https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69991
Bug ID: 69991
Summary: missed tail merge optimization
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vries at gcc dot gnu.org
Target Milestone: ---
Consider testcase test.c:
...
void
foo (int c, int d, int *z, int *y)
{
int res = 0;
if (c)
{
*y = 0;
if (d)
{
*z = 1;
res = *y;
}
}
else
{
*z = 1;
res = *y;
}
}
...
When compiled with -O2 -fno-tree-dce, tail-merge fails, due to this check in
find_duplicate:
...
/* If the incoming vuses are not the same, and the vuse escaped into an
SSA_OP_DEF, then merging the 2 blocks will change the value of the def,
which potentially means the semantics of one of the blocks will be
changed.
TODO: make this check more precise. */
if (vuse_escaped && vuse1 != vuse2)
return;
...
Removing the check allows the testcase to be tail-merged, and the testcase
belonging the check (pr52734.c) still passes. This is due to the fix for
PR62167. With a less conservative fix for PR62167, we'd still need this check
for pr52734.c. So it's not necessarily a good idea to remove the check now.
A quick fix would be to ignore defs with no uses in
gsi_advance_bw_nondebug_nonlocal, in the same way we ignore stmts with zero
operands. But the usability of such a fix would be limited.
Another option is to consider the TODO mentioned in the comment. The test-case
pr52734.c can actually safely be tail-merged, as long as the right bb is
chosen. If we can model the incoming vuse as a dependency for one bb, we should
be able to choose the right bb (the empty one with no depencencies).
