macaw-x86-symbolic: Fix idiv/div semantics
#394
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RyanGlScott
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macaw-x86-symbolic: Fix idiv/div semanticsmacaw-x86-symbolic: Fix idiv/div semantics
langston-barrett
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Jul 12, 2024
When converting a Macaw value with the Macaw type `TupleType [x_1, ..., x_n]` to Crucible, the resulting Crucible value will have the Crucible type `StructType (EmptyCtx ::> ToCrucibleType x_n ::> ... ::> ToCrucibleType x_1)`. (See `macawListToCrucible(M)` in `Data.Macaw.Symbolic.PersistentState` for where this is implemented.) Note that the order of the tuple's fields is reversed in the process of converting it to a Crucible struct. This is a convention that one must keep in mind when dealing with Macaw tuples at the Crucible level. As it turns out, the part of `macaw-x86-symbolic` reponsible for interpreting the semantics of the `idiv` instruction (for signed quotient/remainder) and the `div` instruction (for unsigned quotient/remainder) were _not_ respecting this convention. This is because the `macaw-x86-symbolic` semantics were returning a Crucible struct consisting of `Empty :> quotient :> remainder)`, but at the Macaw level, this was interpreted as the tuple `(remainder, quotient)`, which is the opposite of the intended order. This led to subtle bugs such as those observed in #393. The solution is straightforward: have the `macaw-x86-symbolic` semantics compute `Empty :> remainder :> quotient` instead. Somewhat counterintuitive, but it does work. Fixes #393.
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When converting a Macaw value with the Macaw type
TupleType [x_1, ..., x_n]to Crucible, the resulting Crucible value will have the Crucible typeStructType (EmptyCtx ::> ToCrucibleType x_n ::> ... ::> ToCrucibleType x_1). (SeemacawListToCrucible(M)inData.Macaw.Symbolic.PersistentStatefor where this is implemented.) Note that the order of the tuple's fields is reversed in the process of converting it to a Crucible struct. This is a convention that one must keep in mind when dealing with Macaw tuples at the Crucible level.As it turns out, the part of
macaw-x86-symbolicreponsible for interpreting the semantics of theidivinstruction (for signed quotient/remainder) and thedivinstruction (for unsigned quotient/remainder) were not respecting this convention. This is because themacaw-x86-symbolicsemantics were returning a Crucible struct consisting ofEmpty :> quotient :> remainder), but at the Macaw level, this was interpreted as the tuple(remainder, quotient), which is the opposite of the intended order. This led to subtle bugs such as those observed in #393.The solution is straightforward: have the
macaw-x86-symbolicsemantics computeEmpty :> remainder :> quotientinstead. Somewhat counterintuitive, but it does work.Fixes #393.