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<h4 class="subsection">6.40.1 Simple Constraints</h4>

<p><a name="index-simple-constraints-2534"></a>
The simplest kind of constraint is a string full of letters, each of
which describes one kind of operand that is permitted.  Here are
the letters that are allowed:

     <dl>
<dt>whitespace<dd>Whitespace characters are ignored and can be inserted at any position
except the first.  This enables each alternative for different operands to
be visually aligned in the machine description even if they have different
number of constraints and modifiers.

     <p><a name="index-g_t_0040samp_007bm_007d-in-constraint-2535"></a><a name="index-memory-references-in-constraints-2536"></a><br><dt>&lsquo;<samp><span class="samp">m</span></samp>&rsquo;<dd>A memory operand is allowed, with any kind of address that the machine
supports in general. 
Note that the letter used for the general memory constraint can be
re-defined by a back end using the <code>TARGET_MEM_CONSTRAINT</code> macro.

     <p><a name="index-offsettable-address-2537"></a><a name="index-g_t_0040samp_007bo_007d-in-constraint-2538"></a><br><dt>&lsquo;<samp><span class="samp">o</span></samp>&rsquo;<dd>A memory operand is allowed, but only if the address is
<dfn>offsettable</dfn>.  This means that adding a small integer (actually,
the width in bytes of the operand, as determined by its machine mode)
may be added to the address and the result is also a valid memory
address.

     <p><a name="index-autoincrement_002fdecrement-addressing-2539"></a>For example, an address which is constant is offsettable; so is an
address that is the sum of a register and a constant (as long as a
slightly larger constant is also within the range of address-offsets
supported by the machine); but an autoincrement or autodecrement
address is not offsettable.  More complicated indirect/indexed
addresses may or may not be offsettable depending on the other
addressing modes that the machine supports.

     <p>Note that in an output operand which can be matched by another
operand, the constraint letter &lsquo;<samp><span class="samp">o</span></samp>&rsquo; is valid only when accompanied
by both &lsquo;<samp><span class="samp">&lt;</span></samp>&rsquo; (if the target machine has predecrement addressing)
and &lsquo;<samp><span class="samp">&gt;</span></samp>&rsquo; (if the target machine has preincrement addressing).

     <p><a name="index-g_t_0040samp_007bV_007d-in-constraint-2540"></a><br><dt>&lsquo;<samp><span class="samp">V</span></samp>&rsquo;<dd>A memory operand that is not offsettable.  In other words, anything that
would fit the &lsquo;<samp><span class="samp">m</span></samp>&rsquo; constraint but not the &lsquo;<samp><span class="samp">o</span></samp>&rsquo; constraint.

     <p><a name="index-g_t_0040samp_007b_003c_007d-in-constraint-2541"></a><br><dt>&lsquo;<samp><span class="samp">&lt;</span></samp>&rsquo;<dd>A memory operand with autodecrement addressing (either predecrement or
postdecrement) is allowed.

     <p><a name="index-g_t_0040samp_007b_003e_007d-in-constraint-2542"></a><br><dt>&lsquo;<samp><span class="samp">&gt;</span></samp>&rsquo;<dd>A memory operand with autoincrement addressing (either preincrement or
postincrement) is allowed.

     <p><a name="index-g_t_0040samp_007br_007d-in-constraint-2543"></a><a name="index-registers-in-constraints-2544"></a><br><dt>&lsquo;<samp><span class="samp">r</span></samp>&rsquo;<dd>A register operand is allowed provided that it is in a general
register.

     <p><a name="index-constants-in-constraints-2545"></a><a name="index-g_t_0040samp_007bi_007d-in-constraint-2546"></a><br><dt>&lsquo;<samp><span class="samp">i</span></samp>&rsquo;<dd>An immediate integer operand (one with constant value) is allowed. 
This includes symbolic constants whose values will be known only at
assembly time or later.

     <p><a name="index-g_t_0040samp_007bn_007d-in-constraint-2547"></a><br><dt>&lsquo;<samp><span class="samp">n</span></samp>&rsquo;<dd>An immediate integer operand with a known numeric value is allowed. 
Many systems cannot support assembly-time constants for operands less
than a word wide.  Constraints for these operands should use &lsquo;<samp><span class="samp">n</span></samp>&rsquo;
rather than &lsquo;<samp><span class="samp">i</span></samp>&rsquo;.

     <p><a name="index-g_t_0040samp_007bI_007d-in-constraint-2548"></a><br><dt>&lsquo;<samp><span class="samp">I</span></samp>&rsquo;, &lsquo;<samp><span class="samp">J</span></samp>&rsquo;, &lsquo;<samp><span class="samp">K</span></samp>&rsquo;, <small class="dots">...</small> &lsquo;<samp><span class="samp">P</span></samp>&rsquo;<dd>Other letters in the range &lsquo;<samp><span class="samp">I</span></samp>&rsquo; through &lsquo;<samp><span class="samp">P</span></samp>&rsquo; may be defined in
a machine-dependent fashion to permit immediate integer operands with
explicit integer values in specified ranges.  For example, on the
68000, &lsquo;<samp><span class="samp">I</span></samp>&rsquo; is defined to stand for the range of values 1 to 8. 
This is the range permitted as a shift count in the shift
instructions.

     <p><a name="index-g_t_0040samp_007bE_007d-in-constraint-2549"></a><br><dt>&lsquo;<samp><span class="samp">E</span></samp>&rsquo;<dd>An immediate floating operand (expression code <code>const_double</code>) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).

