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<h4 class="subsection">17.7.1 Basic Characteristics of Registers</h4>
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<p>Registers have various characteristics.
<div class="defun">
— Macro: <b>FIRST_PSEUDO_REGISTER</b><var><a name="index-FIRST_005fPSEUDO_005fREGISTER-4045"></a></var><br>
<blockquote><p>Number of hardware registers known to the compiler. They receive
numbers 0 through <code>FIRST_PSEUDO_REGISTER-1</code>; thus, the first
pseudo register's number really is assigned the number
<code>FIRST_PSEUDO_REGISTER</code>.
</p></blockquote></div>
<div class="defun">
— Macro: <b>FIXED_REGISTERS</b><var><a name="index-FIXED_005fREGISTERS-4046"></a></var><br>
<blockquote><p><a name="index-fixed-register-4047"></a>An initializer that says which registers are used for fixed purposes
all throughout the compiled code and are therefore not available for
general allocation. These would include the stack pointer, the frame
pointer (except on machines where that can be used as a general
register when no frame pointer is needed), the program counter on
machines where that is considered one of the addressable registers,
and any other numbered register with a standard use.
<p>This information is expressed as a sequence of numbers, separated by
commas and surrounded by braces. The <var>n</var>th number is 1 if
register <var>n</var> is fixed, 0 otherwise.
<p>The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either automatically,
by the actions of the macro <code>CONDITIONAL_REGISTER_USAGE</code>, or by
the user with the command options <samp><span class="option">-ffixed-</span><var>reg</var></samp>,
<samp><span class="option">-fcall-used-</span><var>reg</var></samp> and <samp><span class="option">-fcall-saved-</span><var>reg</var></samp>.
</p></blockquote></div>
<div class="defun">
— Macro: <b>CALL_USED_REGISTERS</b><var><a name="index-CALL_005fUSED_005fREGISTERS-4048"></a></var><br>
<blockquote><p><a name="index-call_002dused-register-4049"></a><a name="index-call_002dclobbered-register-4050"></a><a name="index-call_002dsaved-register-4051"></a>Like <code>FIXED_REGISTERS</code> but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers. This macro therefore identifies the registers that are not
available for general allocation of values that must live across
function calls.
<p>If a register has 0 in <code>CALL_USED_REGISTERS</code>, the compiler
automatically saves it on function entry and restores it on function
exit, if the register is used within the function.
</p></blockquote></div>
<div class="defun">
— Macro: <b>CALL_REALLY_USED_REGISTERS</b><var><a name="index-CALL_005fREALLY_005fUSED_005fREGISTERS-4052"></a></var><br>
<blockquote><p><a name="index-call_002dused-register-4053"></a><a name="index-call_002dclobbered-register-4054"></a><a name="index-call_002dsaved-register-4055"></a>Like <code>CALL_USED_REGISTERS</code> except this macro doesn't require
that the entire set of <code>FIXED_REGISTERS</code> be included.
(<code>CALL_USED_REGISTERS</code> must be a superset of <code>FIXED_REGISTERS</code>).
This macro is optional. If not specified, it defaults to the value
of <code>CALL_USED_REGISTERS</code>.
</p></blockquote></div>
<div class="defun">
— Macro: <b>HARD_REGNO_CALL_PART_CLOBBERED</b> (<var>regno, mode</var>)<var><a name="index-HARD_005fREGNO_005fCALL_005fPART_005fCLOBBERED-4056"></a></var><br>
<blockquote><p><a name="index-call_002dused-register-4057"></a><a name="index-call_002dclobbered-register-4058"></a><a name="index-call_002dsaved-register-4059"></a>A C expression that is nonzero if it is not permissible to store a
value of mode <var>mode</var> in hard register number <var>regno</var> across a
call without some part of it being clobbered. For most machines this
macro need not be defined. It is only required for machines that do not
preserve the entire contents of a register across a call.
