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76 lines
3.5 KiB
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76 lines
3.5 KiB
HTML
4 years ago
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<title>Size of an asm - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" title="Using Assembly Language with C">
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<link rel="prev" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" title="Explicit Reg Vars">
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<div class="node">
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<a name="Size-of-an-asm"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>,
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Up: <a rel="up" accesskey="u" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>
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<hr>
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</div>
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<h4 class="subsection">6.43.6 Size of an <code>asm</code></h4>
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<p>Some targets require that GCC track the size of each instruction used
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in order to generate correct code. Because the final length of the
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code produced by an <code>asm</code> statement is only known by the
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assembler, GCC must make an estimate as to how big it will be. It
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does this by counting the number of instructions in the pattern of the
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<code>asm</code> and multiplying that by the length of the longest
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instruction supported by that processor. (When working out the number
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of instructions, it assumes that any occurrence of a newline or of
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whatever statement separator character is supported by the assembler –
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typically ‘<samp><span class="samp">;</span></samp>’ — indicates the end of an instruction.)
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<p>Normally, GCC's estimate is adequate to ensure that correct
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code is generated, but it is possible to confuse the compiler if you use
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pseudo instructions or assembler macros that expand into multiple real
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instructions, or if you use assembler directives that expand to more
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space in the object file than is needed for a single instruction.
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If this happens then the assembler may produce a diagnostic saying that
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a label is unreachable.
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</body></html>
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