5.6

book.tex

@@ -16,6 +16,7 @@
 \usepackage{color}
 %\usepackage{ifthen}
 \usepackage{upquote}
+\usepackage[all]{xy}
 
 
 \definecolor{lightgray}{gray}{1}
@@ -10620,21 +10621,22 @@ $\Rightarrow$
 
 Recall that in the \code{explicate\_control} pass we define one helper
 function for each kind of position in the program.  For the \LangVar{}
-language of integers and variables we needed kinds of positions:
-assignment and tail. The \code{if} expressions of \LangIf{} introduced
-predicate positions. For \LangLoop{}, the \code{begin} expression introduces
-yet another kind of position: effect position. Except for the last
+language of integers and variables we needed assignment and tail
+positions. The \code{if} expressions of \LangIf{} introduced predicate
+positions. For \LangLoop{}, the \code{begin} expression introduces yet
+another kind of position: effect position. Except for the last
 subexpression, the subexpressions inside a \code{begin} are evaluated
 only for their effect. Their result values are discarded. We can
 generate better code by taking this fact into account.
 
 The output language of \code{explicate\_control} is \LangCLoop{}
 (Figure~\ref{fig:c7-syntax}), which is nearly identical to
-\LangCIf{}. The only syntactic difference is that \code{read} may also
-appear as a statement.  The most significant difference between the
-programs generated by \code{explicate\_control} in
-Chapter~\ref{ch:Lif} versus \code{explicate\_control} in this chapter
-is that the control-flow graphs of the later may contain cycles.
+\LangCIf{}. The only syntactic difference is the addition of \VOID{}
+and that \code{read} may appear as a statement.  The most significant
+difference between the programs generated by \code{explicate\_control}
+in Chapter~\ref{ch:Lif} versus \code{explicate\_control} in this
+chapter is that the control-flow graphs of the later may contain
+cycles.
 
 \begin{figure}[tp]
 \fbox{
@@ -10657,21 +10659,33 @@ is that the control-flow graphs of the later may contain cycles.
 \end{figure}
 
 The new auxiliary function \code{explicate\_effect} takes an
-expression (in an effect position) and a continuation. The function
-returns a $\Tail$ that includes the generated code for the input
-expression followed by the continuation. If the expression is
-obviously pure, that is, never causes side effects, then the
-expression can be removed, so the result is just the continuation.
-%
-The $\WHILE{\itm{cnd}}{\itm{body}}$ expression is the most interesting
-case.  First, you will need a fresh label $\itm{loop}$ for the top of
-the loop.  Recursively process the \itm{body} (in effect position)
-with the a \code{goto} to $\itm{loop}$ as the continuation, producing
-\itm{body'}. Next, process the \itm{cnd} (in predicate position) with
+expression (in an effect position) and the code for its
+continuation. The function returns a $\Tail$ that includes the
+generated code for the input expression followed by the
+continuation. If the expression is obviously pure, that is, never
+causes side effects, then the expression can be removed, so the result
+is just the continuation.
+%
+The case for $\WHILE{\itm{cnd}}{\itm{body}}$ expressions is
+interesting; the generated code is depicted in the following diagram.
+\begin{center}
+  \begin{minipage}{0.3\textwidth}
+\xymatrix{
+  *+[F=]{\txt{\code{goto} \itm{loop}}} \ar[r]
+    & *+[F]{\txt{\itm{loop}: \\ \itm{cnd'}}} \ar[r]^{else} \ar[d]^{then}
+    & *+[F]{\txt{\itm{cont}}} \\
+ & *+[F]{\txt{\itm{body'} \\ \code{goto} \itm{loop}}} \ar@/^50pt/[u]
+}
+\end{minipage}
+\end{center}
+We start by creating a fresh label $\itm{loop}$ for the top of the
+loop.  Next, recursively process the \itm{body} (in effect position)
+with a \code{goto} to $\itm{loop}$ as the continuation, producing
+\itm{body'}. Process the \itm{cnd} (in predicate position) with
 \itm{body'} as the then-branch and the continuation block as the
-else-branch. The result should be added to the control-flow graph with
-the label \itm{loop}. The result for the whole \code{while} loop is a
-\code{goto} to the \itm{loop} label.
+else-branch. The result should be added to the dictionary of
+\code{basic-blocks} with the label \itm{loop}. The result for the
+whole \code{while} loop is a \code{goto} to the \itm{loop} label.
 
 The auxiliary functions for tail, assignment, and predicate positions
 need to be updated. The three new language forms, \code{while},
@@ -10767,7 +10781,8 @@ The \code{analyze\_dataflow} function has four parameters.
 %\node (F1-2) at (9,0)  {\large \LangLoopFunRef{}};
 %\node (F1-3) at (6,0)  {\large \LangLoopFunRef{}};
 \node (F1-4) at (6,2)  {\large \LangLoop{}};
-\node (F1-5) at (9,2)  {\large \LangLoopANF{}};
+\node (F1-5) at (9,2)  {\large \LangLoop{}};
+\node (F1-6) at (12,2)  {\large \LangLoopANF{}};
 \node (C3-2) at (3,0)  {\large \racket{\LangCLoop{}}\python{\LangCIf{}}};
 
 \node (x86-2) at (3,-2)  {\large \LangXIfVar{}};
@@ -10795,8 +10810,10 @@ The \code{analyze\_dataflow} function has four parameters.
 %% \path[->,bend right=15] (F1-3) edge [above] node
 %%      {\ttfamily\footnotesize expose-alloc.} (F1-4);
 \path[->,bend left=15] (F1-4) edge [above] node
-     {\ttfamily\footnotesize remove\_complex.} (F1-5);
-\path[->,bend left=15] (F1-5) edge [right] node
+     {\ttfamily\footnotesize uncover\_get!} (F1-5);
+\path[->,bend left=15] (F1-5) edge [above] node
+     {\ttfamily\footnotesize remove\_complex.} (F1-6);
+\path[->,bend left=15] (F1-6) edge [right] node
      {\ttfamily\footnotesize explicate\_control} (C3-2);
 \path[->,bend left=15] (C3-2) edge [left] node
      {\ttfamily\footnotesize select\_instr.} (x86-2);