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\section{MLRisc Generation}
  Every compiler will eventually compile down to an abstract machine
  that it believes will execute source programs efficiently. The
  abstract machine will typically consists of abstract machine
  registers and instructions, one or more stacks, and parameter
  passing conventions.  The hope is that all this will map down
  efficiently onto the target machine. Indeed, the abstract machine
  should be reasonably close to architectures that are envisioned as
  possible targets. Several step need to be followed in the generation
  of MLRisc.

  \begin{enumerate}
   \item The first step in generating target machine code is to define
   the MLRisc intermediate representation after it has been
   appropriately specialized. The interfaces that describe the
   dimensions of specialization are quite simple. Depending on the
   compiler, these may be target dependent; for example, in the SML/NJ
   compiler, the encoding of registers used to indicate the roots of
   garbage collection depend on how the runtime system decodes the
   information.

  \item The only real connection between the MLRisc intermediate
  representation and the target machine is that the first
  $0..K-1$ MLRisc registers map onto the first $K$
  physical registers on the target machine. Thus some mapping of
  dedicated abstract machine registers to physical target registers is
  required. It is not always necessary to map abstract machine
  registers to physical machine registers. For example, on
  architectures like the x86 with few registers, some abstract machine
  registers may be mapped to fixed memory locations. Thus an abstract
  machine register like the \sml{maskReg} may have something like:
\begin{SML}
  LOAD(32, LABEL maskRegLab) 
\end{SML}
spliced instead.

  \item The unit of compilation is called a 
   \href{cluster.html}{cluster} which
  is the smallest unit for inter-procedural optimizations. A cluster
  will typically consist of several entry points that may call each
  other, as well as call local functions in the module. For maximum
  flexibility, the parameter passing convention for local functions
  should be specialized by the \href{mlrisc-ra.html}{register allocator}.

   Once the MLRisc trees for a cluster have been built, they must
  be converted into target assembly or machine code. This is done by
  building up a function (\newdef{codegen}) that
  glues together optimizations modules that have been specialized. For
  example, the target instruction set must be specialized to hold the
  MLRisc constants; the flowgraph must be specialized to carry these
  instructions as well as the MLRisc pseudo-ops; the optimization
  modules must know about several front end constraints such as how to
  spill registers.
  \end{enumerate}

   If the module that translates the abstract machine instructions
  into MLRisc instructions has been appropriately parameterized, then
  it can be reused for multiple target architectures. For high level
  languages it is better to generate MLRisc instructions from the high
  level intermediate form used by the front end of the compiler.

root@smlnj-gforge.cs.uchicago.edu
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