Mainline space problem

[James E Wilson]

Daniel Berlin wrote:
> Unless global is doing something completely weird (it may be, i haven't 
> looked at since i worked on new-ra), the conflict matrix is 
> unidirectional (IE if x conflicts with y, then y must conflict with x).
> Thus you can use a lower-triangular bitmatrix, and save half the memory.

If we save half the memory, then we still need over a gigabyte, and we 
still run out of memory.  Constant factors don't help much with 
quadratic problems.  They are still quadratic.

If we try using a sparse bitmaps, then we are adding unnecessary 
overhead for the most common case which will make the compiler run even 
slower than it already does.  If we try to use simple arrays for the 
common case, and sparse bitmaps for the big function case, then we are 
adding complexity, which will make maintenance harder.

I did of course consider the possibility of using some kind of sparse 
matrix representation here, but I didn't think it was worth mentioning. 
  I think we should instead be looking at reducing the number of pseudo 
regs generated for this testcase.

By the way, I did try -fnew-ra.  It used over 980MB and then died with a 
segfault.  I didn't bother looking at why, but I figured if -fnew-ra 
couldn't handle it, then fixing global to handle it was probably not 
feasible.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com