[patch] tree-ssa-dom.c: Speed up tree_ssa_dominator_optimize.

[Kazu Hirata]

Hi Mark,

> > Attached is a patch to speed up tree_ssa_dominator_optimize by
> > removing a call to mark_dfs_back_edges.
> > 
> > Jeff removed all uses of EDGE_DFS_BACK in tree-ssa-dom.c, so we don't
> > need to call mark_dfs_back_edges.
> > 
> > Tested on i686-pc-linux-gnu.  OK to apply?
> 
> This series of patches looks plausible, though I'll leave them to others 
> to approve.  However, you could make this kind of patch more attractive 
> if you would indicate what kind of improvement you've seen when 
> compiling a benchmark of your choice.  If the improvement is truly tiny, 
> then it may not be worth (a) taking the risk of changing the code, even 
> in an obvious way, and (b) taking up the time of approvers to review the 
> patch.  On the other hand, if the improvements are significant, you'll 
> attract more interest from reviewers.

Good point.

With this particular patch, we consistently get about a 0.04% - 0.06%
improvement in compile time on various cc1-i files.  I used the rdtsc
instruction on IA-32 to measure the time spent in mark_dfs_back_edges,
but the wall clock figure also agrees with the number from rdtsc.

I'll try to post numbers for other speed-up patches of mine as well.

Kazu Hirata