 
> > Why was this committed?
> > 
> > commit ac76bc1145dd7f4476e5d982ce8f355f71015713
> > Author: Dimitry Andric <dim@FreeBSD.org>
> > Date:   Tue Feb 9 22:06:51 2021 +0100
> > 
> >    Fix lib/msun's ctrig_test/test_inf_inputs test case with clang >= 10
> > 
> >    This sprinkles a few strategic volatiles in an attempt to defeat clang's
> >    optimization interfering with the expected floating-point exception
> >    flags.
> > 
> > There is nothing, and I mean, nothing strategic about "sprinkling"
> > volatile onto the declaration of "float x, y, h;"  These variables
> > are referenced in all floating pointing operations in the file,
> > which means that there are needless reloading of x, y, and h
> > from memory.
> 
> There was more context in https://bugs.freebsd.org/244732, but the gist
> was that with clang >= 10, ctanh() and ctanhf() had FE_INVALID set after
> calling them with {inf,inf}. The reasons for this were obscure to me at
> the time, since it regressed with an llvm commit that seemed to have
> very little to do with floating point.
> 
> However, in 3b00222f156d we added -fp-exception-behavior=maytrap to
> clang's compile flags for lib/msun, for https://bugs.freebsd.org/254911,
> to force it to use stricter floating point semantics. This turns out to
> also make the admittedly ugly volatile fixes unnecessary.
> 
> So I have reverted ac76bc1145dd (minus the ctrig_test.c part) in:
> https://cgit.freebsd.org/src/commit/?id=e2157cd0000f6dbb6465d7a885f2dcfd4d3596cb
> 

Thanks for the explanation!  This would indeed be strange.
The evaluation of ctanhf does not invoke ccoshf(z).

The relevant section of code from s_ctanh.c (similar code in s_ctanhf.c)
is 

	if (ix >= 0x7ff00000) {
		if ((ix & 0xfffff) | lx)	/* x is NaN */
			return (CMPLX(nan_mix(x, y),
			    y == 0 ? y : nan_mix(x, y)));
		SET_HIGH_WORD(x, hx - 0x40000000);	/* x = copysign(1, x) */
		return (CMPLX(x, copysign(0, isinf(y) ? y : sin(y) * cos(y))));
	}

The testcase should get to the second return statement.  So,
either isinf(y) or copysign(0, inf) was raising the exception.

-- 
Steve