[PATCH] Fix multiply-add regressions after expand-from-SSA

[Michael Matz]

Hi Adam,

On Thu, 30 Apr 2009, Adam Nemet wrote:

> > I think we should start to _not_ re-build trees here, but instead of
> 
> I am not rebuilding any tree here, just as operands are looked at I look 
> at the TER expression in available.  Maybe the name of the helper is 
> wrong.  I don't mean subsitute as in walking the tree and substituting 
> subtrees but as in return the original expr or subsitute it with 
> something "better".

The overall plan should be to get rid of rebuilding trees, so your current 
helper would be a step in the wrong direction.  Richard showed the IMO 
right way to solve this, instead of explicitely building trees return the 
gimple statements and decompose them at the point where you need the 
parts, i.e. in expand itself.

That at least followed also my ideas how to implement something similar to 
TER with expand-from-SSA.

On that topic it would also be very intersting to know other cases where 
TER really helped.  Widening mult is one case, but there only if there's a 
single use chain, so it would be strictly more powerful to implement that 
transformation on gimple itself, instead of in expand.

In an ideal world we would get rid of TER altogether, that's why knowing 
which cases are currently really helped by it is useful.


Ciao,
Michael.