On 2/23/22 02:33, Richard Biener wrote:
On Tue, Feb 22, 2022 at 5:42 PM Andrew MacLeod via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
This patch simply leverages the existing computation machinery to
re-evaluate values dependent on a newly found non-null value
Ranger associates a monotonically increasing temporal value with every
def as it is defined. When that value is used, we check if any of the
values used in the definition have been updated, making the current
cached global value stale. This makes the evaluation lazy, if there are
no more uses, we will never re-evaluate.
When an ssa-name is marked non-null it does not change the global value,
and thus will not invalidate any global values. This patch marks any
definitions in the block which are dependent on the non-null value as
stale. This will cause them to be re-evaluated when they are next used.
Imports: b.0_1 d.3_7
Exports: b.0_1 _2 _3 d.3_7 _8
_2 : b.0_1(I)
_3 : b.0_1(I) _2
_8 : b.0_1(I) _2 _3 d.3_7(I)
b.0_1 = b;
_2 = b.0_1 == 0B;
_3 = (int) _2;
c = _3;
_5 = *b.0_1; <<-- from this point b.0_1 is [+1, +INF]
a = _5;
d.3_7 = d;
_8 = _3 % d.3_7;
if (_8 != 0)
when _5 is defined, and n.0_1 becomes non-null, we mark the dependent
names that are exports and defined in this block as stale. so _2, _3
and _8.
When _8 is being calculated, _3 is stale, and causes it to be
recomputed. it is dependent on _2, alsdo stale, so it is also
recomputed, and we end up with
_2 == [0, 0]
_3 == [0 ,0]
and _8 = [0, 0]
And then we can fold away the condition.
The side effect is that _2 and _3 are globally changed to be [0, 0], but
this is OK because it is the definition block, so it dominates all other
uses of these names, and they should be [0,0] upon exit anyway. The
previous patch ensure that the global values written to
SSA_NAME_RANGE_INFO is the correct [0,1] for both _2 and _3.
The patch would have been even smaller if I already had a mark_stale
method. I thought there was one, but I guess it never made it in from
lack of need at the time. The only other tweak was to make the value
stale if the dependent value was the same as the definitions.
This bootstraps on x86_64-pc-linux-gnu with no regressions. Re-running
to ensure.
@@ -1475,6 +1488,15 @@ ranger_cache::update_to_nonnull (basic_block
bb, tree name)
{
r.set_nonzero (type);
m_on_entry.set_bb_range (name, bb, r);
+ // Mark consumers of name stale so they can be recomputed.
+ if (m_gori.is_import_p (name, bb) || m_gori.is_export_p (name, bb))
+ {
+ tree x;
+ FOR_EACH_GORI_EXPORT_NAME (m_gori, bb, x)
+ if (m_gori.in_chain_p (name, x)
+ && gimple_bb (SSA_NAME_DEF_STMT (x)) == bb)
+ m_temporal->set_stale (x);
+ }
}
so if we have a BB that exports N names and each of those is updated to nonnull
this is going to be quadratic? It also looks like the gimple_bb check
is cheaper
than the bitmap test done in in_chain_p. What comes to my mind is why we need
to mark "consumers"? Can't consumers check their uses defs when they look
at their timestamp? This whole set_stale thing doesn't seem to be
They do. The timestamps only look at direct uses. Any use of _2 should
look at the def and notice it is stale relative to b.0_1 automatically.
We miss the opportunity in the example which uses _3 to compute _8. _3
is directly dependent on _2 whose def is not stale relative to _3, so we
miss the transitive staleness via b.0_1. This marks all the consumers
whose calculation is derived from the now non-null value as stale.
Within the block, it is fully transitive and anything potentially
derived from NAME will be recalculated if it is used. In old EVRP
terms, it would be like updating the current value vector for any
ssa-names derived from NAME when it becomes non-null, except it is done
lazily.
transitive anyway,
consider:
_1 = ...
<bb>
_2 = _1 + ..;
<bb>
_3 = _2 + ...;
so when _1 is updated to non-null we mark _2 as stale but _3 should
also be stale, no?
When we visit _3 before eventually getting to _2 (to see whether it
updates and thus
we more precisely we know if it makes _3 stale) we won't re-evaluate it?
That said, the change looks somewhat ad-hoc to get to 1-level deep second-level
opportunities?
The patch applies only to dom-walks, and primarily targets definitions
in the current block that we have already seen that we now know are
stale. It is one approach to applying non-null later in the same block
without resorting to much of an algorithmic change. It's not really
intended to affect anything cross block as that is handled differently
via the GORI engine. It would provide better on-exit ranges in the
definition block for some of the names involved.
That said, I'm not crazy about putting anything else into this release
anyway, so if the regressions isn't serious enough, then I'd simply wait
for the revamp of side-effects in the next release to deal with it.
Andrew
