How to prevent optimizer from reordering stuff?

[Dan Olson via digitalmars-d-ldc]

Ok, I have stumbled into an old problem it seems.

C99 invented "#pragma STDC FENV_ACCESS ON" to prevent optimizer from
reordering instructions that affect float environment.  See note [2]
here:

http://en.wikipedia.org/wiki/C99#Example

And clang (LLVM) does not support this pragma:

https://llvm.org/bugs/show_bug.cgi?id=10409

Work around in C is to use volatile vars to force ordering.

And one more reference:

http://wiki.musl-libc.org/wiki/Mathematical_Library#Fenv_and_error_handling