John Regehrfun superoptimizer result from @hyperoptimizer, this occurs in ImageMagick
I Can't Believe It's Not Zero!... hold up. This is checking |a - b| < 0.0, which is always false. Is there a typo or a bug in the checker, or did I misread something?
@steve @hyperoptimizer I don't think typo-- I'm cutting and pasting.
but we've seen Alive2 and Z3 bugs wrt FP....
Alexander Monakov@regehr @steve @hyperoptimizer but the godbolt link has
%cmp = fcmp olt double %ext, 1.000000e-15
which is reasonable, so 0.0000 shown in the screenshot is the result of printing with %f?
@amonakov @steve @hyperoptimizer ah.... the screenshot is Alive2's output, which may not be sufficiently careful about floats.
indeed the godbolt link is the correct code
void friend@regehr with C-style formatters, IIRC, either of %.16e or %.17g will print differently two doubles that are different, whereas obviously %f does not.
(assuming a modicum of quality of the libc doing the printing)
@void_friend @regehr these days I would try to use zmij (https://github.com/vitaut/zmij/) or similar for a self-contained portable very fast option that prints exactly as many digits as necessary to differentiate doubles, and not depend on libc.