On 11/18/22 09:39, Richard Biener wrote:
On Thu, Nov 17, 2022 at 8:38 PM Jakub Jelinek via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
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.
Yes, I don't think singling out the singleton case will help.
There is also simplify_using_ranges::fold_cond() which is used by VRP
and DOM to fold conditionals. So twiddling frange::singleton_p will
have no effect here since FP conditionals results are integers (f > 3.0
is true or false).
And now that we're on this subject...
We are very careful in frange (range-op-floats.o) to avoid returning
true/false in relational which may have a NAN. This keeps us from
folding conditionals that may result in a trapping NAN.
For example, if we see [if (x_5 unord_lt 10.0)...] and we know x_5 is
[-INF, -8.0] +-NAN, this conditional is always true, but we return
VARYING to avoid folding a NAN producing conditional. I wonder whether
we're being too conservative?
An alternative woudld be:
z_8 = x_5 unord_lt 10.0
goto true_side;
But if DCE is going to clean that up anyhow without regards to
exceptions, then maybe we can fold these conditionals altogether? If
not in this release, then in the next one.
ISTM that range-ops should always tell the truth of what it knows,
instead of being conservative wrt exceptions. It should be up to the
clients (VRP or simplify_using_ranges::fold_cond) to use the information
correctly.
Practically strictly
preserving IEEE exceptions is only important for a very small audience, and
for that even INEXACT will matter (but we still have -ftrapping-math
by default).
For that audience likely all constant / range propagation is futile and thus the
easiest thing might be to simply cut that off completely?
I'd say what ranger does is reasonable with -ftrapping-math given the current
practice of handling this option. There's no point in trying to preserve the
(by accident) "better" handling without ranger. Instead as Joseph says somebody
would need to sit down, split -ftrapping-math, adjust the default and thorougly
document things (also with -fnon-call-exceptions which magically makes
IEEE flag raising operations possibly throw exceptions). As there's currently
no code motion barriers for FP code with respect to exception flag inspection
any dead code we preserve is likely going to be unhelpful.
So for now simply amend the documentation as to what -ftrapping-math
currently means with respect to range/constant propagation?
So something like "Even in the presence of -ftrapping-math, VRP may fold
operations that may cause exceptions For example, an addition that is
guaranteed to produce a NAN, may be replaced with a NAN, thus eliding
the addition. This may cause any exception that may have been generated
by the addition to not appear in the final program."
??
Aldy
