------- 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. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42286
