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