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diff --git a/share/doc/gdb/General-Bytecode-Design.html b/share/doc/gdb/General-Bytecode-Design.html new file mode 100644 index 0000000..db834f8 --- /dev/null +++ b/share/doc/gdb/General-Bytecode-Design.html @@ -0,0 +1,222 @@ +<!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 "Free Software" and "Free Software Needs +Free Documentation", with the Front-Cover Texts being "A GNU Manual," +and with the Back-Cover Texts as in (a) below. + +(a) The FSF's Back-Cover Text is: "You are free to copy and modify +this GNU Manual. Buying copies from GNU Press supports the FSF in +developing GNU and promoting software freedom." --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: General Bytecode Design</title> + +<meta name="description" content="Debugging with GDB: General Bytecode Design"> +<meta name="keywords" content="Debugging with GDB: General Bytecode Design"> +<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="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Agent-Expressions.html#Agent-Expressions" rel="up" title="Agent Expressions"> +<link href="Bytecode-Descriptions.html#Bytecode-Descriptions" rel="next" title="Bytecode Descriptions"> +<link href="Agent-Expressions.html#Agent-Expressions" rel="previous" title="Agent Expressions"> +<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="General-Bytecode-Design"></a> +<div class="header"> +<p> +Next: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="n" rel="next">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="General-Bytecode-Design-1"></a> +<h3 class="section">F.1 General Bytecode Design</h3> + +<p>The agent represents bytecode expressions as an array of bytes. Each +instruction is one byte long (thus the term <em>bytecode</em>). Some +instructions are followed by operand bytes; for example, the <code>goto</code> +instruction is followed by a destination for the jump. +</p> +<p>The bytecode interpreter is a stack-based machine; most instructions pop +their operands off the stack, perform some operation, and push the +result back on the stack for the next instruction to consume. Each +element of the stack may contain either a integer or a floating point +value; these values are as many bits wide as the largest integer that +can be directly manipulated in the source language. Stack elements +carry no record of their type; bytecode could push a value as an +integer, then pop it as a floating point value. However, GDB will not +generate code which does this. In C, one might define the type of a +stack element as follows: +</p><div class="example"> +<pre class="example">union agent_val { + LONGEST l; + DOUBLEST d; +}; +</pre></div> +<p>where <code>LONGEST</code> and <code>DOUBLEST</code> are <code>typedef</code> names for +the largest integer and floating point types on the machine. +</p> +<p>By the time the bytecode interpreter reaches the end of the expression, +the value of the expression should be the only value left on the stack. +For tracing applications, <code>trace</code> bytecodes in the expression will +have recorded the necessary data, and the value on the stack may be +discarded. For other applications, like conditional breakpoints, the +value may be useful. +</p> +<p>Separate from the stack, the interpreter has two registers: +</p><dl compact="compact"> +<dt><code>pc</code></dt> +<dd><p>The address of the next bytecode to execute. +</p> +</dd> +<dt><code>start</code></dt> +<dd><p>The address of the start of the bytecode expression, necessary for +interpreting the <code>goto</code> and <code>if_goto</code> instructions. +</p> +</dd> +</dl> +<p>Neither of these registers is directly visible to the bytecode language +itself, but they are useful for defining the meanings of the bytecode +operations. +</p> +<p>There are no instructions to perform side effects on the running +program, or call the program’s functions; we assume that these +expressions are only used for unobtrusive debugging, not for patching +the running code. +</p> +<p>Most bytecode instructions do not distinguish between the various sizes +of values, and operate on full-width values; the upper bits of the +values are simply ignored, since they do not usually make a difference +to the value computed. The exceptions to this rule are: +</p><dl compact="compact"> +<dt>memory reference instructions (<code>ref</code><var>n</var>)</dt> +<dd><p>There are distinct instructions to fetch different word sizes from +memory. Once on the stack, however, the values are treated as full-size +integers. They may need to be sign-extended; the <code>ext</code> instruction +exists for this purpose. +</p> +</dd> +<dt>the sign-extension instruction (<code>ext</code> <var>n</var>)</dt> +<dd><p>These clearly need to know which portion of their operand is to be +extended to occupy the full length of the word. +</p> +</dd> +</dl> + +<p>If the interpreter is unable to evaluate an expression completely for +some reason (a memory location is inaccessible, or a divisor is zero, +for example), we say that interpretation “terminates with an error”. +This means that the problem is reported back to the interpreter’s caller +in some helpful way. In general, code using agent expressions should +assume that they may attempt to divide by zero, fetch arbitrary memory +locations, and misbehave in other ways. +</p> +<p>Even complicated C expressions compile to a few bytecode instructions; +for example, the expression <code>x + y * z</code> would typically produce +code like the following, assuming that <code>x</code> and <code>y</code> live in +registers, and <code>z</code> is a global variable holding a 32-bit +<code>int</code>: +</p><div class="example"> +<pre class="example">reg 1 +reg 2 +const32 <i>address of z</i> +ref32 +ext 32 +mul +add +end +</pre></div> + +<p>In detail, these mean: +</p><dl compact="compact"> +<dt><code>reg 1</code></dt> +<dd><p>Push the value of register 1 (presumably holding <code>x</code>) onto the +stack. +</p> +</dd> +<dt><code>reg 2</code></dt> +<dd><p>Push the value of register 2 (holding <code>y</code>). +</p> +</dd> +<dt><code>const32 <i>address of z</i></code></dt> +<dd><p>Push the address of <code>z</code> onto the stack. +</p> +</dd> +<dt><code>ref32</code></dt> +<dd><p>Fetch a 32-bit word from the address at the top of the stack; replace +the address on the stack with the value. Thus, we replace the address +of <code>z</code> with <code>z</code>’s value. +</p> +</dd> +<dt><code>ext 32</code></dt> +<dd><p>Sign-extend the value on the top of the stack from 32 bits to full +length. This is necessary because <code>z</code> is a signed integer. +</p> +</dd> +<dt><code>mul</code></dt> +<dd><p>Pop the top two numbers on the stack, multiply them, and push their +product. Now the top of the stack contains the value of the expression +<code>y * z</code>. +</p> +</dd> +<dt><code>add</code></dt> +<dd><p>Pop the top two numbers, add them, and push the sum. Now the top of the +stack contains the value of <code>x + y * z</code>. +</p> +</dd> +<dt><code>end</code></dt> +<dd><p>Stop executing; the value left on the stack top is the value to be +recorded. +</p> +</dd> +</dl> + + +<hr> +<div class="header"> +<p> +Next: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="n" rel="next">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |