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   6 <TITLE>The SUIF Version 1 Library - Loop Nodes</TITLE>
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  17 <H3><A NAME="SEC16" HREF="suif1_toc.html#TOC16">Loop Nodes</A></H3>
  18 <P>
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  20 <A NAME="IDX65"></A>
  21
  22 </P>
  23 <P>
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  29 A <CODE>tree_loop</CODE> node represents a "do-while" loop.  It contains two
  30 tree node lists: the <CODE>body</CODE> and the <CODE>test</CODE>.  The <CODE>body</CODE>
  31 list comes first and holds the loop body.  The <CODE>test</CODE> list contains
  32 code to evaluate the "while" expression and conditionally branch back
  33 to the beginning of the body.
  34
  35 </P>
  36 <P>
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  43 There are three labels associated with a <CODE>tree_loop</CODE> node:
  44 <CODE>toplab</CODE>, <CODE>contlab</CODE>, and <CODE>brklab</CODE>.  The <CODE>toplab</CODE>
  45 label marks the beginning of the loop <CODE>body</CODE>; the <CODE>test</CODE> list
  46 typically contains a conditional branch back to <CODE>toplab</CODE>.  A
  47 "continue" statement in the loop body requires a jump over the rest of
  48 the body to the beginning of the test code.  The <CODE>contlab</CODE> label is
  49 positioned at the beginning of the <CODE>test</CODE> list for this purpose.
  50 Similarly, a "break" statement in the loop is translated to a jump to
  51 the <CODE>brklab</CODE> label which is located immediately after the loop.
  52 These <CODE>tree_loop</CODE> labels must be defined in the scope of the loop
  53 node, but the label positions are implicit rather than being marked by
  54 label instructions.  When the <CODE>tree_loop</CODE> is expanded to low-SUIF
  55 form, the label instructions are inserted into the tree node lists.
  56
  57 </P>
  58 <P>
  59 Because the loop nodes are only intended for use with structured control
  60 flow, certain restrictions on the contents of the <CODE>tree_loop</CODE>
  61 lists are required.  The <CODE>test</CODE> part has exactly the same
  62 restrictions as the <CODE>header</CODE> of a <CODE>tree_if</CODE> except that it
  63 is the <CODE>toplab</CODE> label instead of the <CODE>jumpto</CODE> label to which
  64 jumps and branches are allowed and expected.  See section <A HREF="suif1_15.html#SEC15">If Nodes</A>.  Note
  65 in particular that use of the <CODE>brklab</CODE> and <CODE>contlab</CODE> labels
  66 is not allowed in the <CODE>test</CODE> list.
  67
  68 </P>
  69 <P>
  70 The <CODE>body</CODE> list has restrictions analogous to the restriction on
  71 <CODE>then_part</CODE> and <CODE>else_part</CODE> lists of a <CODE>tree_if</CODE>:
  72 arbitrary nesting of other <CODE>tree_node</CODE>s is allowed, but control
  73 flow into or out of the <CODE>body</CODE> is not allowed.  The <CODE>body</CODE>
  74 list is allowed slightly more leeway, though: jumps or branches are
  75 allowed from anywhere in the <CODE>body</CODE> to the <CODE>brklab</CODE> or the
  76 <CODE>contlab</CODE> labels.  This is the only place where the <CODE>brklab</CODE>
  77 or <CODE>contlab</CODE> labels can be used--they cannot be used outside the
  78 <CODE>tree_loop</CODE> or in the <CODE>test</CODE> list.
  79
  80 </P>
  81 <P>
  82 Note that either or both of the <CODE>test</CODE> and <CODE>body</CODE> parts may
  83 be empty lists, but if the <CODE>test</CODE> part is empty, the loop is
  84 degenerate--it will always execute exactly once.
  85
  86 </P>
  87 <P>
  88 For example, the following C code could be translated into the SUIF code
  89 shown in a simplified form below.  Note that because the <CODE>test</CODE>
  90 code is at the <EM>bottom</EM> of the loop, the "while" loop must be
  91 converted to a "do-while" loop guarded by an "if" node.  See section <A HREF="suif1_15.html#SEC15">If Nodes</A>.
  92
  93 </P>
  94
  95 <PRE>
  96 while (k &#62; 0) {
  97     if (k = 1) break;
  98     k = k - 1;
  99     if (k &#60; 10) continue;
 100     k = k - 10;
 101 }
 102 </PRE>
 103
 104
 105 <PRE>
 106 IF (Jumpto=L:__L1)
 107 IF HEADER
 108     bfalse e1, L:__L1
 109       e1: sl e2, k
 110         e2: ldc 0
 111 IF THEN
 112     LOOP (Top=L:__L2 Cont=L:__L3 Brk=L:__L4)
 113     LOOP BODY
 114         btrue e1, L:__L4
 115           e1: seq k, e2
 116             e2: ldc 1
 117         sub k = k, e1
 118           e1: ldc 1
 119         btrue e1, L:__L3
 120           e1: sl k, e2
 121             e2: ldc 10
 122         sub k = k, e1
 123           e1: ldc 10
 124     LOOP TEST
 125         btrue e1, L:__L2
 126           e1: sl e2, k
 127             e2: ldc 0
 128     LOOP END
 129 IF ELSE
 130 IF END
 131 </PRE>
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