C++: Prevent AliasedVirtualVariable from overlapping string literals

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We were hitting a combinatorial explosion in `hasDefinitionAtRank` for functions that contain a large number of string literals. The problem was that every `Chi` instruction for `AliasedVirtualVariable` was treated as a definition of every string literal. We already mark string literals as `isReadOnly()`, but we were allowing `AliasedVirtualVariable` to define read-only locations so that the `AliasedDefinition` instruction would provide the initial definition for all string literals.

To fix this, I've introduced the new `InitializeNonLocal` instruction, which is inserted in the prologue of every function right after `AliasedDefinition`. It provides the initial definition for every non-stack memory location, including read-only locations, but is never written to anywhere else. It is the conterpart of the `AliasedUse` instruction in the function epilogue, which represents the use of all non-stack memory after the function returns. I considered renaming `AliasedUse` to `ReturnNonLocal`, to match the `InitializeXXX`/`ReturnXXX` pattern we already use for parameters and indirections, but held off to avoid unnecessary churn. Any thoughts on whether I should make this name change?

This change has a significant speedup in evaluation time for a few of our troublesome databases:
`attnam/ivan`: 13%
`awslabs/s2n`: 26%
`SinaMostafanejad/OpenRDM`: 7%
`zcoinofficial/zcoin`: 8%

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This commit is contained in:

Dave Bartolomeo

2020-01-31 10:51:46 -07:00

parent 3b81c3b95c

commit e27a0fe504

13 changed files with 3652 additions and 3282 deletions

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cpp/ql/test/library-tests/ir/ir/raw_ir.expected

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cpp/ql/test/library-tests/ir/ssa/aliased_ssa_ir.expected

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cpp/ql/test/library-tests/ir/ssa/aliased_ssa_ir_unsound.expected

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cpp/ql/test/library-tests/ir/ssa/unaliased_ssa_ir.expected

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cpp/ql/test/library-tests/ir/ssa/unaliased_ssa_ir_unsound.expected

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