------- Comment #9 from rguenther at suse dot de 2010-01-05 16:03 ------- Subject: Re: October 23rd change to tree-ssa-pre.c breaks calculix on powerpc with -ffast-math
On Tue, 5 Jan 2010, segher at kernel dot crashing dot org wrote: > ------- Comment #7 from segher at kernel dot crashing dot org 2010-01-05 > 15:57 ------- > With -fno-signed-zeroes, a-b*c is transformed to -(b*c-a), which is a machine > instruction. If the result would have been +0 before, it now is -0. > The code then takes the sqrt() of that; sqrt(-0) is -0. This then is > passed to atan2(), which has a discontinuity at 0, and we get wildly > diverging results. > > The compiler does nothing wrong here; the transformation is perfectly > valid. > > A solution might be to transform e.g. atan2(x,y) into atan2(+0.+x,+0.+y) > when -fno-signed-zeroes is in effect (and of course somehow make sure > those additions aren't optimised away). Similar for other math library > functions with discontinuities at +/- 0. Right. Just it might be simpler with -fno-signed-zeros to transform a-b*c to 0 + -(b*c-a). Of course if the result was -0 before then it will be +0 after either variant (and the atan2 discontinuity would still happen even with your fix). Thus whatever "fix" the underlying problem is surely that calculix is not really -fno-signed-zeros safe. Can't we get lucky again as before by trying to recover the PRE code change? Richard. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42286
