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C++: Speed up variableLiveOnEntryToBlock in IR

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This predicate computed a local CP between all defs and uses of the same
virtual variable in a basic block. This wasn't a problem in
`unaliased_ssa`, but it became a huge problem in `aliased_ssa`, probably
because many variables can be modelled with a single virtual variable
there.

Before this commit, evaluation of `aliased_ssa`'s
`variableLiveOnEntryToBlock#ff#antijoin_rhs` on Wireshark took 80
_minutes_. After this commit, that predicate and its immediate
dependencies take around 5 _seconds_.
This commit is contained in:
Jonas Jensen
2019-02-02 15:19:51 +01:00
parent 6243c722c6
commit 4ac22253eb
2 changed files with 18 additions and 6 deletions
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12
cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/SSAConstruction.qll
View File

@@ -392,9 +392,15 @@ cached private module Cached {
}
private predicate variableLiveOnEntryToBlock(Alias::VirtualVariable vvar, OldBlock block) {
exists (int index | hasUse(vvar, _, block, index) |
not exists (int j | ssa_variableUpdate(vvar, _, block, j) | j < index)
) or
exists(int firstAccess |
hasUse(vvar, _, block, firstAccess) and
firstAccess = min(int index |
hasUse(vvar, _, block, index)
or
ssa_variableUpdate(vvar, _, block, index)
)
)
or
(variableLiveOnExitFromBlock(vvar, block) and not ssa_variableUpdate(vvar, _, block, _))
}

12
cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/internal/SSAConstruction.qll
View File

@@ -392,9 +392,15 @@ cached private module Cached {
}
private predicate variableLiveOnEntryToBlock(Alias::VirtualVariable vvar, OldBlock block) {
exists (int index | hasUse(vvar, _, block, index) |
not exists (int j | ssa_variableUpdate(vvar, _, block, j) | j < index)
) or
exists(int firstAccess |
hasUse(vvar, _, block, firstAccess) and
firstAccess = min(int index |
hasUse(vvar, _, block, index)
or
ssa_variableUpdate(vvar, _, block, index)
)
)
or
(variableLiveOnExitFromBlock(vvar, block) and not ssa_variableUpdate(vvar, _, block, _))
}