https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109184
--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note even when promoting _2 to an array there wouldn't be any data dependence
between the actual refs. There's a dependence between stmts, but that doesn't
exactly capture the issue either. Consider
int a[1024], b[1024];
void foo()
{
for (int i = 0; i < 32; ++i)
for (int j = 0; j < 32; ++j)
for (int k = 0; k < 32; ++k)
a[j] = b[k];
}
where interchange of the two outer loops would be valid since b[k] doesn't
change. Likewise
for (int i = 0; i < 32; ++i)
for (int j = 0; j < 32; ++j)
for (int k = 0; k < 32; ++k)
{
b[k] = b[k] + 1;
a[j] = 0;
}
would be valid to interchange. For the testcase at hand
tree_loop_interchange::valid_data_dependences only gets the l_1930[] update
as data-dependence that's not statically resolved.
If we'd have that ominous _2[i] and
_2[i] = l_1930[k_26] + -1;
l_1930[k_26] = _2[i];
g_1731[_6] = _2[i];
we'd only have extra zero-distance DDRs here and we ignore those.
