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C++: Performance tweak for 1-field struct loads

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On kamailio/kamailio the `DataFlowUtil::simpleInstructionLocalFlowStep`
predicate was slow because of the case for single-field structs, where
there was a large tuple-count bulge when joining with
`getFieldSizeOfClass`:

    3552902   ~2%       {2} r1 = SCAN Instruction::CopyInstruction::getSourceValueOperand_dispred#3#ff AS I OUTPUT I.<1>, I.<0>
    2065347   ~2%       {2} r35 = JOIN r1 WITH Operand::NonPhiMemoryOperand::getAnyDef_dispred#3#ff AS R ON FIRST 1 OUTPUT r1.<1>, R.<1>
    2065827   ~2%       {3} r36 = JOIN r35 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 1 OUTPUT R.<1>, r35.<1>, r35.<0>
    2065825   ~3%       {3} r37 = JOIN r36 WITH Type::Type::getSize_dispred#ff AS R ON FIRST 1 OUTPUT r36.<1>, r36.<2>, R.<1>
    2068334   ~2%       {4} r38 = JOIN r37 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 1 OUTPUT R.<1>, r37.<2>, r37.<0>, r37.<1>
    314603817 ~0%       {3} r39 = JOIN r38 WITH DataFlowUtil::getFieldSizeOfClass#fff_120#join_rhs AS R ON FIRST 2 OUTPUT r38.<3>, R.<2>, r38.<2>
    8         ~0%       {2} r40 = JOIN r39 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 2 OUTPUT r39.<2>, r39.<0>

That's 314M tuples.

Strangely, there is no such bulge on more well-behaved snapshots like
mysql/mysql-server.

With this commit the explosion is gone:

    ...
    2065825  ~0%       {4} r37 = JOIN r36 WITH Type::Type::getSize_dispred#ff AS R ON FIRST 1 OUTPUT r36.<0>, R.<1>, r36.<1>, r36.<2>
    1521     ~1%       {3} r38 = JOIN r37 WITH DataFlowUtil::getFieldSizeOfClass#fff_021#join_rhs AS R ON FIRST 2 OUTPUT r37.<2>, R.<2>, r37.<3>
    8        ~0%       {2} r39 = JOIN r38 WITH Instruction::Instruction::getResultType_dispred#3#ff AS R ON FIRST 2 OUTPUT r38.<0>, r38.<2>
This commit is contained in:
Jonas Jensen
2020-06-09 14:33:32 +02:00
parent 398678a28b
commit a341912da9
1 changed files with 4 additions and 3 deletions
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7
cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll
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@@ -564,10 +564,11 @@ private predicate simpleInstructionLocalFlowStep(Instruction iFrom, Instruction
or
// Flow from stores to structs with a single field to a load of that field.
iTo.(LoadInstruction).getSourceValueOperand().getAnyDef() = iFrom and
exists(int size, Type type |
exists(int size, Type type, Class cTo |
type = iFrom.getResultType() and
iTo.getResultType().getSize() = size and
getFieldSizeOfClass(iTo.getResultType(), type, size)
cTo = iTo.getResultType() and
cTo.getSize() = size and
getFieldSizeOfClass(cTo, type, size)
)
or
// Flow through modeled functions