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185 lines
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HTML
4 years ago
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<html lang="en">
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<head>
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<title>Register Basics - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Registers.html#Registers" title="Registers">
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<link rel="next" href="Allocation-Order.html#Allocation-Order" title="Allocation Order">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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</head>
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<body>
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<div class="node">
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<a name="Register-Basics"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Allocation-Order.html#Allocation-Order">Allocation Order</a>,
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Up: <a rel="up" accesskey="u" href="Registers.html#Registers">Registers</a>
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<hr>
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</div>
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<h4 class="subsection">17.7.1 Basic Characteristics of Registers</h4>
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<!-- prevent bad page break with this line -->
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<p>Registers have various characteristics.
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<div class="defun">
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— Macro: <b>FIRST_PSEUDO_REGISTER</b><var><a name="index-FIRST_005fPSEUDO_005fREGISTER-4045"></a></var><br>
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<blockquote><p>Number of hardware registers known to the compiler. They receive
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numbers 0 through <code>FIRST_PSEUDO_REGISTER-1</code>; thus, the first
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pseudo register's number really is assigned the number
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<code>FIRST_PSEUDO_REGISTER</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>FIXED_REGISTERS</b><var><a name="index-FIXED_005fREGISTERS-4046"></a></var><br>
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<blockquote><p><a name="index-fixed-register-4047"></a>An initializer that says which registers are used for fixed purposes
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all throughout the compiled code and are therefore not available for
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general allocation. These would include the stack pointer, the frame
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pointer (except on machines where that can be used as a general
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register when no frame pointer is needed), the program counter on
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machines where that is considered one of the addressable registers,
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and any other numbered register with a standard use.
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<p>This information is expressed as a sequence of numbers, separated by
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commas and surrounded by braces. The <var>n</var>th number is 1 if
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register <var>n</var> is fixed, 0 otherwise.
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<p>The table initialized from this macro, and the table initialized by
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the following one, may be overridden at run time either automatically,
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by the actions of the macro <code>CONDITIONAL_REGISTER_USAGE</code>, or by
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the user with the command options <samp><span class="option">-ffixed-</span><var>reg</var></samp>,
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<samp><span class="option">-fcall-used-</span><var>reg</var></samp> and <samp><span class="option">-fcall-saved-</span><var>reg</var></samp>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>CALL_USED_REGISTERS</b><var><a name="index-CALL_005fUSED_005fREGISTERS-4048"></a></var><br>
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<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
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clobbered (in general) by function calls as well as for fixed
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registers. This macro therefore identifies the registers that are not
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available for general allocation of values that must live across
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function calls.
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<p>If a register has 0 in <code>CALL_USED_REGISTERS</code>, the compiler
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automatically saves it on function entry and restores it on function
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exit, if the register is used within the function.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>CALL_REALLY_USED_REGISTERS</b><var><a name="index-CALL_005fREALLY_005fUSED_005fREGISTERS-4052"></a></var><br>
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<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
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that the entire set of <code>FIXED_REGISTERS</code> be included.
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(<code>CALL_USED_REGISTERS</code> must be a superset of <code>FIXED_REGISTERS</code>).
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This macro is optional. If not specified, it defaults to the value
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of <code>CALL_USED_REGISTERS</code>.
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</p></blockquote></div>
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<div class="defun">
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— 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>
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<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
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value of mode <var>mode</var> in hard register number <var>regno</var> across a
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call without some part of it being clobbered. For most machines this
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macro need not be defined. It is only required for machines that do not
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preserve the entire contents of a register across a call.
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</p></blockquote></div>
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<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>
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<div class="defun">
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— Target Hook: void <b>TARGET_CONDITIONAL_REGISTER_USAGE</b> (<var>void</var>)<var><a name="index-TARGET_005fCONDITIONAL_005fREGISTER_005fUSAGE-4065"></a></var><br>
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<blockquote><p>This hook may conditionally modify five variables
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<code>fixed_regs</code>, <code>call_used_regs</code>, <code>global_regs</code>,
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<code>reg_names</code>, and <code>reg_class_contents</code>, to take into account
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any dependence of these register sets on target flags. The first three
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of these are of type <code>char []</code> (interpreted as Boolean vectors).
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<code>global_regs</code> is a <code>const char *[]</code>, and
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<code>reg_class_contents</code> is a <code>HARD_REG_SET</code>. Before the macro is
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called, <code>fixed_regs</code>, <code>call_used_regs</code>,
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<code>reg_class_contents</code>, and <code>reg_names</code> have been initialized
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from <code>FIXED_REGISTERS</code>, <code>CALL_USED_REGISTERS</code>,
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<code>REG_CLASS_CONTENTS</code>, and <code>REGISTER_NAMES</code>, respectively.
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<code>global_regs</code> has been cleared, and any <samp><span class="option">-ffixed-</span><var>reg</var></samp>,
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<samp><span class="option">-fcall-used-</span><var>reg</var></samp> and <samp><span class="option">-fcall-saved-</span><var>reg</var></samp>
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command options have been applied.
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<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
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flags, you may indicate this to GCC by using this macro to modify
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<code>fixed_regs</code> and <code>call_used_regs</code> to 1 for each of the
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registers in the classes which should not be used by GCC. Also make
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<code>define_register_constraint</code>s return <code>NO_REGS</code> for constraints
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that shouldn't be used.
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<p>(However, if this class is not included in <code>GENERAL_REGS</code> and all
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of the insn patterns whose constraints permit this class are
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controlled by target switches, then GCC will automatically avoid using
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these registers when the target switches are opposed to them.)
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>INCOMING_REGNO</b> (<var>out</var>)<var><a name="index-INCOMING_005fREGNO-4068"></a></var><br>
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<blockquote><p>Define this macro if the target machine has register windows. This C
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expression returns the register number as seen by the called function
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corresponding to the register number <var>out</var> as seen by the calling
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function. Return <var>out</var> if register number <var>out</var> is not an
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outbound register.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>OUTGOING_REGNO</b> (<var>in</var>)<var><a name="index-OUTGOING_005fREGNO-4069"></a></var><br>
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<blockquote><p>Define this macro if the target machine has register windows. This C
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expression returns the register number as seen by the calling function
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corresponding to the register number <var>in</var> as seen by the called
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function. Return <var>in</var> if register number <var>in</var> is not an inbound
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register.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>LOCAL_REGNO</b> (<var>regno</var>)<var><a name="index-LOCAL_005fREGNO-4070"></a></var><br>
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<blockquote><p>Define this macro if the target machine has register windows. This C
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expression returns true if the register is call-saved but is in the
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register window. Unlike most call-saved registers, such registers
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need not be explicitly restored on function exit or during non-local
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gotos.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>PC_REGNUM</b><var><a name="index-PC_005fREGNUM-4071"></a></var><br>
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<blockquote><p>If the program counter has a register number, define this as that
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register number. Otherwise, do not define it.
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</p></blockquote></div>
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</body></html>
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