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diff --git a/share/doc/gccint/Insn-Lengths.html b/share/doc/gccint/Insn-Lengths.html new file mode 100644 index 0000000..738735c --- /dev/null +++ b/share/doc/gccint/Insn-Lengths.html @@ -0,0 +1,181 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +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. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Insn Lengths</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Insn Lengths"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Insn Lengths"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Insn-Attributes.html#Insn-Attributes" rel="up" title="Insn Attributes"> +<link href="Constant-Attributes.html#Constant-Attributes" rel="next" title="Constant Attributes"> +<link href="Attr-Example.html#Attr-Example" rel="previous" title="Attr Example"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Insn-Lengths"></a> +<div class="header"> +<p> +Next: <a href="Constant-Attributes.html#Constant-Attributes" accesskey="n" rel="next">Constant Attributes</a>, Previous: <a href="Attr-Example.html#Attr-Example" accesskey="p" rel="previous">Attr Example</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Computing-the-Length-of-an-Insn"></a> +<h4 class="subsection">17.19.5 Computing the Length of an Insn</h4> +<a name="index-insn-lengths_002c-computing"></a> +<a name="index-computing-the-length-of-an-insn"></a> + +<p>For many machines, multiple types of branch instructions are provided, each +for different length branch displacements. In most cases, the assembler +will choose the correct instruction to use. However, when the assembler +cannot do so, GCC can when a special attribute, the <code>length</code> +attribute, is defined. This attribute must be defined to have numeric +values by specifying a null string in its <code>define_attr</code>. +</p> +<p>In the case of the <code>length</code> attribute, two additional forms of +arithmetic terms are allowed in test expressions: +</p> +<dl compact="compact"> +<dd><a name="index-match_005fdup-and-attributes"></a> +</dd> +<dt><code>(match_dup <var>n</var>)</code></dt> +<dd><p>This refers to the address of operand <var>n</var> of the current insn, which +must be a <code>label_ref</code>. +</p> +<a name="index-pc-and-attributes"></a> +</dd> +<dt><code>(pc)</code></dt> +<dd><p>For non-branch instructions and backward branch instructions, this refers +to the address of the current insn. But for forward branch instructions, +this refers to the address of the next insn, because the length of the +current insn is to be computed. +</p></dd> +</dl> + +<a name="index-addr_005fvec_002c-length-of"></a> +<a name="index-addr_005fdiff_005fvec_002c-length-of"></a> +<p>For normal insns, the length will be determined by value of the +<code>length</code> attribute. In the case of <code>addr_vec</code> and +<code>addr_diff_vec</code> insn patterns, the length is computed as +the number of vectors multiplied by the size of each vector. +</p> +<p>Lengths are measured in addressable storage units (bytes). +</p> +<p>Note that it is possible to call functions via the <code>symbol_ref</code> +mechanism to compute the length of an insn. However, if you use this +mechanism you must provide dummy clauses to express the maximum length +without using the function call. You can see an example of this in the +<code>pa</code> machine description for the <code>call_symref</code> pattern. +</p> +<p>The following macros can be used to refine the length computation: +</p> +<dl compact="compact"> +<dd><a name="index-ADJUST_005fINSN_005fLENGTH"></a> +</dd> +<dt><code>ADJUST_INSN_LENGTH (<var>insn</var>, <var>length</var>)</code></dt> +<dd><p>If defined, modifies the length assigned to instruction <var>insn</var> as a +function of the context in which it is used. <var>length</var> is an lvalue +that contains the initially computed length of the insn and should be +updated with the correct length of the insn. +</p> +<p>This macro will normally not be required. A case in which it is +required is the ROMP. On this machine, the size of an <code>addr_vec</code> +insn must be increased by two to compensate for the fact that alignment +may be required. +</p></dd> +</dl> + +<a name="index-get_005fattr_005flength"></a> +<p>The routine that returns <code>get_attr_length</code> (the value of the +<code>length</code> attribute) can be used by the output routine to +determine the form of the branch instruction to be written, as the +example below illustrates. +</p> +<p>As an example of the specification of variable-length branches, consider +the IBM 360. If we adopt the convention that a register will be set to +the starting address of a function, we can jump to labels within 4k of +the start using a four-byte instruction. Otherwise, we need a six-byte +sequence to load the address from memory and then branch to it. +</p> +<p>On such a machine, a pattern for a branch instruction might be specified +as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">(define_insn "jump" + [(set (pc) + (label_ref (match_operand 0 "" "")))] + "" +{ + return (get_attr_length (insn) == 4 + ? "b %l0" : "l r15,=a(%l0); br r15"); +} + [(set (attr "length") + (if_then_else (lt (match_dup 0) (const_int 4096)) + (const_int 4) + (const_int 6)))]) +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Constant-Attributes.html#Constant-Attributes" accesskey="n" rel="next">Constant Attributes</a>, Previous: <a href="Attr-Example.html#Attr-Example" accesskey="p" rel="previous">Attr Example</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |