------- Comment #10 from segher at kernel dot crashing dot org 2010-01-05 16:42 ------- (In reply to comment #9) > Right. Just it might be simpler with -fno-signed-zeros to > transform a-b*c to 0 + -(b*c-a).
a-b*c is two machine instructions; -(b*c-a) is one. Adding zero again makes it two, no good :-( > 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). Sure, it's not a fix, more a band-aid: it will produce more intuitive results when -fno-signed-zeroes is in effect, since atan2() does care about the sign of zero, and that compiler flag says we do not want that. > Thus whatever "fix" the underlying problem is surely that calculix > is not really -fno-signed-zeros safe. Yes, certainly. It seems to me this will happen with more sloppy code though, not just calculix. > Can't we get lucky again > as before by trying to recover the PRE code change? Well, the code change actually improved the generated code here, do we want to undo that? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42286
