Serious performance regression -- some tree optimizer questions

[Daniel Berlin]

> there appear to be some optimizations that would appear quite
> straightforward to me that no pass is currently performing.
>
> For example, the gimple code contains sequences of the form:
>
> s1 = a + b
> s2 = s1 + c
> t1 = a + c
> t2 = t1 + b
>
> but no pass recognizes that s2 == t2 ...


Uh, both of the value numbering passes (DOM and PRE) should recognize 
this.

> Any suggestions how to further investigate this?  Is this just
> the way things are with the current tree optimizers, or is this
> supposed to work and we just need to find the bug?

File bug reports with code we can compile.

Also, can you send me the dump file for 
-fdump-tree-pre-vops-details-blocks-stats?

I'd like to see these code sequences you don't think it is optimizing.