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C++: IR: faster definitionReachesRank

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On Wireshark with 6GB RAM, I've observed `definitionReachesRank` to be
the slowest predicate in the IR. It seems that the implementation was
slow because the optimizer failed to eliminate the common
`reachesRank - 1` subexpression. This led to context being pushed into
the `not`, which got implemented as `MATERIALIZE`. That wouldn't
normally be a disaster, but this is one of the largest predicates in the
IR SSA construction, and iteration 2 was very slow.

Before:

    (1505s) Starting to evaluate predicate SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta/4[1]@93f592 (iteration 1)
    (1535s) Tuple counts for SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta:
    130670697 ~0%     {4} r1 = SCAN project#SSAConstruction::DefUse::hasDefinitionAtRank#fffff AS I OUTPUT I.<0>, I.<1>, I.<2>, (I.<2> + 1)
    130670697 ~6%     {5} r2 = JOIN r1 WITH SSAConstruction::DefUse::exitRank#fff AS R ON FIRST 2 OUTPUT r1.<0>, r1.<1>, r1.<2>, r1.<3>, R.<2>
    130670697 ~6%     {5} r3 = SELECT r2 ON r2.<3> <= r2.<4>
    130670697 ~0%     {4} r4 = SCAN r3 OUTPUT r3.<0>, r3.<1>, r3.<2>, r3.<3>
                      return r4
    (1535s) 			 - SSAConstruction::DefUse::definitionReachesRank#ffff_delta has 130670697 rows (order for disjuncts: delta=<standard>).

    (1535s) Starting to evaluate predicate SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta/4[2]@866c14 (iteration 2)
    (1626s) Tuple counts for SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta:
    261341394 ~107%     {4} r1 = JOIN SSAConstruction::DefUse::definitionReachesRank#ffff#prev_delta AS L WITH SSAConstruction::DefUse::definitionReachesRank#ffff#join_rhs AS R ON FIRST 3 OUTPUT R.<0>, R.<1>, R.<2>, (1 + L.<3>)
    261341394 ~107%     {4} r2 = r1 AND NOT SSAConstruction::DefUse::definitionReachesRank#ffff#prev AS R(r1.<0>, r1.<1>, r1.<2>, r1.<3>)
    130670697 ~0%       {5} r3 = SCAN r2 OUTPUT r2.<0>, r2.<1>, (r2.<3> - 1), r2.<2>, r2.<3>
    106034590 ~1%       {4} r4 = JOIN r3 WITH project#SSAConstruction::DefUse::hasDefinitionAtRank#fffff AS R ON FIRST 3 OUTPUT r3.<0>, r3.<1>, r3.<3>, r3.<4>
    106034590           {4} r5 = MATERIALIZE r4 AS antijoin_rhs
    24636107  ~3%       {4} r6 = r2 AND NOT r5(r2.<0>, r2.<1>, r2.<2>, r2.<3>)
    24636107  ~0%       {5} r7 = JOIN r6 WITH SSAConstruction::DefUse::exitRank#fff AS R ON FIRST 2 OUTPUT r6.<0>, r6.<1>, r6.<2>, r6.<3>, R.<2>
    2749441   ~0%       {5} r8 = SELECT r7 ON r7.<3> <= r7.<4>
    2749441   ~4%       {4} r9 = SCAN r8 OUTPUT r8.<0>, r8.<1>, r8.<2>, r8.<3>
                        return r9
    (1626s) 			 - SSAConstruction::DefUse::definitionReachesRank#ffff_delta has 2749441 rows (order for disjuncts: delta=<standard>).

