c11a3aedec
Extend range query done by loop-ch to also support basic blocks
that are not headers of the loop. This is easy to do, since we already
do path specific query so we only need to extend the path by headers we decided
to dulicate earlier.
This makes it possible to track situations where exit that is always false in
the first iteration (wihch is a common case) is not in the first iteration of
the loop. Doing so lets us to update profile better and do better heuristics.
In particular I changed logic as follows
1) should_duplicate_loop_header_p counts size of duplicated region. When we
know that a given conditional will be constant true or constant false either
in the duplicated region, by range query, or in the loop body after
duplication (since it is loop invariant), we do not account it to code size
costs
2) don't need account loop invariant compuations that will be duplicated
as they will become fully invariant
(maybe we want to have some cap for register pressure eventually?)
3) optimize_size logic is now different. Originally we started duplicating
iff the first conditional was known to be true by ranger query, but then
we used same limits as for -O2.
I now simply lower limits to 0. This means that every conditional
in duplicated sequence must be either loop invariant or constant when
duplicated and we only duplicate statements computing loop invariants
and those we account to 0 size anyway,
This makes code IMO more logical, but makes little difference in practice.
The problem is that in loop:
void test2();
void test(int n)
{
for (int i = 0; n && i < 10; i++)
test2();
}
We produce:
<bb 4> [local count: 1073741824 freq: 9.090909]:
# i_4 = PHI <0(2), i_9(3)>
_1 = n_7(D) != 0;
_2 = i_4 <= 9;
_3 = _1 & _2;
if (_3 != 0)
goto <bb 3>; [89.00%]
else
goto <bb 5>; [11.00%]
and do not understand that the final conditional is a combination of a conditional
that is always true in first iteration and a conditional that is loop invariant.
This is also the case of
void test2();
void test(int n)
{
for (int i = 0; n; i++)
{
if (i > 10)
break;
test2();
}
}
Which we turn to the earlier case in ifcombine.
With disabled ifcombine things however works as exepcted. This is something
I plan to handle incrementally. However extending loop-ch and peeling passes
to understand such combined conditionals is still not good enough: at the time ifcombine
merged the two conditionals we lost profile information on how often n is 0,
so we can't recover correct profile or know what is expected number of iterations
after the transofrm.
gcc/ChangeLog:
* tree-ssa-loop-ch.cc (edge_range_query): Take loop argument; be ready
for queries not in headers.
(static_loop_exit): Add basic blck parameter; update use of
edge_range_query
(should_duplicate_loop_header_p): Add ranger and static_exits
parameter. Do not account statements that will be optimized
out after duplicaiton in overall size. Add ranger query to
find static exits.
(update_profile_after_ch): Take static_exits has set instead of
single eliminated_edge.
(ch_base::copy_headers): Do all analysis in the first pass;
remember invariant_exits and static_exits.
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