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  25. <title>Liveness information (GNU Compiler Collection (GCC) Internals)</title>

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  70. <a name="Liveness-information"></a>

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  72. <p>

  73. Previous: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="p" rel="prev">Maintaining the CFG</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

  74. </div>

  75. <hr>

  76. <a name="Liveness-information-1"></a>

  77. <h3 class="section">15.5 Liveness information</h3>

  78. <a name="index-Liveness-representation"></a>

  79. <p>Liveness information is useful to determine whether some register is

  80. &ldquo;live&rdquo; at given point of program, i.e. that it contains a value that

  81. may be used at a later point in the program.  This information is

  82. used, for instance, during register allocation, as the pseudo

  83. registers only need to be assigned to a unique hard register or to a

  84. stack slot if they are live.  The hard registers and stack slots may

  85. be freely reused for other values when a register is dead.

  86. </p>

  87. <p>Liveness information is available in the back end starting with

  88. <code>pass_df_initialize</code> and ending with <code>pass_df_finish</code>.  Three

  89. flavors of live analysis are available: With <code>LR</code>, it is possible

  90. to determine at any point <code>P</code> in the function if the register may be

  91. used on some path from <code>P</code> to the end of the function.  With

  92. <code>UR</code>, it is possible to determine if there is a path from the

  93. beginning of the function to <code>P</code> that defines the variable.

  94. <code>LIVE</code> is the intersection of the <code>LR</code> and <code>UR</code> and a

  95. variable is live at <code>P</code> if there is both an assignment that reaches

  96. it from the beginning of the function and a use that can be reached on

  97. some path from <code>P</code> to the end of the function.

  98. </p>

  99. <p>In general <code>LIVE</code> is the most useful of the three.  The macros

  100. <code>DF_[LR,UR,LIVE]_[IN,OUT]</code> can be used to access this information.

  101. The macros take a basic block number and return a bitmap that is indexed

  102. by the register number.  This information is only guaranteed to be up to

  103. date after calls are made to <code>df_analyze</code>.  See the file

  104. <code>df-core.c</code> for details on using the dataflow.

  105. </p>

  106. 

  107. <a name="index-REG_005fDEAD_002c-REG_005fUNUSED"></a>

  108. <p>The liveness information is stored partly in the RTL instruction stream

  109. and partly in the flow graph.  Local information is stored in the

  110. instruction stream: Each instruction may contain <code>REG_DEAD</code> notes

  111. representing that the value of a given register is no longer needed, or

  112. <code>REG_UNUSED</code> notes representing that the value computed by the

  113. instruction is never used.  The second is useful for instructions

  114. computing multiple values at once.

  115. </p>

  116. 

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  121. Previous: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="p" rel="prev">Maintaining the CFG</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

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