[Bug middle-end/93806] Wrong optimization: instability of floating-point results with -funsafe-math-optimizations leads to nonsense

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93806

--- Comment #12 from Rich Felker <bugdal at aerifal dot cx> ---
To me the meaning of internal consistency is very clear: that the semantics of
the C language specification are honored and that the only valid
transformations are those that follow the "as-if rule". Since C without Annex F
allows arbitrarily awful floating point results, your example in comment 11 is
fine. Each instance of 1/a can evaluate to a different value. They could even
evaluate to random values. However, if you had written:

  int b = 1/a == 1/0.;
  int c = b;
  return b == c;

then the function must necessarily return 1, because the single instance of
1/a==1/0. in the abstract machine has a single value, either 0 or 1, and in the
abstract machine that value is stored to b, then copied to c, and b and c
necessarily have the same value. While I don't think it's likely that GCC would
mess up this specific example, it seems that it currently _can_ make
transformations such that a more elaborate version of the same idea would be
broken.