[Bug tree-optimization/61441] ARM aarch64 fails to quiet signaling NaN

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61441

--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Thu, 13 Aug 2015, ssaraswati at gmail dot com wrote:

> With -fno-signaling-nans (which is the default), we need to fix the ccp so that
> the sNaN is converted to qNaN when the value is propagated.

With -fno-signaling-nans we don't really care about the result value.

> With -fsignaling-nans, we need to ensure that we disable the folding since it
> reduces the number of exceptions visible with sNaNs. I notice that even if I

More precisely, exceptions are relevant with -fsignaling-nans 
-ftrapping-math but even with -fno-trapping-math it's still visible if the 
result is wrongly a sNaN instead of a qNaN.  (-ftrapping-math is the 
default.  It's not clear there's any use for -fsignaling-nans 
-fno-trapping-math.  But -fsignaling-nans is the option that says there 
may be signaling NaN arguments, and -ftrapping-math is the option that 
says we care about exceptions.)

The *number* of exceptions isn't relevant, only the set raised at any 
point (e.g. function call) where the raised exceptions may be tested or 
modified; it's valid to change one nonzero number of times raising a given 
exception between two such points to a different nonzero number of times 
raising that exception.

> disable ccp, other optimizations like dead code elimination with cddce also
> needs to be disabled. Let me know if you agree.

I'm sure there are lots of optimizations that wrongly discard exceptions, 
whether for signaling NaNs or other operands.

Roughly, the present -ftrapping-math seems to be implemented as meaning 
that an expression shouldn't be transformed to change the exceptions 
raised, but exceptions aren't treated as side effects so it can still be 
discarded if apparently unused, or moved past code that might test or 
modify exceptions.  Something stricter that actually follows Annex F would 
treat exceptions more like side effects and eliminate a lot of code 
movement / dead code removal.  It might be less suitable for the default 
compilation mode, however.

I expect there are plenty of bugs even with that more limited description 
of what -ftrapping-math tries to do.