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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>You have freedom to copy and modify this GNU Manual, like GNU     software.  Copies published by the Free Software Foundation raise     funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!--  pre.display { font-family:inherit }  pre.format  { font-family:inherit }  pre.smalldisplay { font-family:inherit; font-size:smaller }  pre.smallformat  { font-family:inherit; font-size:smaller }  pre.smallexample { font-size:smaller }  pre.smalllisp    { font-size:smaller }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Simple%20Constraints">Simple Constraints</a>,Next:&nbsp;<a rel="next" accesskey="n" href="Multi-Alternative.html#Multi-Alternative">Multi-Alternative</a>,Up:&nbsp;<a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a><hr><br></div><h3 class="subsection">Simple Constraints</h4><p>The simplest kind of constraint is a string full of letters, each ofwhich describes one kind of operand that is permitted.  Here arethe letters that are allowed:     <dl><dt>whitespace     <dd>Whitespace characters are ignored and can be inserted at any positionexcept the first.  This enables each alternative for different operands tobe visually aligned in the machine description even if they have differentnumber of constraints and modifiers.     <br><dt><code>m</code>     <dd>A memory operand is allowed, with any kind of address that the machinesupports in general.     <br><dt><code>o</code>     <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 memoryaddress.     <p>For example, an address which is constant is offsettable; so is anaddress that is the sum of a register and a constant (as long as aslightly larger constant is also within the range of address-offsetssupported by the machine); but an autoincrement or autodecrementaddress is not offsettable.  More complicated indirect/indexedaddresses may or may not be offsettable depending on the otheraddressing modes that the machine supports.     <p>Note that in an output operand which can be matched by anotheroperand, the constraint letter <code>o</code> is valid only when accompaniedby both <code>&lt;</code> (if the target machine has predecrement addressing)and <code>&gt;</code> (if the target machine has preincrement addressing).     <br><dt><code>V</code>     <dd>A memory operand that is not offsettable.  In other words, anything thatwould fit the <code>m</code> constraint but not the <code>o</code> constraint.     <br><dt><code>&lt;</code>     <dd>A memory operand with autodecrement addressing (either predecrement orpostdecrement) is allowed.     <br><dt><code>&gt;</code>     <dd>A memory operand with autoincrement addressing (either preincrement orpostincrement) is allowed.     <br><dt><code>r</code>     <dd>A register operand is allowed provided that it is in a generalregister.     <br><dt><code>i</code>     <dd>An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only atassembly time or later.     <br><dt><code>n</code>     <dd>An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands lessthan a word wide.  Constraints for these operands should use <code>n</code>rather than <code>i</code>.     <br><dt><code>I</code>, <code>J</code>, <code>K</code>, <small class="dots">...</small> <code>P</code>     <dd>Other letters in the range <code>I</code> through <code>P</code> may be defined ina machine-dependent fashion to permit immediate integer operands withexplicit integer values in specified ranges.  For example, on the68000, <code>I</code> is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shiftinstructions.     <br><dt><code>E</code>     <dd>An immediate floating operand (expression code <code>const_double</code>) isallowed, but only if the target floating point format is the same asthat of the host machine (on which the compiler is running).     <br><dt><code>F</code>     <dd>An immediate floating operand (expression code <code>const_double</code> or<code>const_vector</code>) is allowed.     <br><dt><code>G</code>, <code>H</code>     <dd><code>G</code> and <code>H</code> may be defined in a machine-dependent fashion topermit immediate floating operands in particular ranges of values.     <br><dt><code>s</code>     <dd>An immediate integer operand whose value is not an explicit integer isallowed.     <p>This might appear strange; if an insn allows a constant operand with avalue not known at compile time, it certainly must allow any knownvalue.  So why use <code>s</code> instead of <code>i</code>?  Sometimes it allowsbetter code to be generated.     <p>For example, on the 68000 in a fullword instruction it is possible touse an immediate operand; but if the immediate value is between -128and 127, better code results from loading the value into a register andusing the register.  This is because the load into the register can bedone with a <code>moveq</code> instruction.  We arrange for this to happenby defining the letter <code>K</code> to mean "any integer outside therange -128 to 127", and then specifying <code>Ks</code> in the operandconstraints.     <br><dt><code>g</code>     <dd>Any register, memory or immediate integer operand is allowed, except forregisters that are not general registers.     <br><dt><code>X</code>     <dd>Any operand whatsoever is allowed.     <br><dt><code>0</code>, <code>1</code>, <code>2</code>, <small class="dots">...</small> <code>9</code>     <dd>An operand that matches the specified operand number is allowed.  If adigit is used together with letters within the same alternative, thedigit should come last.     <p>This number is allowed to be more than a single digit.  If multipledigits are encountered consecutively, they are interpreted as a singledecimal integer.  There is scant chance for ambiguity, since to-dateit has never been desirable that <code>10</code> be interpreted as matchingeither operand 1 <em>or</em> operand 0.  Should this be desired, onecan use multiple alternatives instead.     <p>This is called a <dfn>matching constraint</dfn> and what it really means isthat the assembler has only a single operand that fills two roleswhich <code>asm</code> distinguishes.  For example, an add instruction usestwo input operands and an output operand, but on most CISCmachines an add instruction really has only two operands, one of them aninput-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-onlyoperand and one output-only operand.  Moreover, the digit must be asmaller number than the number of the operand that uses it in theconstraint.     <br><dt><code>p</code>     <dd>An operand that is a valid memory address is allowed.  This isfor "load address" and "push address" instructions.     <p><code>p</code> in the constraint must be accompanied by <code>address_operand</code>as the predicate in the <code>match_operand</code>.  This predicate interpretsthe mode specified in the <code>match_operand</code> as the mode of the memoryreference for which the address would be valid.     <br><dt><var>other-letters</var>     <dd>Other letters can be defined in machine-dependent fashion to stand forparticular classes of registers or other arbitrary operand types. <code>d</code>, <code>a</code> and <code>f</code> are defined on the 68000/68020 to standfor data, address and floating point registers.   </dl>   </body></html>
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