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85 lines
3.2 KiB
85 lines
3.2 KiB
3 years ago
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This directory contains a model of the semantics of LLVM, of LLAIR, and of the
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LLVM to LLAIR translation, along with some theorems about them. It is written
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in the HOL4 theorem prover.
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# Building
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To build the formalisation, which just gets HOL4 to check the proofs, run the
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`Holmake` command in this directory. This requires having HOL4 installed, which
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itself depends on PolyML.
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The directions for installing HOL4 are here
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<https://hol-theorem-prover.org/#get>, but essentially it is
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git clone https://github.com/HOL-Theorem-Prover/HOL.git
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cd HOL
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poly < tools/smart-configure.sml
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bin/build
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PolyML is available here <https://www.polyml.org/download.html>, but it is
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widely available from various package managers and easiest to install that way,
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e.g., on MacOS `brew install polyml`.
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This has been tested on PolyML 5.8, and on HOL4 Github commit e2d0fafdb3ccdd098d2bd6e0aaaacbc9468ae4b1.
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# What is modelled
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The model of the translation is not complete: function calls and exceptions are
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two notable omissions. However, it covers the other important aspects of the
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translation:
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- the SSA restrictions on LLVM,
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- building compound expressions to reduce the number of temporary stores in
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LLAIR,
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- and how the SSA restrictions imply that a simple linear traversal of the
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(topologically sorted) CFG suffices for the translation,
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- the treatment of LLVM's bitwise 2s complement arithmetic on LLAIR's
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mathematical integers.
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The semantics of LLVM and LLAIR are given as small step abstract state
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machines, and the translation between them as a pure recursive function.
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The semantics of LLVM uses a concrete memory model, i.e., indices into an array
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of bytes. This means that it does not account for all possible forms of
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undefined behaviour that could occur with LLVM's more complicated pointer
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provenance model.
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The proof that if an LLVM program doesn't get stuck and has a certain
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observable behaviour, then the translation to LLAIR also doesn't get stuck and
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has the same behaviour is complete, except for a few minor cheats. The proof in
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the other direction, from LLAIR behaviour to LLVM behaviour, is not attempted.
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# Contents
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`llairScript.sml` : the syntax and semantics of LLAIR
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`llair_propScript.sml` : theorems about various properties of LLAIR's semantics
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`llvmScript.sml` : the syntax and semantics of LLVM
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`llvm_propScript.sml` : theorems about various properties of LLVM's semantics
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`llvm_ssaScript.sml` : the definition of SSA form for LLVM, along with theorems
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about SSA, dominance, and variable liveness
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`llvm_to_llairScript.sml` : the translation function's definition
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`llvm_to_llair_propScript.sml` : theorems about the LLVM to LLAIR translation
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that don't mention the semantics, so just those that establish invariants about
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the translation functions themselves.
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`llvm_to_llair_sem_propScript.sml` : the overall correctness theorems for the
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translation
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`memory_modelScript.sml` : an abstract model of memory as an array of bytes. It
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includes reading and writing multi-byte words in little-endian encoding
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`miscScript.sml` : various generic theorems that could in principle be moved
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into HOL4's libraries
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`settingsScript.sml` : set up HOL4, replacing many of it's ugly UPPERCASE
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constants with lower case ones
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