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