OpenE2K/gcc
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

dominance.c: Add comment about why we use DFS numbering of dominance tree.

Browse Source 
2006-01-03  Daniel Berlin  <dberlin@dberlin.org>

	* dominance.c: Add comment about why we use DFS numbering
	of dominance tree.

From-SVN: r109308
This commit is contained in:
Daniel Berlin 2006-01-04 02:03:19 +00:00 committed by Daniel Berlin
parent 862e1e62fc
commit b629276a48
2 changed files with 79 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
gcc/ChangeLog
View File

@ -1,3 +1,8 @@
2006-01-03 Daniel Berlin <dberlin@dberlin.org>
* dominance.c: Add comment about why we use DFS numbering
of dominance tree.
2006-01-03 Jakub Jelinek <jakub@redhat.com>
Richard Henderson <rth@redhat.com>

74
gcc/dominance.c
View File

@ -814,6 +814,80 @@ nearest_common_dominator_for_set (enum cdi_direction dir, bitmap blocks)
return dom;
}
/* Given a dominator tree, we can determine whether one thing
dominates another in constant time by using two DFS numbers:
1. The number for when we visit a node on the way down the tree
2. The number for when we visit a node on the way back up the tree
You can view these as bounds for the range of dfs numbers the
nodes in the subtree of the dominator tree rooted at that node
will contain.
The dominator tree is always a simple acyclic tree, so there are
only three possible relations two nodes in the dominator tree have
to each other:
1. Node A is above Node B (and thus, Node A dominates node B)
A
|
C
/ \
B D
In the above case, DFS_Number_In of A will be <= DFS_Number_In of
B, and DFS_Number_Out of A will be >= DFS_Number_Out of B. This is
because we must hit A in the dominator tree *before* B on the walk
down, and we will hit A *after* B on the walk back up
2. Node A is below node B (and thus, node B dominates node B)
B
|
A
/ \
C D
In the above case, DFS_Number_In of A will be >= DFS_Number_In of
B, and DFS_Number_Out of A will be <= DFS_Number_Out of B.
This is because we must hit A in the dominator tree *after* B on
the walk down, and we will hit A *before* B on the walk back up
3. Node A and B are siblings (and thus, neither dominates the other)
C
|
D
/ \
A B
In the above case, DFS_Number_In of A will *always* be <=
DFS_Number_In of B, and DFS_Number_Out of A will *always* be <=
DFS_Number_Out of B. This is because we will always finish the dfs
walk of one of the subtrees before the other, and thus, the dfs
numbers for one subtree can't intersect with the range of dfs
numbers for the other subtree. If you swap A and B's position in
the dominator tree, the comparison changes direction, but the point
is that both comparisons will always go the same way if there is no
dominance relationship.
Thus, it is sufficient to write
A_Dominates_B (node A, node B)
{
return DFS_Number_In(A) <= DFS_Number_In(B)
&& DFS_Number_Out (A) >= DFS_Number_Out(B);
}
A_Dominated_by_B (node A, node B)
{
return DFS_Number_In(A) >= DFS_Number_In(A)
&& DFS_Number_Out (A) <= DFS_Number_Out(B);
} */
/* Return TRUE in case BB1 is dominated by BB2. */
bool