[Bug middle-end/19988] [4.3/4.4/4.5 Regression] pessimizes fp multiply-add/subtract combo

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #21 from rguenth at gcc dot gnu dot org  2010-01-16 20:30 -------
(In reply to comment #20)
> It obviously could do the same.  It all depends on the answer to this
> question:
>    Do we want to deal with this via canonicalization of expressions, and if
>    yes, then also on the outcome of our discussion of where that should happen
>    (fold or special code in VN dealing with just value numbers).
> If no, then something like your patch can be done.
> 
> In this case I'm leaning towards Paolo (comment #14), we possibly might want
> to canonicalize towards fewer constants (hence towards positive constants),
> when otherwise the number of operations remains the same (hence treating
> PLUS_EXPR and MINUS_EXPR as equivalent).

Yes, that sounds reasonable.  Note that most existing foldings only look
for PLUS_EXPR once the 2nd operand is a constant, so auditing of them is
required.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19988