RFA: vectorizer patches 2/2: reduction splitting

[Joern Wolfgang Rennecke <joern dot rennecke at riscy-ip dot com>]

It's nice to use the processors vector arithmetic to good effect, but 
it's all for naught when
there are too many moves from/to general registers cluttering up the 
loop.  With a
double-vector reduction variable, the standard final reduction code got 
so awkward that
the register allocator decided that the reduction variable must live in 
general purpose
registers, not only after the loop, but across the loop patch.
Splitting the reduction to force the first step to be done as a vector 
operation
seemed the obvious solution. The hook was called, but the vectorizer still
generated the vanilla final reduction code.  It turns out that the 
reduction splitting
was calculated, but the result not used, and the calculation started anew.

The attached patch fixes this.

bootstrapped and regression tested on x86_64-pc-linux-gnu .

Attachment: reduction-split-patch-266008.txt
Description: Text document