Re: [autovect] [committed] vectorizer idiom recognition support andsaturating subtraction

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> This patch adds an analysis pass that searches for computation idioms in
> the program. It is a tentative implementation, that invokes the
> pattern-recognition pass from within the vectorizer, only on loops that are
> candidate for vectorization. The pattern is replaced only if it can be
> vectorized. We may want instead to invoke it as a general pass independent
> of the vectorizer that searches for such idioms anywhere in the program (as
> part of a tree-combine pass?).

Or of phiopt.  In fact your patch looks like a different implementation 
of phiopt.  I had a patch that built a COND_EXPR out of a phi and let 
fold recognize/simplify it.  Maybe phiopt can become a special case of 
the tree-combine pass.

For the particular case of unsigned saturation, it may be worthwhile to 
recognize it in fold and (in case the optab does not have an entry for a 
particular mode) have different expansions of it depending on the 
BRANCH_COST.

Paolo