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C++: use shortestDistances in PrimitiveBasicBlocks

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The use of transitive closure for BB index calculation has been the
cause of an out-of-memory error. This commit switches the calculation to
use the `shortestDistances` HOP, which still has the problem that the
result needs to fit in RAM, but at least the RAM requirements are sure
to be linear in the size of the result. The `shortestDistances` HOP is
already used for BB index calculation for the C++ IR and for C#.

We could guard even better against OOM by switching the calculation to
use manual recursion, but that would undo the much-needed performance
improvements we got from #123.

This change improves performance on Wireshark, which is notorious for
having long basic blocks. When I benchmarked `shortestDistances`
for #123, it was slower than TC. With the current evaluator, it looks
like `shortestDistances` is faster. Performance before was:

    PrimitiveBasicBlocks::Cached::getMemberIndex#ff ................... 9.7s (executed 8027 times)
    #PrimitiveBasicBlocks::Cached::member_step#ffPlus ................. 6.6s
    PrimitiveBasicBlocks::Cached::primitive_basic_block_entry_node#f .. 3.5s
    PrimitiveBasicBlocks::Cached::primitive_basic_block_member#fff .... 2.3s

Performance with this commit is:

    PrimitiveBasicBlocks::Cached::primitive_basic_block_entry_node#f ................................................................... 3.5s
    shortestDistances@PrimitiveBasicBlocks::Cached::primitive_basic_block_entry_node#1@PrimitiveBasicBlocks::Cached::member_step#2#fff . 3s
    PrimitiveBasicBlocks::Cached::primitive_basic_block_member#fff ..................................................................... 963ms
This commit is contained in:
Jonas Jensen
2019-06-11 09:44:50 +02:00
parent 61f7c78fa9
commit 16b151745b
1 changed files with 2 additions and 21 deletions
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23
cpp/ql/src/semmle/code/cpp/controlflow/internal/PrimitiveBasicBlocks.qll
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@@ -60,29 +60,10 @@ private cached module Cached {
not n2 instanceof PrimitiveBasicBlock
}
/** Returns the index of `node` in its `PrimitiveBasicBlock`. */
private int getMemberIndex(Node node) {
primitive_basic_block_entry_node(node) and
result = 0
or
exists(Node prev |
member_step(prev, node) and
result = getMemberIndex(prev) + 1
)
}
/** Holds if `node` is the `pos`th control-flow node in primitive basic block `bb`. */
cached
predicate primitive_basic_block_member(Node node, PrimitiveBasicBlock bb, int pos) {
primitive_basic_block_entry_node(bb) and
(
pos = 0 and
node = bb
or
pos = getMemberIndex(node) and
member_step+(bb, node)
)
}
predicate primitive_basic_block_member(Node node, PrimitiveBasicBlock bb, int pos) =
shortestDistances(primitive_basic_block_entry_node/1, member_step/2)(bb, node, pos)
/** Gets the number of control-flow nodes in the primitive basic block `bb`. */
cached