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\section{Annotations}
\subsection{Overview}
A compiler front-end has to be propagate information to
the back-end. An optimization phase may have to leave behind information
at various places of the IR so that other phases can reuse such information.
MLRISC uses the \newdef{annotations}
mechanism for these functions.
Individual instructions, basic blocks, and flow graph edges,
can be attached one or more annotations.
The basic MLRISC system understands many annotations. Some examples are:
\begin{description}
\item[COMMENT]
these can be used to attach comments. If attached to
an instruction, the assemblers will output
them as part of their assembly output.
\item[BRANCH\_PROB]
these can be attached to a branch instruction to indicate
the probability in which is it taken.
\item[EXECUTION\_FREQ]
these can be attached to a basic block to indicate
its expected execution frequency
\end{description}
\subsection{Details}
The primitive annotations datatype is defined
to have this \mlrischref{library/annotations.sig}{signature}.
In addition, MLRISC predefined a few primitive annotations that are
recognized by the core system. This signature is
\mlrischref{instructions/mlriscAnnotations.sig}{MLRISC\_ANNOTATIONS}.
More detailed documentation can be found in this
\href{http://cm.bell-labs.com/cm/cs/what/smlnj/compiler-notes/annotations.ps}{paper}.
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