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C++: Fix coversEntireVariable perf in AliasedSSA

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This predicate got an unfortunate join order, leading to these tuple
counts on ElektraInitiative/libelektra:

    (290s) Tuple counts for AliasedSSA::VariableMemoryLocation::coversEntireVariable_dispred#f:
    57117     ~0%     {3} r1 = SCAN IRType::IRType::getByteSize_dispred#ff AS I OUTPUT 0, (I.<1> * 8), I.<0>
    421445272 ~0%     {3} r2 = JOIN r1 WITH AliasedSSA::VariableMemoryLocation#fffffff_5601#join_rhs AS R ON FIRST 2 OUTPUT R.<3>, r1.<2>, R.<2>
    103282    ~2%     {1} r3 = JOIN r2 WITH AliasConfiguration::Allocation::getIRType_dispred#ff AS R ON FIRST 2 OUTPUT r2.<2>
                      return r3

With this commit, we get these tuple counts instead:

    (0s) Tuple counts for AliasedSSA::VariableMemoryLocation::varIRTypeHasBitRange#bff:
    361874 ~0%     {3} r1 = SCAN AliasedSSA::VariableMemoryLocation#fffffff AS I OUTPUT I.<1>, 0, I.<0>
    361874 ~0%     {3} r2 = JOIN r1 WITH AliasConfiguration::Allocation::getIRType_dispred#ff AS R ON FIRST 1 OUTPUT R.<1>, 0, r1.<2>
    361874 ~1%     {3} r3 = JOIN r2 WITH IRType::IRType::getByteSize_dispred#ff AS R ON FIRST 1 OUTPUT r2.<2>, 0, (R.<1> * 8)
                   return r3

    (0s) Tuple counts for AliasedSSA::VariableMemoryLocation::coversEntireVariable_dispred#f:
    103282 ~2%     {1} r1 = JOIN AliasedSSA::VariableMemoryLocation#fffffff_056#join_rhs AS L WITH AliasedSSA::VariableMemoryLocation::varIRTypeHasBitRange#bff AS R ON FIRST 3 OUTPUT L.<0>
    103282 ~2%     {1} r2 = STREAM DEDUP r1
                   return r2
This commit is contained in:
Jonas Jensen
2020-02-10 13:42:42 +01:00
parent 09960e0a7d
commit cf1bc693b4
1 changed files with 6 additions and 3 deletions
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9
cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/AliasedSSA.qll
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@@ -220,9 +220,12 @@ class VariableMemoryLocation extends TVariableMemoryLocation, AllocationMemoryLo
/**
* Holds if this memory location covers the entire variable.
*/
final predicate coversEntireVariable() {
startBitOffset = 0 and
endBitOffset = var.getIRType().getByteSize() * 8
final predicate coversEntireVariable() { varIRTypeHasBitRange(startBitOffset, endBitOffset) }
pragma[noinline]
private predicate varIRTypeHasBitRange(int start, int end) {
start = 0 and
end = var.getIRType().getByteSize() * 8
}
}