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<a name="Liveness-information"></a>
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<p>
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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> [<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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<a name="Liveness-information-1"></a>
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<h3 class="section">14.5 Liveness information</h3>
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<a name="index-Liveness-representation"></a>
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<p>Liveness information is useful to determine whether some register is
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“live” at given point of program, i.e. that it contains a value that
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may be used at a later point in the program. This information is
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used, for instance, during register allocation, as the pseudo
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registers only need to be assigned to a unique hard register or to a
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stack slot if they are live. The hard registers and stack slots may
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be freely reused for other values when a register is dead.
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</p>
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<p>Liveness information is available in the back end starting with
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<code>pass_df_initialize</code> and ending with <code>pass_df_finish</code>. Three
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flavors of live analysis are available: With <code>LR</code>, it is possible
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to determine at any point <code>P</code> in the function if the register may be
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used on some path from <code>P</code> to the end of the function. With
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<code>UR</code>, it is possible to determine if there is a path from the
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beginning of the function to <code>P</code> that defines the variable.
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<code>LIVE</code> is the intersection of the <code>LR</code> and <code>UR</code> and a
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variable is live at <code>P</code> if there is both an assignment that reaches
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it from the beginning of the function and a use that can be reached on
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some path from <code>P</code> to the end of the function.
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</p>
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<p>In general <code>LIVE</code> is the most useful of the three. The macros
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<code>DF_[LR,UR,LIVE]_[IN,OUT]</code> can be used to access this information.
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The macros take a basic block number and return a bitmap that is indexed
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by the register number. This information is only guaranteed to be up to
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date after calls are made to <code>df_analyze</code>. See the file
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<code>df-core.c</code> for details on using the dataflow.
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</p>
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<a name="index-REG_005fDEAD_002c-REG_005fUNUSED"></a>
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<p>The liveness information is stored partly in the RTL instruction stream
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and partly in the flow graph. Local information is stored in the
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instruction stream: Each instruction may contain <code>REG_DEAD</code> notes
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representing that the value of a given register is no longer needed, or
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<code>REG_UNUSED</code> notes representing that the value computed by the
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instruction is never used. The second is useful for instructions
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computing multiple values at once.
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</p>
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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> [<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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