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diff --git a/share/doc/gcc/Basic-Asm.html b/share/doc/gcc/Basic-Asm.html new file mode 100644 index 0000000..540ab4f --- /dev/null +++ b/share/doc/gcc/Basic-Asm.html @@ -0,0 +1,224 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +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). 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For +the C++ language, <code>asm</code> is a standard keyword, but <code>__asm__</code> +can be used for code compiled with <samp>-fno-asm</samp>. +</p> +<a name="Qualifiers-1"></a> +<h4 class="subsubheading">Qualifiers</h4> +<dl compact="compact"> +<dt><code>volatile</code></dt> +<dd><p>The optional <code>volatile</code> qualifier has no effect. +All basic <code>asm</code> blocks are implicitly volatile. +</p> +</dd> +<dt><code>inline</code></dt> +<dd><p>If you use the <code>inline</code> qualifier, then for inlining purposes the size +of the <code>asm</code> statement is taken as the smallest size possible (see <a href="Size-of-an-asm.html#Size-of-an-asm">Size of an asm</a>). +</p></dd> +</dl> + +<a name="Parameters"></a> +<h4 class="subsubheading">Parameters</h4> +<dl compact="compact"> +<dt><var>AssemblerInstructions</var></dt> +<dd><p>This is a literal string that specifies the assembler code. The string can +contain any instructions recognized by the assembler, including directives. +GCC does not parse the assembler instructions themselves and +does not know what they mean or even whether they are valid assembler input. +</p> +<p>You may place multiple assembler instructions together in a single <code>asm</code> +string, separated by the characters normally used in assembly code for the +system. A combination that works in most places is a newline to break the +line, plus a tab character (written as ‘<samp>\n\t</samp>’). +Some assemblers allow semicolons as a line separator. However, +note that some assembler dialects use semicolons to start a comment. +</p></dd> +</dl> + +<a name="Remarks"></a> +<h4 class="subsubheading">Remarks</h4> +<p>Using extended <code>asm</code> (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>) typically produces +smaller, safer, and more efficient code, and in most cases it is a +better solution than basic <code>asm</code>. However, there are two +situations where only basic <code>asm</code> can be used: +</p> +<ul> +<li> Extended <code>asm</code> statements have to be inside a C +function, so to write inline assembly language at file scope (“top-level”), +outside of C functions, you must use basic <code>asm</code>. +You can use this technique to emit assembler directives, +define assembly language macros that can be invoked elsewhere in the file, +or write entire functions in assembly language. +Basic <code>asm</code> statements outside of functions may not use any +qualifiers. + +</li><li> Functions declared +with the <code>naked</code> attribute also require basic <code>asm</code> +(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>). +</li></ul> + +<p>Safely accessing C data and calling functions from basic <code>asm</code> is more +complex than it may appear. To access C data, it is better to use extended +<code>asm</code>. +</p> +<p>Do not expect a sequence of <code>asm</code> statements to remain perfectly +consecutive after compilation. If certain instructions need to remain +consecutive in the output, put them in a single multi-instruction <code>asm</code> +statement. Note that GCC’s optimizers can move <code>asm</code> statements +relative to other code, including across jumps. +</p> +<p><code>asm</code> statements may not perform jumps into other <code>asm</code> statements. +GCC does not know about these jumps, and therefore cannot take +account of them when deciding how to optimize. Jumps from <code>asm</code> to C +labels are only supported in extended <code>asm</code>. +</p> +<p>Under certain circumstances, GCC may duplicate (or remove duplicates of) your +assembly code when optimizing. This can lead to unexpected duplicate +symbol errors during compilation if your assembly code defines symbols or +labels. +</p> +<p><strong>Warning:</strong> The C standards do not specify semantics for <code>asm</code>, +making it a potential source of incompatibilities between compilers. These +incompatibilities may not produce compiler warnings/errors. +</p> +<p>GCC does not parse basic <code>asm</code>’s <var>AssemblerInstructions</var>, which +means there is no way to communicate to the compiler what is happening +inside them. GCC has no visibility of symbols in the <code>asm</code> and may +discard them as unreferenced. It also does not know about side effects of +the assembler code, such as modifications to memory or registers. Unlike +some compilers, GCC assumes that no changes to general purpose registers +occur. This assumption may change in a future release. +</p> +<p>To avoid complications from future changes to the semantics and the +compatibility issues between compilers, consider replacing basic <code>asm</code> +with extended <code>asm</code>. See +<a href="https://gcc.gnu.org/wiki/ConvertBasicAsmToExtended">How to convert +from basic asm to extended asm</a> for information about how to perform this +conversion. +</p> +<p>The compiler copies the assembler instructions in a basic <code>asm</code> +verbatim to the assembly language output file, without +processing dialects or any of the ‘<samp>%</samp>’ operators that are available with +extended <code>asm</code>. This results in minor differences between basic +<code>asm</code> strings and extended <code>asm</code> templates. For example, to refer to +registers you might use ‘<samp>%eax</samp>’ in basic <code>asm</code> and +‘<samp>%%eax</samp>’ in extended <code>asm</code>. +</p> +<p>On targets such as x86 that support multiple assembler dialects, +all basic <code>asm</code> blocks use the assembler dialect specified by the +<samp>-masm</samp> command-line option (see <a href="x86-Options.html#x86-Options">x86 Options</a>). +Basic <code>asm</code> provides no +mechanism to provide different assembler strings for different dialects. +</p> +<p>For basic <code>asm</code> with non-empty assembler string GCC assumes +the assembler block does not change any general purpose registers, +but it may read or write any globally accessible variable. +</p> +<p>Here is an example of basic <code>asm</code> for i386: +</p> +<div class="example"> +<pre class="example">/* Note that this code will not compile with -masm=intel */ +#define DebugBreak() asm("int $3") +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Extended-Asm.html#Extended-Asm" accesskey="n" rel="next">Extended Asm</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |