<html lang="en"> <head> <title>Condition Code - 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="Target-Macros.html#Target-Macros" title="Target Macros"> <link rel="prev" href="Anchored-Addresses.html#Anchored-Addresses" title="Anchored Addresses"> <link rel="next" href="Costs.html#Costs" title="Costs"> <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="Condition-Code"></a> <p> Next: <a rel="next" accesskey="n" href="Costs.html#Costs">Costs</a>, Previous: <a rel="previous" accesskey="p" href="Anchored-Addresses.html#Anchored-Addresses">Anchored Addresses</a>, Up: <a rel="up" accesskey="u" href="Target-Macros.html#Target-Macros">Target Macros</a> <hr> </div> <h3 class="section">17.15 Condition Code Status</h3> <p><a name="index-condition-code-status-4427"></a> The macros in this section can be split in two families, according to the two ways of representing condition codes in GCC. <p>The first representation is the so called <code>(cc0)</code> representation (see <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>), where all instructions can have an implicit clobber of the condition codes. The second is the condition code register representation, which provides better schedulability for architectures that do have a condition code register, but on which most instructions do not affect it. The latter category includes most RISC machines. <p>The implicit clobbering poses a strong restriction on the placement of the definition and use of the condition code. In the past the definition and use were always adjacent. However, recent changes to support trapping arithmatic may result in the definition and user being in different blocks. Thus, there may be a <code>NOTE_INSN_BASIC_BLOCK</code> between them. Additionally, the definition may be the source of exception handling edges. <p>These restrictions can prevent important optimizations on some machines. For example, on the IBM RS/6000, there is a delay for taken branches unless the condition code register is set three instructions earlier than the conditional branch. The instruction scheduler cannot perform this optimization if it is not permitted to separate the definition and use of the condition code register. <p>For this reason, it is possible and suggested to use a register to represent the condition code for new ports. If there is a specific condition code register in the machine, use a hard register. If the condition code or comparison result can be placed in any general register, or if there are multiple condition registers, use a pseudo register. Registers used to store the condition code value will usually have a mode that is in class <code>MODE_CC</code>. <p>Alternatively, you can use <code>BImode</code> if the comparison operator is specified already in the compare instruction. In this case, you are not interested in most macros in this section. <ul class="menu"> <li><a accesskey="1" href="CC0-Condition-Codes.html#CC0-Condition-Codes">CC0 Condition Codes</a>: Old style representation of condition codes. <li><a accesskey="2" href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes">MODE_CC Condition Codes</a>: Modern representation of condition codes. </ul> </body></html>