After:

    (12s) Tuple counts for SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta:
    130670697 ~0%     {4} r1 = SCAN project#SSAConstruction::DefUse::hasDefinitionAtRank#fffff AS I OUTPUT I.<0>, I.<1>, I.<2>, (I.<2> + 1)
                      return r1
    (12s) 			 - SSAConstruction::DefUse::definitionReachesRank#ffff_delta has 130670697 rows (order for disjuncts: delta=<standard>).
    (12s) Starting to evaluate predicate SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta/4[2]@fff64c (iteration 2)
    (34s) Tuple counts for SSAConstruction::DefUse::definitionReachesRank#ffff#cur_delta:
    108784031 ~0%     {4} r1 = SSAConstruction::DefUse::definitionReachesRank#ffff#prev_delta AS L AND NOT SSAConstruction::DefUse::exitRank#fff AS R(L.<0>, L.<1>, L.<3>)
    2749441   ~5%     {4} r2 = r1 AND NOT project#SSAConstruction::DefUse::hasDefinitionAtRank#fffff AS R(r1.<0>, r1.<1>, r1.<3>)
    2749441   ~4%     {4} r3 = SCAN r2 OUTPUT r2.<0>, r2.<1>, r2.<2>, (r2.<3> + 1)
    2749441   ~4%     {4} r4 = r3 AND NOT SSAConstruction::DefUse::definitionReachesRank#ffff#prev AS R(r3.<0>, r3.<1>, r3.<2>, r3.<3>)
                      return r4
    (34s) 			 - SSAConstruction::DefUse::definitionReachesRank#ffff_delta has 2749441 rows (order for disjuncts: delta=<standard>).

Note that the row counts are exactly the same before and after.
This commit is contained in:
Jonas Jensen
2020-03-04 14:31:10 +01:00
parent 88c74b2a4b
commit 6b2fd17f03
3 changed files with 33 additions and 30 deletions
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21
cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/SSAConstruction.qll
View File

@@ -665,17 +665,18 @@ module DefUse {
private predicate definitionReachesRank(
Alias::MemoryLocation useLocation, OldBlock block, int defRank, int reachesRank
) {
// The def always reaches the next use, even if there is also a def on the
// use instruction.
hasDefinitionAtRank(useLocation, _, block, defRank, _) and
reachesRank <= exitRank(useLocation, block) and // Without this, the predicate would be infinite.
(
// The def always reaches the next use, even if there is also a def on the
// use instruction.
reachesRank = defRank + 1
or
// If the def reached the previous rank, it also reaches the current rank,
// unless there was another def at the previous rank.
definitionReachesRank(useLocation, block, defRank, reachesRank - 1) and
not hasDefinitionAtRank(useLocation, _, block, reachesRank - 1, _)
reachesRank = defRank + 1
or
// If the def reached the previous rank, it also reaches the current rank,
// unless there was another def at the previous rank.
exists(int prevRank |
reachesRank = prevRank + 1 and
definitionReachesRank(useLocation, block, defRank, prevRank) and
not prevRank = exitRank(useLocation, block) and
not hasDefinitionAtRank(useLocation, _, block, prevRank, _)
)
}

21
cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/internal/SSAConstruction.qll
View File

@@ -665,17 +665,18 @@ module DefUse {
private predicate definitionReachesRank(
Alias::MemoryLocation useLocation, OldBlock block, int defRank, int reachesRank
) {
// The def always reaches the next use, even if there is also a def on the
// use instruction.
hasDefinitionAtRank(useLocation, _, block, defRank, _) and
reachesRank <= exitRank(useLocation, block) and // Without this, the predicate would be infinite.
(
// The def always reaches the next use, even if there is also a def on the
// use instruction.
reachesRank = defRank + 1
or
// If the def reached the previous rank, it also reaches the current rank,
// unless there was another def at the previous rank.
definitionReachesRank(useLocation, block, defRank, reachesRank - 1) and
not hasDefinitionAtRank(useLocation, _, block, reachesRank - 1, _)
reachesRank = defRank + 1
or
// If the def reached the previous rank, it also reaches the current rank,
// unless there was another def at the previous rank.
exists(int prevRank |
reachesRank = prevRank + 1 and
definitionReachesRank(useLocation, block, defRank, prevRank) and
not prevRank = exitRank(useLocation, block) and
not hasDefinitionAtRank(useLocation, _, block, prevRank, _)
)
}