On Thu, Nov 17, 2022 at 06:59:45PM +0000, Joseph Myers wrote:
> On Thu, 17 Nov 2022, Aldy Hernandez via Gcc-patches wrote:
>
> > So... is the optimization wrong? Are we not allowed to substitute
> > that NAN if we know it's gonna happen? Should we also allow F F F F F
> > in the test? Or something else?
>
> This seems like the usual ambiguity about what transformations
> -ftrapping-math (on by default) is meant to prevent.
>
> Generally it's understood to prevent transformations that add *or remove*
> exceptions, so folding a case that raises "invalid" to a NaN (with
> "invalid" no longer raised) is invalid with -ftrapping-math. But that
> doesn't tend to be applied if the operation raising the exceptions has a
> result that is otherwise unused - in such a case the operation may still
> be removed completely (the exception isn't properly treated as a side
> effect to avoid dead code elimination; cf. Marc Glisse's -ffenv-access
> patches from August 2020). And it may often also not be applied to
> "inexact".
The problem is that the above model I'm afraid is largely incompatible with
the optimizations ranger provides.
A strict model where no operations that could raise exceptions are discarded
is easy, we let frange optimize as much as it wants and just tell DCE not to
eliminate operations that can raise exceptions.
But in the model where some exceptions can be discarded if results are unused
but not others where they are used, there is no way to distinguish between
the result of the operation really isn't needed and ranger figured out a
result (or usable range of something) and therefore the result of the
operation isn't needed.
Making frange more limited with -ftrapping-math, making it punt for
operations that could raise an exception would be quite drastic
pessimization. Perhaps for -ftrapping-math we could say no frange value is
singleton and so at least for most of operations we actually wouldn't
optimize out the whole computation when we know the result? Still, we could
also just have
r = long_computation (x, y, z);
if (r > 42.0)
and if frange figures out that r must be [256.0, 1024.0] and never NAN, we'd
still happily optimize away the comparison.
Jakub
