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<h4 class="subsection">17.7.2 Order of Allocation of Registers</h4>
<p><a name="index-order-of-register-allocation-4072"></a><a name="index-register-allocation-order-4073"></a>
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Registers are allocated in order.
<div class="defun">
— Macro: <b>REG_ALLOC_ORDER</b><var><a name="index-REG_005fALLOC_005fORDER-4074"></a></var><br>
<blockquote><p>If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which GCC should prefer
to use them (from most preferred to least).
<p>If this macro is not defined, registers are used lowest numbered first
(all else being equal).
<p>One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On such
machines, define <code>REG_ALLOC_ORDER</code> to be an initializer that lists
the highest numbered allocable register first.
</p></blockquote></div>
<div class="defun">
— Macro: <b>ADJUST_REG_ALLOC_ORDER</b><var><a name="index-ADJUST_005fREG_005fALLOC_005fORDER-4075"></a></var><br>
<blockquote><p>A C statement (sans semicolon) to choose the order in which to allocate
hard registers for pseudo-registers local to a basic block.
<p>Store the desired register order in the array <code>reg_alloc_order</code>.
Element 0 should be the register to allocate first; element 1, the next
register; and so on.
<p>The macro body should not assume anything about the contents of
<code>reg_alloc_order</code> before execution of the macro.
<p>On most machines, it is not necessary to define this macro.
</p></blockquote></div>
<div class="defun">
— Macro: <b>HONOR_REG_ALLOC_ORDER</b><var><a name="index-HONOR_005fREG_005fALLOC_005fORDER-4076"></a></var><br>
<blockquote><p>Normally, IRA tries to estimate the costs for saving a register in the
prologue and restoring it in the epilogue. This discourages it from
using call-saved registers. If a machine wants to ensure that IRA
allocates registers in the order given by REG_ALLOC_ORDER even if some
call-saved registers appear earlier than call-used ones, then define this
macro as a C expression to nonzero. Default is 0.
</p></blockquote></div>
<div class="defun">
— Macro: <b>IRA_HARD_REGNO_ADD_COST_MULTIPLIER</b> (<var>regno</var>)<var><a name="index-IRA_005fHARD_005fREGNO_005fADD_005fCOST_005fMULTIPLIER-4077"></a></var><br>
<blockquote><p>In some case register allocation order is not enough for the
Integrated Register Allocator (<acronym>IRA</acronym>) to generate a good code.
If this macro is defined, it should return a floating point value
based on <var>regno</var>. The cost of using <var>regno</var> for a pseudo will
be increased by approximately the pseudo's usage frequency times the
value returned by this macro. Not defining this macro is equivalent
to having it always return <code>0.0</code>.
<p>On most machines, it is not necessary to define this macro.
</p></blockquote></div>
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