------- Comment #3 from steven at gcc dot gnu dot org 2007-10-20 10:54 -------
There is a discrepancy between the code in gcse.c:hoist_expr_reaches_here_p()
and the comment before it. Quoting:
/* Determine if the expression identified by EXPR_INDEX would
reach BB unimpared if it was placed at the end of EXPR_BB.
It's unclear exactly what Muchnick meant by "unimpared". It seems
to me that the expression must either be computed or transparent in
*every* block in the path(s) from EXPR_BB to BB. Any other definition
would allow the expression to be hoisted out of loops, even if
the expression wasn't a loop invariant.
Contrast this to reachability for PRE where an expression is
considered reachable if *any* path reaches instead of *all*
paths. */
static int
hoist_expr_reaches_here_p (basic_block expr_bb, int expr_index, basic_block bb,
char *visited)
{
...
/* Does this predecessor generate this expression? */
else if (TEST_BIT (comp[pred_bb->index], expr_index))
break;
else if (! TEST_BIT (transp[pred_bb->index], expr_index))
break;
...
The comment says that the expression is hoistable if pred_bb computes the
expression identified by EXPR_INDEX. But the code makes the function return
false if a basic block on the path from EXPR_BB to BB computes this expression.
It seems to me that the comment is right, and the code needs fixing.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33828
