birthpoints in rtl.

[Andrew MacLeod]

Kenneth Zadeck wrote:
>
> Birthpoints are not nearly as useful as phi-functions because the
> algorithms that use birthpoints do not generally leave the birthpoints
> in the right places when they are finished.  There is a lot of value
> added by the operand of phi-functions.  But they do solve the n**2
> case for DU and UD chains (and because of the better SSA building
> algorithms than were available when Reif and Lewis first proposed
> their technique, will be much faster).
>   
I wonder if we could use these factored copies with a threshold value 
instead,  such that we only use them when they will replace X uses or 
defs, something that is tunable and start with high values for X. This 
means the copies won't get in the way normally, and will make a big 
difference when large/pathological cases come along.  Experiments could 
then determine a good value for X.

The copies will then only interfere with optimizations when the 
situation is already horrible.  In fact, it wouldn't surprise me if this 
could actually *help* a number of passes, including RA, for larger 
values of X  :-)

Does RTL have an efficient copy prop? That could then naturally remove 
these copies when desired, and then they could be added back in as 
needed the next time DU/UD chains are built. If they weren't removed, 
the next DU/UD build wouldn't trigger the threshold machinery since the 
copy is already there, and we still end up with the same results.

> There is the complication of how to add the noop move in the presence
> of SUBREGs, and given the amount of pain that I suffered in adding the
>   

If we used thresholding, you could try simply punting on these initially 
and see what happens, and only deal with it when it becomes an issue.

Anyway, just a thought on how to make it less intrusive.

Andrew