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   6 <TITLE>The SUIF Version 1 Library - Variable Features</TITLE>
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  17 <H3><A NAME="SEC56" HREF="suif1_toc.html#TOC56">Basic Features of Variables</A></H3>
  18
  19 <P>
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  21 <A NAME="IDX374"></A>
  22 The type of a variable can be accessed via the <CODE>type</CODE> and
  23 <CODE>set_type</CODE> methods.  The type must be visible in the scope where
  24 the variable is defined.  Since the type specifies the amount of storage
  25 occupied by the variable, the size of the type must not be zero.  This
  26 means that function types, void types, incomplete structure and union
  27 types, and array types with unknown bounds cannot be used for variables
  28 <A NAME="DOCF4" HREF="suif1_foot.html#FOOT4">(4)</A>.  Pointers to those types are acceptable, of course.
  29 Sub-variables may have any type so long as the size of that type and
  30 the sub-variable's offset allow it to remain within the parent
  31 variable.
  32
  33 </P>
  34 <P>
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  40 Variable symbols may also be allocated to machine registers.  A flag is
  41 used to indicate variables that are contained in registers; the
  42 <CODE>is_reg</CODE> method returns the value of this flag and the
  43 <CODE>set_reg</CODE> and <CODE>reset_reg</CODE> methods change its value.  The
  44 <CODE>reg_num</CODE> annotation can be attached to a register variable to
  45 specify the corresponding machine register number (see section <A HREF="suif1_78.html#SEC78">Miscellaneous Annotations</A>).  Variables cannot be registers if they have variable
  46 definitions.  Whether a sub-variable is a register or not is slaved to
  47 this property in its parent.
  48
  49 </P>
  50 <P>
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  52 <A NAME="IDX381"></A>
  53 Symbols for variables that are not allocated on the stack must have
  54 separate variable definitions (see section <A HREF="suif1_58.html#SEC58">Variable Definitions</A>) or else
  55 they are merely declarations of external symbols.  The
  56 <CODE>has_var_def</CODE> method checks if a variable symbol has a separate
  57 definition, and the <CODE>definition</CODE> method returns a pointer to the
  58 definition.  It is an error to call <CODE>definition</CODE> for a variable
  59 that does not have a definition, so be sure to check first.
  60 Sub-variables are never allowed to have their own definitions.
  61
  62 </P>
  63 <P>
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  65 <A NAME="IDX383"></A>
  66 The <CODE>var_sym</CODE> class includes several methods to check various
  67 attributes and flags for variables.  The <CODE>is_auto</CODE> method checks if
  68 a variable is in the automatic storage class, i.e. if it will be
  69 allocated on the stack.  Global variables and static local variables
  70 (those with variable definitions) are not automatic and <CODE>is_auto</CODE>
  71 returns <CODE>FALSE</CODE> for them; other variables are automatic.  The
  72 <CODE>is_static</CODE> method is the opposite of <CODE>is_auto</CODE>, except that
  73 register variables are not considered to be static.
  74
  75 </P>
  76 <P>
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  80 The formal parameters of a procedure are represented by variables that
  81 have a special flag set.  The <CODE>is_param</CODE> method returns the value
  82 of this flag, and the <CODE>set_param</CODE> and <CODE>reset_param</CODE> methods
  83 change its value.  Identifying the formals with flags does not allow
  84 their order to be specified, so the procedure symbol table maintains a
  85 list of the formals (see section <A HREF="suif1_44.html#SEC44">Procedure Symbol Tables</A>).  All of the
  86 variables for which <CODE>is_param</CODE> returns <CODE>TRUE</CODE> should be on the
  87 list of parameters, and conversely, all variables on the parameter list
  88 should have the flag set.  Sub-variables are not allowed to be
  89 parameters; sub-variables of parameters are allowed, but these are not
  90 considered parameters themselves.
  91
  92 </P>
  93 <P>
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  97 Because many compiler optimizations are only applicable to variables
  98 that are not aliased, it is important to identify such variables.  Each
  99 variable symbol includes a flag to indicate whether the variable has its
 100 address taken, and thus whether it could be aliased.  The
 101 <CODE>is_addr_taken</CODE> method returns the value of this flag.  The
 102 front-end should set these flags, but they may also need to be updated
 103 as the code is transformed.  The <CODE>set_addr_taken</CODE> and
 104 <CODE>reset_addr_taken</CODE> methods can be used to change the flag for a
 105 particular variable symbol.  For sub-variables, this property is
 106 slaved to this property in the parent variable.
 107
 108 </P>
 109 <P>
 110 <A NAME="IDX390"></A>
 111 Besides requiring that variables not be aliased, most optimizations also
 112 do not apply to aggregate data structures.  The <CODE>is_scalar</CODE> method
 113 checks the type of a variable symbol and returns <CODE>TRUE</CODE> if it is
 114 not an aggregate type.  Arrays, structures, and unions are considered to
 115 be aggregates.
 116
 117 </P>
 118 <P>
 119 <A NAME="IDX391"></A>
 120 In general, most optimizations target scalar variables that are
 121 guaranteed not to be aliased.  Since it is difficult to guarantee that
 122 global variables with external linkage are not addressed, those
 123 variables may be aliased even if their flags indicate that they are not
 124 addressed.  Rather than checking these conditions separately, the
 125 library provides one method to check them all.  The <CODE>is_spilled</CODE>
 126 method returns <CODE>TRUE</CODE> for variables that are static, addressed, or
 127 aggregates.  Variables that are not spilled can then be targeted for
 128 optimization.
 129
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