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<h3 class="section">14.5 Liveness information</h3>
<p><a name="index-Liveness-representation-3227"></a>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>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>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-3228"></a>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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