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136 lines
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136 lines
6.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Insn Lengths - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Insn-Attributes.html#Insn-Attributes" title="Insn Attributes">
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<link rel="prev" href="Attr-Example.html#Attr-Example" title="Attr Example">
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<link rel="next" href="Constant-Attributes.html#Constant-Attributes" title="Constant Attributes">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Insn-Lengths"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Constant-Attributes.html#Constant-Attributes">Constant Attributes</a>,
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Previous: <a rel="previous" accesskey="p" href="Attr-Example.html#Attr-Example">Attr Example</a>,
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Up: <a rel="up" accesskey="u" href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>
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<hr>
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</div>
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<h4 class="subsection">16.19.5 Computing the Length of an Insn</h4>
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<p><a name="index-insn-lengths_002c-computing-3768"></a><a name="index-computing-the-length-of-an-insn-3769"></a>
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For many machines, multiple types of branch instructions are provided, each
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for different length branch displacements. In most cases, the assembler
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will choose the correct instruction to use. However, when the assembler
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cannot do so, GCC can when a special attribute, the <code>length</code>
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attribute, is defined. This attribute must be defined to have numeric
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values by specifying a null string in its <code>define_attr</code>.
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<p>In the case of the <code>length</code> attribute, two additional forms of
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arithmetic terms are allowed in test expressions:
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<a name="index-g_t_0040code_007bmatch_005fdup_007d-and-attributes-3770"></a>
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<dl><dt><code>(match_dup </code><var>n</var><code>)</code><dd>This refers to the address of operand <var>n</var> of the current insn, which
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must be a <code>label_ref</code>.
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<p><a name="index-g_t_0040code_007bpc_007d-and-attributes-3771"></a><br><dt><code>(pc)</code><dd>For non-branch instructions and backward branch instructions, this refers
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to the address of the current insn. But for forward branch instructions,
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this refers to the address of the next insn, because the length of the
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current insn is to be computed.
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</dl>
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<p><a name="index-g_t_0040code_007baddr_005fvec_007d_002c-length-of-3772"></a><a name="index-g_t_0040code_007baddr_005fdiff_005fvec_007d_002c-length-of-3773"></a>For normal insns, the length will be determined by value of the
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<code>length</code> attribute. In the case of <code>addr_vec</code> and
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<code>addr_diff_vec</code> insn patterns, the length is computed as
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the number of vectors multiplied by the size of each vector.
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<p>Lengths are measured in addressable storage units (bytes).
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<p>Note that it is possible to call functions via the <code>symbol_ref</code>
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mechanism to compute the length of an insn. However, if you use this
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mechanism you must provide dummy clauses to express the maximum length
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without using the function call. You can an example of this in the
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<code>pa</code> machine description for the <code>call_symref</code> pattern.
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<p>The following macros can be used to refine the length computation:
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<a name="index-ADJUST_005fINSN_005fLENGTH-3774"></a>
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<dl><dt><code>ADJUST_INSN_LENGTH (</code><var>insn</var><code>, </code><var>length</var><code>)</code><dd>If defined, modifies the length assigned to instruction <var>insn</var> as a
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function of the context in which it is used. <var>length</var> is an lvalue
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that contains the initially computed length of the insn and should be
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updated with the correct length of the insn.
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<p>This macro will normally not be required. A case in which it is
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required is the ROMP. On this machine, the size of an <code>addr_vec</code>
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insn must be increased by two to compensate for the fact that alignment
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may be required.
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</dl>
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<p><a name="index-get_005fattr_005flength-3775"></a>The routine that returns <code>get_attr_length</code> (the value of the
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<code>length</code> attribute) can be used by the output routine to
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determine the form of the branch instruction to be written, as the
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example below illustrates.
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<p>As an example of the specification of variable-length branches, consider
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the IBM 360. If we adopt the convention that a register will be set to
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the starting address of a function, we can jump to labels within 4k of
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the start using a four-byte instruction. Otherwise, we need a six-byte
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sequence to load the address from memory and then branch to it.
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<p>On such a machine, a pattern for a branch instruction might be specified
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as follows:
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<pre class="smallexample"> (define_insn "jump"
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[(set (pc)
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(label_ref (match_operand 0 "" "")))]
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""
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{
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return (get_attr_length (insn) == 4
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? "b %l0" : "l r15,=a(%l0); br r15");
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}
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[(set (attr "length")
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(if_then_else (lt (match_dup 0) (const_int 4096))
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(const_int 4)
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(const_int 6)))])
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</pre>
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</body></html>
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