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C++: Further performance improvement for the null termination queries #7018

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C++: Further performance improvement for the null termination queries #7018

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@geoffw0 @MathiasVP
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@geoffw0 geoffw0 commented Nov 1, 2021

Follow-up to #6915.

The goal here was to eliminate the mutual recursion between mayAddNullTerminator and mayAddNullTerminatorHelper, which was still causing unnecessarily slow performance on a few projects. I'm not sure if I've found the best solution to this - in particular mayAddNullTerminatorHelper looks quite large to me, though I haven't seen it blow up in practice. Happy to take further suggestions.

The change has a small positive effect on performance locally. I ran DCA on this (actually a variant with the two queries enabled) and only saw a 29s overall improvement there, which is likely not statistically significant.

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geoffw0 added 2 commits Nov 1, 2021
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C++: Improve comments.
d6a714c
@geoffw0
C++: Eliminate mutual recursion.
dec7f93
@geoffw0 geoffw0 added the C++ label Nov 1, 2021
@geoffw0 geoffw0 requested a review from as a code owner Nov 1, 2021
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MathiasVP reviewed Nov 2, 2021
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@MathiasVP MathiasVP left a comment

LGTM (with the exception of a small QLDoc nitpick).

cpp/ql/lib/semmle/code/cpp/commons/NullTermination.qll
mayAddNullTerminatorHelper(e, va, v0) and
mayAddNullTerminator(pragma[only_bind_into](e0), pragma[only_bind_into](v0.getAnAccess())) and
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@MathiasVP MathiasVP Nov 2, 2021

Ah, I see why this speeds it up. Previously, we were computing an SCC consisting of {mayAddNullTerminator, mayAddNullTerminatorHelper}, but now we're computing mayAddNullTerminatorHelper first, and then evaluating an SCC just containing mayAddNullTerminator. So this doesn't change any query results. Is that a correct description of what's going on?

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@geoffw0 geoffw0 Nov 2, 2021

Yes, that's my understanding as well. The tricky part is that we may be computing a bigger (less constrained) mayAddNullTerminatorHelper relation in the first place, but I haven't seen that be a problem.

cpp/ql/lib/semmle/code/cpp/commons/NullTermination.qll Outdated Show resolved Hide resolved
@geoffw0 @MathiasVP
Update cpp/ql/lib/semmle/code/cpp/commons/NullTermination.qll
c1de416 
Co-authored-by: Mathias Vorreiter Pedersen <mathiasvp@github.com>
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