| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- 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. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>GNU Compiler Collection (GCC) Internals: Calls</title> |
| |
| <meta name="description" content="GNU Compiler Collection (GCC) Internals: Calls"> |
| <meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Calls"> |
| <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="RTL.html#RTL" rel="up" title="RTL"> |
| <link href="Sharing.html#Sharing" rel="next" title="Sharing"> |
| <link href="Insns.html#Insns" rel="prev" title="Insns"> |
| <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="Calls"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Sharing.html#Sharing" accesskey="n" rel="next">Sharing</a>, Previous: <a href="Insns.html#Insns" accesskey="p" rel="prev">Insns</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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="RTL-Representation-of-Function_002dCall-Insns"></a> |
| <h3 class="section">13.20 RTL Representation of Function-Call Insns</h3> |
| <a name="index-calling-functions-in-RTL"></a> |
| <a name="index-RTL-function_002dcall-insns"></a> |
| <a name="index-function_002dcall-insns"></a> |
| |
| <p>Insns that call subroutines have the RTL expression code <code>call_insn</code>. |
| These insns must satisfy special rules, and their bodies must use a special |
| RTL expression code, <code>call</code>. |
| </p> |
| <a name="index-call-usage"></a> |
| <p>A <code>call</code> expression has two operands, as follows: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(call (mem:<var>fm</var> <var>addr</var>) <var>nbytes</var>) |
| </pre></div> |
| |
| <p>Here <var>nbytes</var> is an operand that represents the number of bytes of |
| argument data being passed to the subroutine, <var>fm</var> is a machine mode |
| (which must equal as the definition of the <code>FUNCTION_MODE</code> macro in |
| the machine description) and <var>addr</var> represents the address of the |
| subroutine. |
| </p> |
| <p>For a subroutine that returns no value, the <code>call</code> expression as |
| shown above is the entire body of the insn, except that the insn might |
| also contain <code>use</code> or <code>clobber</code> expressions. |
| </p> |
| <a name="index-BLKmode_002c-and-function-return-values"></a> |
| <p>For a subroutine that returns a value whose mode is not <code>BLKmode</code>, |
| the value is returned in a hard register. If this register’s number is |
| <var>r</var>, then the body of the call insn looks like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(set (reg:<var>m</var> <var>r</var>) |
| (call (mem:<var>fm</var> <var>addr</var>) <var>nbytes</var>)) |
| </pre></div> |
| |
| <p>This RTL expression makes it clear (to the optimizer passes) that the |
| appropriate register receives a useful value in this insn. |
| </p> |
| <p>When a subroutine returns a <code>BLKmode</code> value, it is handled by |
| passing to the subroutine the address of a place to store the value. |
| So the call insn itself does not “return” any value, and it has the |
| same RTL form as a call that returns nothing. |
| </p> |
| <p>On some machines, the call instruction itself clobbers some register, |
| for example to contain the return address. <code>call_insn</code> insns |
| on these machines should have a body which is a <code>parallel</code> |
| that contains both the <code>call</code> expression and <code>clobber</code> |
| expressions that indicate which registers are destroyed. Similarly, |
| if the call instruction requires some register other than the stack |
| pointer that is not explicitly mentioned in its RTL, a <code>use</code> |
| subexpression should mention that register. |
| </p> |
| <p>Functions that are called are assumed to modify all registers listed in |
| the configuration macro <code>CALL_USED_REGISTERS</code> (see <a href="Register-Basics.html#Register-Basics">Register Basics</a>) and, with the exception of <code>const</code> functions and library |
| calls, to modify all of memory. |
| </p> |
| <p>Insns containing just <code>use</code> expressions directly precede the |
| <code>call_insn</code> insn to indicate which registers contain inputs to the |
| function. Similarly, if registers other than those in |
| <code>CALL_USED_REGISTERS</code> are clobbered by the called function, insns |
| containing a single <code>clobber</code> follow immediately after the call to |
| indicate which registers. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Sharing.html#Sharing" accesskey="n" rel="next">Sharing</a>, Previous: <a href="Insns.html#Insns" accesskey="p" rel="prev">Insns</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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> |