</p></blockquote></div>
<p><a name="index-fixed_005fregs-4060"></a><a name="index-call_005fused_005fregs-4061"></a><a name="index-global_005fregs-4062"></a><a name="index-reg_005fnames-4063"></a><a name="index-reg_005fclass_005fcontents-4064"></a>
<div class="defun">
— Target Hook: void <b>TARGET_CONDITIONAL_REGISTER_USAGE</b> (<var>void</var>)<var><a name="index-TARGET_005fCONDITIONAL_005fREGISTER_005fUSAGE-4065"></a></var><br>
<blockquote><p>This hook may conditionally modify five variables
<code>fixed_regs</code>, <code>call_used_regs</code>, <code>global_regs</code>,
<code>reg_names</code>, and <code>reg_class_contents</code>, to take into account
any dependence of these register sets on target flags. The first three
of these are of type <code>char []</code> (interpreted as Boolean vectors).
<code>global_regs</code> is a <code>const char *[]</code>, and
<code>reg_class_contents</code> is a <code>HARD_REG_SET</code>. Before the macro is
called, <code>fixed_regs</code>, <code>call_used_regs</code>,
<code>reg_class_contents</code>, and <code>reg_names</code> have been initialized
from <code>FIXED_REGISTERS</code>, <code>CALL_USED_REGISTERS</code>,
<code>REG_CLASS_CONTENTS</code>, and <code>REGISTER_NAMES</code>, respectively.
<code>global_regs</code> has been cleared, and any <samp><span class="option">-ffixed-</span><var>reg</var></samp>,
<samp><span class="option">-fcall-used-</span><var>reg</var></samp> and <samp><span class="option">-fcall-saved-</span><var>reg</var></samp>
command options have been applied.
<p><a name="index-disabling-certain-registers-4066"></a><a name="index-controlling-register-usage-4067"></a>If the usage of an entire class of registers depends on the target
flags, you may indicate this to GCC by using this macro to modify
<code>fixed_regs</code> and <code>call_used_regs</code> to 1 for each of the
registers in the classes which should not be used by GCC. Also make
<code>define_register_constraint</code>s return <code>NO_REGS</code> for constraints
that shouldn't be used.
<p>(However, if this class is not included in <code>GENERAL_REGS</code> and all
of the insn patterns whose constraints permit this class are
controlled by target switches, then GCC will automatically avoid using
these registers when the target switches are opposed to them.)
</p></blockquote></div>
<div class="defun">
— Macro: <b>INCOMING_REGNO</b> (<var>out</var>)<var><a name="index-INCOMING_005fREGNO-4068"></a></var><br>
<blockquote><p>Define this macro if the target machine has register windows. This C
expression returns the register number as seen by the called function
corresponding to the register number <var>out</var> as seen by the calling
function. Return <var>out</var> if register number <var>out</var> is not an
outbound register.
</p></blockquote></div>
<div class="defun">
— Macro: <b>OUTGOING_REGNO</b> (<var>in</var>)<var><a name="index-OUTGOING_005fREGNO-4069"></a></var><br>
<blockquote><p>Define this macro if the target machine has register windows. This C
expression returns the register number as seen by the calling function
corresponding to the register number <var>in</var> as seen by the called
function. Return <var>in</var> if register number <var>in</var> is not an inbound
register.
</p></blockquote></div>
<div class="defun">
— Macro: <b>LOCAL_REGNO</b> (<var>regno</var>)<var><a name="index-LOCAL_005fREGNO-4070"></a></var><br>
<blockquote><p>Define this macro if the target machine has register windows. This C
expression returns true if the register is call-saved but is in the
register window. Unlike most call-saved registers, such registers
need not be explicitly restored on function exit or during non-local
gotos.
</p></blockquote></div>
<div class="defun">
— Macro: <b>PC_REGNUM</b><var><a name="index-PC_005fREGNUM-4071"></a></var><br>
<blockquote><p>If the program counter has a register number, define this as that
register number. Otherwise, do not define it.
</p></blockquote></div>
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