On 8/21/21 10:19 PM, Stefan Kanthak wrote:
Jakub Jelinek <ja...@redhat.com> wrote:
On Sat, Aug 21, 2021 at 09:40:16PM +0200, Stefan Kanthak wrote:
I believe your example doesn't take into account that the values can be NaN
which compares false in all situations.
That's a misbelief!
Please notice the first if-clause, which rules out NaNs for both arguments.
Also notice that GCC did NOT generate JP after the 4 COMISD instructions
in question, i.e. it knew that NaNs had been ruled out.
GCC doesn't do value range propagation of floating point values, not even
the special ones like NaNs, infinities, +/- zeros etc., and without that the
earlier ifs aren't taken into account for the earlier code.
Ouch, to bad! That was but the very least expectation I had.
And indeed, removing the 3 other if-clauses doesn't change how the
final if-clause gets translated.
Sorry Matt, we were both wrong with our assumptions.
Also, as you have been told already, sending these mails to gcc mailing list
is a very bad idea, unless somebody jumps on it immediately and implements
it, they will be effectively forgotten. You should file missed
optimizations into gcc bugzilla where they can be seen any time.
I don't have a bugzilla account, and I don't use GCC for anything serious.
Stefan
It's *that* demanding for you to create a Bugzilla account ? From my
experience, creating a Bugzilla account takes around 30 seconds (though
I suppose you'd have to wait for a bit if automatic account creation is
temporarily disabled), it quite honestly feels as though you've spent
far more time writing out these e-mails and talking here on the mailing
list than that.
On Bugzilla I've had some optimization bug reports of mine take months
to get a response and more to get fixed, but at least something happened
after a while: if I just went on the mailing list it's incredibly likely
they'd have been completely and utterly lost to time (it's not that GCC
devs don't like the mailing list, but there's a reason a *bug tracking
system* is used...)
