Re: optimization/7719: gcc with -O2 generates wrong code

[Petr dot Savicky at ff dot cuni dot cz]

On Wed, Feb 19, 2003 at 08:35:13AM -0000, ebotcazou at gcc dot gnu dot org wrote:
> Synopsis: gcc with -O2 generates wrong code
> 
> State-Changed-From-To: open->feedback
> State-Changed-By: ebotcazou
> State-Changed-When: Wed Feb 19 08:35:13 2003
> State-Changed-Why:
>     Probably not a bug. On x86, at -O2, floating point values
>     may be kept in FP registers when doing comparison with
>     others stored in memory. Now x86 FP registers have
>     extra-precision over a 'double', which may invalid
>     an equality comparison. Compile your code with '-ffloat-store'
>     if it relies on exact IEEE floating-point semantics.

Yes, using -O2 together with -ffloat-store eliminates the problem.
Moreover, the fact that a double value may change just by storing
and reloading from memory explains the behaviour of the program.

OK, this is not a bug of the compiler itself, but it is a bug in
the default settings of gcc on x86. Everybody knows that the results
of computer arithmetic are not safe, may have different results on
different processors and definitely do not satisfy things like
(a+b)+c = a+(b+c). Here, however, the problem is not in the operations
with the numbers, but with keeping their values untouched. The value of a
variable changes during a sequence of operations which do not involve it.
The change may influence not only equality tests, but also inequality
tests, which cannot be avoided. This is hard to accept as a correct
behaviour even on a processor with a conceptually buggy design.

The current success of GNU Project heavily relies on using its
software on x86. Perhaps, this may be a reason to be more friendly
to this processor and put -ffloat-store into the definition of -O2,
which is frequently used as a default.

For me as a Linux user, it was not nice to have the bug only under Linux
and not under Windows, just because the Windows version was compiled
by a different compiler.

Best wishes

  Petr Savicky