diff options
| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
|---|---|---|
| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gccint/Basic-Blocks.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
Change-Id: I7303388733328cd98ab9aa3c30236db67f2e9e9c
Diffstat (limited to 'share/doc/gccint/Basic-Blocks.html')
| -rw-r--r-- | share/doc/gccint/Basic-Blocks.html | 217 |
1 files changed, 217 insertions, 0 deletions
diff --git a/share/doc/gccint/Basic-Blocks.html b/share/doc/gccint/Basic-Blocks.html new file mode 100644 index 0000000..a816119 --- /dev/null +++ b/share/doc/gccint/Basic-Blocks.html @@ -0,0 +1,217 @@ +<!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: Basic Blocks</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Basic Blocks"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Basic Blocks"> +<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="Control-Flow.html#Control-Flow" rel="up" title="Control Flow"> +<link href="Edges.html#Edges" rel="next" title="Edges"> +<link href="Control-Flow.html#Control-Flow" rel="previous" title="Control Flow"> +<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="Basic-Blocks"></a> +<div class="header"> +<p> +Next: <a href="Edges.html#Edges" accesskey="n" rel="next">Edges</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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="Basic-Blocks-1"></a> +<h3 class="section">15.1 Basic Blocks</h3> + +<a name="index-basic-block"></a> +<a name="index-basic_005fblock-1"></a> +<p>A basic block is a straight-line sequence of code with only one entry +point and only one exit. In GCC, basic blocks are represented using +the <code>basic_block</code> data type. +</p> +<a name="index-ENTRY_005fBLOCK_005fPTR_002c-EXIT_005fBLOCK_005fPTR"></a> +<p>Special basic blocks represent possible entry and exit points of a +function. These blocks are called <code>ENTRY_BLOCK_PTR</code> and +<code>EXIT_BLOCK_PTR</code>. These blocks do not contain any code. +</p> +<a name="index-BASIC_005fBLOCK"></a> +<p>The <code>BASIC_BLOCK</code> array contains all basic blocks in an +unspecified order. Each <code>basic_block</code> structure has a field +that holds a unique integer identifier <code>index</code> that is the +index of the block in the <code>BASIC_BLOCK</code> array. +The total number of basic blocks in the function is +<code>n_basic_blocks</code>. Both the basic block indices and +the total number of basic blocks may vary during the compilation +process, as passes reorder, create, duplicate, and destroy basic +blocks. The index for any block should never be greater than +<code>last_basic_block</code>. The indices 0 and 1 are special codes +reserved for <code>ENTRY_BLOCK</code> and <code>EXIT_BLOCK</code>, the +indices of <code>ENTRY_BLOCK_PTR</code> and <code>EXIT_BLOCK_PTR</code>. +</p> +<a name="index-next_005fbb_002c-prev_005fbb_002c-FOR_005fEACH_005fBB_002c-FOR_005fALL_005fBB"></a> +<p>Two pointer members of the <code>basic_block</code> structure are the +pointers <code>next_bb</code> and <code>prev_bb</code>. These are used to keep +doubly linked chain of basic blocks in the same order as the +underlying instruction stream. The chain of basic blocks is updated +transparently by the provided API for manipulating the CFG. The macro +<code>FOR_EACH_BB</code> can be used to visit all the basic blocks in +lexicographical order, except <code>ENTRY_BLOCK</code> and <code>EXIT_BLOCK</code>. +The macro <code>FOR_ALL_BB</code> also visits all basic blocks in +lexicographical order, including <code>ENTRY_BLOCK</code> and <code>EXIT_BLOCK</code>. +</p> +<a name="index-post_005forder_005fcompute_002c-inverted_005fpost_005forder_005fcompute_002c-walk_005fdominator_005ftree"></a> +<p>The functions <code>post_order_compute</code> and <code>inverted_post_order_compute</code> +can be used to compute topological orders of the CFG. The orders are +stored as vectors of basic block indices. The <code>BASIC_BLOCK</code> array +can be used to iterate each basic block by index. +Dominator traversals are also possible using +<code>walk_dominator_tree</code>. Given two basic blocks A and B, block A +dominates block B if A is <em>always</em> executed