You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
169 lines
8.9 KiB
HTML
169 lines
8.9 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Basic Blocks - GNU Compiler Collection (GCC) Internals</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="Control-Flow.html#Control-Flow" title="Control Flow">
|
||
|
|
<link rel="next" href="Edges.html#Edges" title="Edges">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
Copyright (C) 1988-2015 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.-->
|
||
|
|
<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 }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Basic-Blocks"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Edges.html#Edges">Edges</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Control-Flow.html#Control-Flow">Control Flow</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h3 class="section">14.1 Basic Blocks</h3>
|
||
|
|
|
||
|
|
<p><a name="index-basic-block-3154"></a><a name="index-basic_005fblock-3155"></a>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-3156"></a>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-3157"></a>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-3158"></a>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-3159"></a>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>Each <code>basic_block</code> also contains pointers to the first
|
||
|
|
instruction (the <dfn>head</dfn>) and the last instruction (the <dfn>tail</dfn>)
|
||
|
|
or <dfn>end</dfn> 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-3160"></a><a name="index-NOTE_005fINSN_005fBASIC_005fBLOCK-3161"></a>For RTL, these pointers are <code>BB_HEAD</code> and <code>BB_END</code>.
|
||
|
|
|
||
|
|
<p><a name="index-insn-notes_002c-notes-3162"></a><a name="index-NOTE_005fINSN_005fBASIC_005fBLOCK-3163"></a>In the RTL representation of a function, the instruction stream
|
||
|
|
contains not only the “real” instructions, but also <dfn>notes</dfn>
|
||
|
|
or <dfn>insn notes</dfn> (to distinguish them from <dfn>reg notes</dfn>).
|
||
|
|
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><span class="file">insn-notes.def</span></samp>.
|
||
|
|
|
||
|
|
<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-3164"></a><a name="index-table-jump-3165"></a>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 <dfn>fall-thru</dfn>, 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-3166"></a>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
|
||
|
|
<dfn>GIMPLE statement iterators</dfn> (GSIs). Grep for <code>^gsi</code>
|
||
|
|
in the various <samp><span class="file">gimple-*</span></samp> and <samp><span class="file">tree-*</span></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.
|
||
|
|
|
||
|
|
<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>
|
||
|
|
</body></html>
|
||
|
|
|