C++: Protect the value numbering library from instructions with multiple enclosing functions #20369
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… The next commits will protect the recursion from instructions which has multiple IRFunctions and assign such instructions a unique value number so it won't make sense for them to have an IRFunction column.
…n instructions with a non-unique IRFUnction a unique value number.
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Pull Request Overview
This PR addresses a performance issue in the value numbering library caused by instructions with multiple enclosing IR functions. The fix ensures that such instructions are assigned unique value numbers to prevent tuple count explosion in downstream analysis.
- Modifies the value numbering system to handle instructions with multiple enclosing functions
- Adds filtering logic to detect problematic instructions with multiple enclosing functions
- Changes unique value number generation to be instruction-based rather than function-based
.../lib/semmle/code/cpp/ir/implementation/unaliased_ssa/gvn/internal/ValueNumberingInternal.qll
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We hit a performance problem on https://github.com/openvinotoolkit/oneDNN in the 2.23.0 release, and after looking long and hard at the resulting tuple count explosion in the
IRGuardslibrary I narrowed the problem down to a fewInitializeParameterinstructions with more than one enclosing IR functions.Since each value number has an
IRFunctioncolumn this meant that theseInitializeParameterinstructions mapped to many many value numbers which meant that certain joins inIRGuardwhich were expected to have no fan-out caused a large explosion.Ideally, we should have no instruction with multiple enclosing
IRFunctions. However, to mitigate this problem going forward this PR ensures that instructions with multipleIRFunctions are assigned a unique value number.Commit-by-commit review recommended.