C++: Compute isInCycle only for raw IR #2511
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On wireshark/wireshark, `isInCycle` ran into a low-memory loop on the `aliased_ssa` stage. It shouldn't be necessary to detect cycles after the `raw` stage, so this commit moves cycle detection into the `Construction` modules and makes it a no-op in `SSAConstruction.qll`.
geoffw0
approved these changes
Dec 12, 2019
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LGTM. We've discussed this quite a lot on Slack, in summary:
- we're all happy with the actual changes to code
- several of us have done some ad-hoc testing today
- the issue that the cycle-finding is intended to solve no longer occurs where it did before, nor is there a test case for it; it isn't clear whether or not it's needed any more
- in any case the second and third layers of cycle finding (which this PR removes) were never needed in practice
- the csharp IR is not in use so that side of this PR isn't a big concern
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I finally found hardware and parameters that let me evaluate the IR on the giant Linux snapshot from https://semmle.com/large-oss-projects. There were no cycles, so it looks like the cycle detection isn't needed on recent-ish databases. That means this PR should certainly be safe, and maybe we can go even further and remove the cycle detection. |
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On wireshark/wireshark,
isInCycleran into a low-memory loop on thealiased_ssastage. It shouldn't be necessary to detect cycles after therawstage, so this commit moves cycle detection into theConstructionmodules and makes it a no-op inSSAConstruction.qll.Testing to do: