[Bug target/70976] Useless vectorization leads to degradation of performance

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70976

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|                            |x86_64-*-*, i?86-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-05-06
          Component|rtl-optimization            |target
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  The cost estimate for the unsigned vector multiplication is off.

Confirmed also for the second observation - even the scalar loop could have
been improved by transforming it into

    u = 1;
    for(unsigned long i = 0; i < 1UL<<30 / 2; i++)
        u += 23442*u;
    u *= u;
    u *= 13;

but I hardly think that blaming this on the compiler is ok - the source
is simply using a very inefficient algorithm.  Still we could pattern
match some of the most common summation sequences and transform them
into sth better.

Keeping this bug for the target vectorizer cost model issue which should have
prevented vectorization from being happened.