Under g++ version 4.8.0 (32-bit mingw), the square root of -0.0 (binary representation 0x8000000000000000) is NAN. But I could have sworn that earlier versions (I'm not sure which, but the last time I ran my full test suite) returned simply 0.0, which seems more sensible to me.
Is this right? Has something in the C++ standard changed, or is this a g++ change?
This is a non-standard* behavior of the MinGW environment (that I have also observed, indirectly, through a customer for whom the software I work on was failing unexpectedly).
You may have seen the correct result for
sqrt(-0.)
, i.e.-0.
, with a higher optimization level where the value was computed correctly at compile-time, or with a previous version of the run-time in which the bug was not present.I ended up defining my own “fixed”
sqrt()
as:(*) IEEE 754 defines
sqrt(-0.)
as-0.
. A C or C++ compiler does not have to implement IEEE 754 for floating-point arithmetic, but GCC tries to (and in this particular instance, fails).