* Overview

  This document describes the process for adding support (i.e., semantics) for
  new instructions to a semmc-based macaw backend.

* When do I need this

  Missing instruction semantics will manifest in one of two ways:

  - Pattern match failures in the generated ~execInstruction~ function (e.g.,
    ~Data.Macaw.PPC.Semantics.PPC64.execInstruction~)
  - Basic blocks returned by macaw that are terminated by a ~TranslateError~
    terminator

  The first signal will only appear in debug mode.  Later, we will add a
  fallthrough case to prevent noisy failures and turn them into the second case.

  The second case can happen for two reasons.  First, we may not have support in
  the disassembler for the instruction.  We know we are missing some PowerPC
  instructions, and we don't have any support right now for system management
  instructions.

* Process

  1. Identify the instruction with missing semantics

     This is included in the exception for a missing case in ~execInstruction~.
     If there is no exception there, it can be determined from the address in
     the ~TranslateError~.

  2. Find the semantics for the instruction in the relevant architecture manual

     PowerPC: http://www.iman1.jo/iman1/images/IMAN1-User-Site-Files/Architecure/PPC_Vers202_Book1_public.pdf

  3. Implement the semantics using the semantics DSL

     For PowerPC, this lives in the semmc-ppc package in the semmc repository.
     The relevant module is ~SemMC.Architecture.PPC.Base~.  Find the most
     appropriate sub-module under the ~Base~ hierarchy (or add a new one) and
     add a new opcode definition.  For most instructions, you should use
     ~defineOpcodeWithIP~, which automatically handles updating the instruction
     pointer.

  4. Regenerate the semantics files

     After the definition is added to the DSL, we need to regenerate the
     semantics files to actually put the definition on disk.  To do this, run
     the ~genbase.sh~ script for your architecture.  For PowerPC, this script is
     ~semmc/semmc-ppc/scripts/genbase.sh~.

     Verify that the new expected file is generated.  It should be a file with a
     ~.sem~ extension in the data directory of your architecture.  These files
     are an extended version of the SMTLib format.

  5. Force a rebuild of the opcode module

     We use TH to store the contents of the semantics files as bytestrings.
     This is inconvenient at development time, but important for distribution
     and run-time (so that the locations of the semantics files on disk are not
     important).  Unfortunately, TH can't tell when we add files to a directory,
     so we have to trick it into rebuilding the necessary modules.

     For PowerPC, the relevant modules are:

     - ~SemMC.Architecture.PPC64.Opcodes.Internal~
     - ~SemMC.Architecture.PPC32.Opcodes.Internal~

     The easiest way to force a rebuild is just to add another newline to each
     file and then rebuild.

  6. Rebuild your macaw backend

     This would be macaw-ppc or macaw-arm.  After rebuilding these, try to
     analyze your binary again to make sure the changes worked.

** Adding new operations

   Sometimes, new operations are required in the formula language.  For example,
   we will need to add support for the popcount (population count) instruction
   at some point.  There are two cases:

   1. SimpleBuilder (in crucible) supports the operation
   2. SimpleBuilder does not support the operation

*** SimpleBuilder supports the operation

    In this case, we:

    1. Add a wrapper in the DSL (~SemMC.DSL~ in the semmc package)
    2. Update the formula parser to parse the new operation

       The formula parser is located in the semmc package
       (~SemMC.Formula.Parser~) and converts s-expression formulas into
       SimpleBuilder terms.  The parser will call into SimpleBuilder, mostly via
       the ~IsExprBuilder~ interface, to construct these terms.
    3. Update the TH-macaw IR conversion to handle the new construct

       This code is located in macaw-semmc in the architecture-specific
       backends.  For example, the PowerPC version is in macaw-ppc under
       ~Data.Macaw.PPC.Semantics.TH~.  The function of interest is
       ~crucAppToExprTH~, which converts SimpleBuilder terms into macaw terms.
       Add a new case that performs the conversion as necessary.
    4. Rebuild

*** SimpleBuilder does not support the operation

    Ideally, we can modify SimpleBuilder, but that is more involved.  Sometimes
    we can get away with the following:

    1. Add a wrapper in the DSL (~SemMC.DSL~) that wraps the necessary
       functionality into an uninterpreted function
    2. Add a type signature for the uninterpreted function for your architecture

       The signatures for PowerPC are in ~SemMC.Architecture.PPC.UF~ in the
       semmc-ppc package.  These signatures give types to the uninterpreted
       functions instantiated into SimpleBuilder.

       Note that we don't need to update the parser, since it treats all
       uninterpreted functions uniformly.
    3. Add an interpretation for the uninterpreted function in the TH-macaw translator

       Instantiate the necessary macaw IR fragment to implement the
       uninterpreted function.  Right now, there are a few macaw operations not
       supported in SimpleBuilder, so this might be necessary.