C++ IR: Don't propagate GVN through non-exact Copy #1568
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The `ValueNumbering` library is supposed to propagate value numberings through a `CopyInstruction` only when it's _congruent_, meaning it must have exact overlap with its source. A `CopyInstruction` can be a `LoadInstruction`, a `StoreInstruction`, or a `CopyValueInstruction`. The latter is also a `UnaryInstruction`, and the value numbering rule for `UnaryInstruction` applied to it as well. This meant that value numbering would propagate even through a non-congruent `CopyValueInstruction`. That's semantically wrong but probably only an issue in very rare circumstances, and it should get corrected when we change the definition of `getUnary` to require congruence. What's worse is the performance implications. It meant that the value numbering IPA witness could take two different paths through every `CopyValueInstruction`. If multiple `CopyValueInstruction`s were chained, this would lead to an exponential number of variable numbers for the same `Instruction`, and we would run out of time and space while performing value numbering. This fixes the performance of `ValueNumbering.qll` on https://github.com/asterisk/asterisk, although this project might also require a separate change for fixing an infinite loop in the IR constant analysis.
dave-bartolomeo
approved these changes
Jul 9, 2019
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The
ValueNumberinglibrary is supposed to propagate value numberings through aCopyInstructiononly when it's congruent, meaning it must have exact overlap with its source. ACopyInstructioncan be aLoadInstruction, aStoreInstruction, or aCopyValueInstruction. The latter is also aUnaryInstruction, and the value numbering rule forUnaryInstructionapplied to it as well.This meant that value numbering would propagate even through a non-congruent
CopyValueInstruction. That's semantically wrong but probably only an issue in very rare circumstances, and it should get corrected when we change the definition ofgetUnaryto require congruence.What's worse is the performance implications. It meant that the value numbering IPA witness could take two different paths through every
CopyValueInstruction. If multipleCopyValueInstructions were chained, this would lead to an exponential number of variable numbers for the sameInstruction, and we would run out of time and space while performing value numbering.This fixes the performance of
ValueNumbering.qllon https://github.com/asterisk/asterisk, although this project might also require #1567.