before B. +</p> +<p>Each <code>basic_block</code> also contains pointers to the first +instruction (the <em>head</em>) and the last instruction (the <em>tail</em>) +or <em>end</em> of the instruction stream contained in a basic block. In +fact, since the <code>basic_block</code> data type is used to represent +blocks in both major intermediate representations of GCC (<code>GIMPLE</code> +and RTL), there are pointers to the head and end of a basic block for +both representations, stored in intermediate representation specific +data in the <code>il</code> field of <code>struct basic_block_def</code>. +</p> +<a name="index-CODE_005fLABEL"></a> +<a name="index-NOTE_005fINSN_005fBASIC_005fBLOCK"></a> +<p>For RTL, these pointers are <code>BB_HEAD</code> and <code>BB_END</code>. +</p> +<a name="index-insn-notes_002c-notes"></a> +<a name="index-NOTE_005fINSN_005fBASIC_005fBLOCK-1"></a> +<p>In the RTL representation of a function, the instruction stream +contains not only the “real” instructions, but also <em>notes</em> +or <em>insn notes</em> (to distinguish them from <em>reg notes</em>). +Any function that moves or duplicates the basic blocks needs +to take care of updating of these notes. Many of these notes expect +that the instruction stream consists of linear regions, so updating +can sometimes be tedious. All types of insn notes are defined +in <samp>insn-notes.def</samp>. +</p> +<p>In the RTL function representation, the instructions contained in a +basic block always follow a <code>NOTE_INSN_BASIC_BLOCK</code>, but zero +or more <code>CODE_LABEL</code> nodes can precede the block note. +A basic block ends with a control flow instruction or with the last +instruction before the next <code>CODE_LABEL</code> or +<code>NOTE_INSN_BASIC_BLOCK</code>. +By definition, a <code>CODE_LABEL</code> cannot appear in the middle of +the instruction stream of a basic block. +</p> +<a name="index-can_005ffallthru"></a> +<a name="index-table-jump"></a> +<p>In addition to notes, the jump table vectors are also represented as +“pseudo-instructions” inside the insn stream. These vectors never +appear in the basic block and should always be placed just after the +table jump instructions referencing them. After removing the +table-jump it is often difficult to eliminate the code computing the +address and referencing the vector, so cleaning up these vectors is +postponed until after liveness analysis. Thus the jump table vectors +may appear in the insn stream unreferenced and without any purpose. +Before any edge is made <em>fall-thru</em>, the existence of such +construct in the way needs to be checked by calling +<code>can_fallthru</code> function. +</p> +<a name="index-GIMPLE-statement-iterators"></a> +<p>For the <code>GIMPLE</code> representation, the PHI nodes and statements +contained in a basic block are in a <code>gimple_seq</code> pointed to by +the basic block intermediate language specific pointers. +Abstract containers and iterators are used to access the PHI nodes +and statements in a basic blocks. These iterators are called +<em>GIMPLE statement iterators</em> (GSIs). Grep for <code>^gsi</code> +in the various <samp>gimple-*</samp> and <samp>tree-*</samp> files. +There is a <code>gimple_stmt_iterator</code> type for iterating over +all kinds of statement, and a <code>gphi_iterator</code> subclass for +iterating over PHI nodes. +The following snippet will pretty-print all PHI nodes the statements +of the current function in the GIMPLE representation. +</p> +<div class="smallexample"> +<pre class="smallexample">basic_block bb; + +FOR_EACH_BB (bb) + { + gphi_iterator pi; + gimple_stmt_iterator si; + + for (pi = gsi_start_phis (bb); !gsi_end_p (pi); gsi_next (&pi)) + { + gphi *phi = pi.phi (); + print_gimple_stmt (dump_file, phi, 0, TDF_SLIM); + } + for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si)) + { + gimple stmt = gsi_stmt (si); + print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM); + } + } +</pre></div> + + +<hr> +<div class="header"> +<p> +Next: <a href="Edges.html#Edges" accesskey="n" rel="next">Edges</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</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> |