[Bug rtl-optimization/323] optimized code gives strange floating point results

[jonathan at csse dot uwa dot edu dot au]

------- Additional Comments From jonathan at csse dot uwa dot edu dot au  2004-10-04 10:57 -------
Some musings based on my encounter with this bug...

Comparison of doubles is not just restricted to numerical analysis.  Bug report
17802 (aka #323) arose out of a binary search of a timer queue.  Is it wrong to
assume that if "int secs = 2" and "int usecs = 100000" then "secs + 1e-6 *
usecs" will return the same result every time it is evaluated as an inline function?

Calling a trivial function twice with an identical parameter may return a value
less than, equal to or greater than itself (depending on register allocation in
the context of the calls).  This is a really nasty piece of nondeterministic
behaviour.  Weren't compilers and "high level" languages invented to get around
exactly this kind of hardware dependency?

Compiling with -ffloat-store did not help in my case.  Compiling without -O
isn't always possible either.  Since the function that causes the problem is the
trivial calculation above, inlined for performance, I can't guarrantee that
every user of the header file it appears in will know to compile without -O (or
be willing to).  The outcome will be users reporting a bug in my code.

This bug is being reported regularly, and probably affecting a lot more people
that don't report it.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323