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  17 <H2><A NAME="SEC30" HREF="suif1_toc.html#TOC30">Operands</A></H2>
  18 <P>
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  24 Most SUIF instructions have operands that are represented by objects of
  25 the <CODE>operand</CODE> class, which is implemented in the files
  26 <TT>`operand.h'</TT> and <CODE>operand.cc</CODE>.  An operand may have three
  27 different kinds of values:
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  29 </P>
  30
  31 <UL>
  32 <LI>
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  35 It may be a null operand.  The <CODE>is_null</CODE> method tests for this
  36 case.  Null operands occur when an operand field in an instruction is
  37 unused.
  38
  39 <LI>
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  44 An operand may refer directly to the symbol for a variable.  The
  45 <CODE>is_symbol</CODE> method tests for this case.  The <CODE>symbol</CODE> and
  46 <CODE>set_symbol</CODE> methods access the symbol field.  A variable in a
  47 source operand indicates that the contents of the variable are used.  In
  48 a destination operand, a variable is assigned the result of the
  49 instruction.
  50
  51 <LI>
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  56 An operand may also point to another instruction.  The <CODE>is_instr</CODE>
  57 method tests for this, and the <CODE>instr</CODE> and <CODE>set_instr</CODE> methods
  58 access the pointer field.  This is used to implement expression trees
  59 and, in low-SUIF, to refer to the results of other instructions in a
  60 flat list.  An instruction pointer in a source operand means that the
  61 result of that instruction is used as the operand.  Conversely, an
  62 instruction pointer in a destination operand indicates that the result
  63 value is used by that instruction.
  64 </UL>
  65
  66 <P>
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  68 The <CODE>is_expr</CODE> method is very similar to <CODE>is_instr</CODE>.  Besides
  69 checking if the operand holds an instruction pointer, it also tests if
  70 that instruction is a subexpression that is not contained in a separate
  71 <CODE>tree_instr</CODE> node (i.e. it's not in a flat list).  This method
  72 should not be used for destination operands.
  73
  74 </P>
  75 <P>
  76 Before changing a source operand from an instruction pointer to some
  77 other value, the instruction must be removed.  The <CODE>remove</CODE> method
  78 checks if the operand is an instruction, and if so, calls the
  79 instruction's <CODE>remove</CODE> method.  See section <A HREF="suif1_28.html#SEC28">Source Operands</A>.
  80
  81 </P>
  82 <P>
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  85 The operand class includes several methods to simplify common
  86 operations.  One frequently needs to know the type of a particular
  87 operand.  This can be determined from the variable's type or the
  88 instruction's result type, but checking for these different kinds of
  89 operands and then extracting the type is cumbersome.  Instead, you can
  90 use the <CODE>type</CODE> method which performs this operation for you.  It
  91 returns the SUIF <CODE>void</CODE> type for null operands.  Another common
  92 operation is testing an operand to see if it contains an integer
  93 constant from a load constant instruction.  The <CODE>is_const_int</CODE>
  94 method checks if this is the case, and if so, it also returns the value
  95 of the constant.
  96
  97 </P>
  98 <P>
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 101 Operands have two different methods for printing themselves out to a
 102 file as text.  The <CODE>print</CODE> method is used by the library when
 103 printing instructions.  Because source and destination operands are
 104 handled differently, <CODE>print</CODE> requires that you specify the
 105 instruction that contains the operand so that it can determine how the
 106 operand is being used.  However, when debugging a program, you may not
 107 know which instruction contains an operand.  If you do know that it is
 108 used as a source operand, you can use the <CODE>print_source</CODE> method to
 109 print it without specifying the instruction.
 110
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