The most frequent case that shows up when bootstrapping autovect branch
with BOOT_CFLAGS="-O2 -fcheck-data-deps" is the following:
Dist vectors from the first dependence analyzer:
1 0
Omega dist vectors are not the same:
0 0
Data dependence relation is:
(Data Dep:
access_fn_A: {0, +, 1}_3
access_fn_B: {0, +, 1}_3
(subscript
iterations_that_access_an_element_twice_in_A: 0
last_conflict: scev_not_known;
iterations_that_access_an_element_twice_in_B: 0
last_conflict: scev_not_known;
(Subscript distance: 0
)
)
distance_vect: 0 0
direction_vect: = =
)
This is caused by a loop containing a data ref like the following:
loop_2
loop_3
A[{0, +, 1}_3] = ...
endloop_3
endloop_2
For this pattern, tree-data-ref.c says the following:
/* There is a distance of 1 on all the outer loops:
Example: there is a dependence of distance 1 on loop_1 for the array A.
| loop_1
| A[5] = ...
| endloop
*/
But now that Omega says that dist is (0, 0) I'm not sure anymore whether
this is the standard meaning of distance vectors.
Allen&Kennedy book states:
Definition 2.9. Suppose that there is a dependence from statement
S1 on iteration i of a loop nest and statement S2 on iteration j, then
the dependence distance vector d(i,j) is defined as a vector of
length n such that d(i,j) = j - i.
--
Summary: [data deps] Distance on outer loops for self output deps
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sebastian dot pop at cri dot ensmp dot fr
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23411