     <p><a name="index-g_t_0040samp_007bF_007d-in-constraint-2550"></a><br><dt>&lsquo;<samp><span class="samp">F</span></samp>&rsquo;<dd>An immediate floating operand (expression code <code>const_double</code> or
<code>const_vector</code>) is allowed.

     <p><a name="index-g_t_0040samp_007bG_007d-in-constraint-2551"></a><a name="index-g_t_0040samp_007bH_007d-in-constraint-2552"></a><br><dt>&lsquo;<samp><span class="samp">G</span></samp>&rsquo;, &lsquo;<samp><span class="samp">H</span></samp>&rsquo;<dd>&lsquo;<samp><span class="samp">G</span></samp>&rsquo; and &lsquo;<samp><span class="samp">H</span></samp>&rsquo; may be defined in a machine-dependent fashion to
permit immediate floating operands in particular ranges of values.

     <p><a name="index-g_t_0040samp_007bs_007d-in-constraint-2553"></a><br><dt>&lsquo;<samp><span class="samp">s</span></samp>&rsquo;<dd>An immediate integer operand whose value is not an explicit integer is
allowed.

     <p>This might appear strange; if an insn allows a constant operand with a
value not known at compile time, it certainly must allow any known
value.  So why use &lsquo;<samp><span class="samp">s</span></samp>&rsquo; instead of &lsquo;<samp><span class="samp">i</span></samp>&rsquo;?  Sometimes it allows
better code to be generated.

     <p>For example, on the 68000 in a fullword instruction it is possible to
use an immediate operand; but if the immediate value is between &minus;128
and 127, better code results from loading the value into a register and
using the register.  This is because the load into the register can be
done with a &lsquo;<samp><span class="samp">moveq</span></samp>&rsquo; instruction.  We arrange for this to happen
by defining the letter &lsquo;<samp><span class="samp">K</span></samp>&rsquo; to mean &ldquo;any integer outside the
range &minus;128 to 127&rdquo;, and then specifying &lsquo;<samp><span class="samp">Ks</span></samp>&rsquo; in the operand
constraints.

     <p><a name="index-g_t_0040samp_007bg_007d-in-constraint-2554"></a><br><dt>&lsquo;<samp><span class="samp">g</span></samp>&rsquo;<dd>Any register, memory or immediate integer operand is allowed, except for
registers that are not general registers.

     <p><a name="index-g_t_0040samp_007bX_007d-in-constraint-2555"></a><br><dt>&lsquo;<samp><span class="samp">X</span></samp>&rsquo;<dd>Any operand whatsoever is allowed.

     <p><a name="index-g_t_0040samp_007b0_007d-in-constraint-2556"></a><a name="index-digits-in-constraint-2557"></a><br><dt>&lsquo;<samp><span class="samp">0</span></samp>&rsquo;, &lsquo;<samp><span class="samp">1</span></samp>&rsquo;, &lsquo;<samp><span class="samp">2</span></samp>&rsquo;, <small class="dots">...</small> &lsquo;<samp><span class="samp">9</span></samp>&rsquo;<dd>An operand that matches the specified operand number is allowed.  If a
digit is used together with letters within the same alternative, the
digit should come last.

     <p>This number is allowed to be more than a single digit.  If multiple
digits are encountered consecutively, they are interpreted as a single
decimal integer.  There is scant chance for ambiguity, since to-date
it has never been desirable that &lsquo;<samp><span class="samp">10</span></samp>&rsquo; be interpreted as matching
either operand 1 <em>or</em> operand 0.  Should this be desired, one
can use multiple alternatives instead.

     <p><a name="index-matching-constraint-2558"></a><a name="index-constraint_002c-matching-2559"></a>This is called a <dfn>matching constraint</dfn> and what it really means is
that the assembler has only a single operand that fills two roles
which <code>asm</code> distinguishes.  For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
input-output operand:

     <pre class="smallexample">          addl #35,r12
</pre>
     <p>Matching constraints are used in these circumstances. 
More precisely, the two operands that match must include one input-only
operand and one output-only operand.  Moreover, the digit must be a
smaller number than the number of the operand that uses it in the
constraint.

     <p><a name="index-load-address-instruction-2560"></a><a name="index-push-address-instruction-2561"></a><a name="index-address-constraints-2562"></a><a name="index-g_t_0040samp_007bp_007d-in-constraint-2563"></a><br><dt>&lsquo;<samp><span class="samp">p</span></samp>&rsquo;<dd>An operand that is a valid memory address is allowed.  This is
for &ldquo;load address&rdquo; and &ldquo;push address&rdquo; instructions.

     <p><a name="index-address_005foperand-2564"></a>&lsquo;<samp><span class="samp">p</span></samp>&rsquo; in the constraint must be accompanied by <code>address_operand</code>
as the predicate in the <code>match_operand</code>.  This predicate interprets
the mode specified in the <code>match_operand</code> as the mode of the memory
reference for which the address would be valid.

     <p><a name="index-other-register-constraints-2565"></a><a name="index-extensible-constraints-2566"></a><br><dt><var>other-letters</var><dd>Other letters can be defined in machine-dependent fashion to stand for
particular classes of registers or other arbitrary operand types. 
&lsquo;<samp><span class="samp">d</span></samp>&rsquo;, &lsquo;<samp><span class="samp">a</span></samp>&rsquo; and &lsquo;<samp><span class="samp">f</span></samp>&rsquo; are defined on the 68000/68020 to stand
for data, address and floating point registers. 
</dl>

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