aarch64-linux-gnu-4.9/share/doc/gccint/Sharing.html - toolchains/linux-x86/gcc - Git at Google

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 <a name="Sharing"></a>
 <div class="header">
 <p>
 Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="Calls.html#Calls" accesskey="p" rel="prev">Calls</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</a> &nbsp; [<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>
 <a name="Structure-Sharing-Assumptions"></a>
 <h3 class="section">13.21 Structure Sharing Assumptions</h3>
 <a name="index-sharing-of-RTL-components"></a>
 <a name="index-RTL-structure-sharing-assumptions"></a>

 <p>The compiler assumes that certain kinds of RTL expressions are unique;
 there do not exist two distinct objects representing the same value.
 In other cases, it makes an opposite assumption: that no RTL expression
 object of a certain kind appears in more than one place in the
 containing structure.
 </p>
 <p>These assumptions refer to a single function; except for the RTL
 objects that describe global variables and external functions,
 and a few standard objects such as small integer constants,
 no RTL objects are common to two functions.
 </p>
 <ul>
 <li> <a name="index-reg_002c-RTL-sharing"></a>
 Each pseudo-register has only a single <code>reg</code> object to represent it,
 and therefore only a single machine mode.

 </li><li> <a name="index-symbolic-label"></a>
 <a name="index-symbol_005fref_002c-RTL-sharing"></a>
 For any symbolic label, there is only one <code>symbol_ref</code> object
 referring to it.

 </li><li> <a name="index-const_005fint_002c-RTL-sharing"></a>
 All <code>const_int</code> expressions with equal values are shared.

 </li><li> <a name="index-pc_002c-RTL-sharing"></a>
 There is only one <code>pc</code> expression.

 </li><li> <a name="index-cc0_002c-RTL-sharing"></a>
 There is only one <code>cc0</code> expression.

 </li><li> <a name="index-const_005fdouble_002c-RTL-sharing"></a>
 There is only one <code>const_double</code> expression with value 0 for
 each floating point mode.  Likewise for values 1 and 2.

 </li><li> <a name="index-const_005fvector_002c-RTL-sharing"></a>
 There is only one <code>const_vector</code> expression with value 0 for
 each vector mode, be it an integer or a double constant vector.

 </li><li> <a name="index-label_005fref_002c-RTL-sharing"></a>
 <a name="index-scratch_002c-RTL-sharing"></a>
 No <code>label_ref</code> or <code>scratch</code> appears in more than one place in
 the RTL structure; in other words, it is safe to do a tree-walk of all
 the insns in the function and assume that each time a <code>label_ref</code>
 or <code>scratch</code> is seen it is distinct from all others that are seen.

 </li><li> <a name="index-mem_002c-RTL-sharing"></a>
 Only one <code>mem</code> object is normally created for each static
 variable or stack slot, so these objects are frequently shared in all
 the places they appear.  However, separate but equal objects for these
 variables are occasionally made.

 </li><li> <a name="index-asm_005foperands_002c-RTL-sharing"></a>
 When a single <code>asm</code> statement has multiple output operands, a
 distinct <code>asm_operands</code> expression is made for each output operand.
 However, these all share the vector which contains the sequence of input
 operands.  This sharing is used later on to test whether two
 <code>asm_operands</code> expressions come from the same statement, so all
 optimizations must carefully preserve the sharing if they copy the
 vector at all.

 </li><li> No RTL object appears in more than one place in the RTL structure
 except as described above.  Many passes of the compiler rely on this
 by assuming that they can modify RTL objects in place without unwanted
 side-effects on other insns.

 </li><li> <a name="index-unshare_005fall_005frtl"></a>
 During initial RTL generation, shared structure is freely introduced.
 After all the RTL for a function has been generated, all shared
 structure is copied by <code>unshare_all_rtl</code> in <samp>emit-rtl.c</samp>,
 after which the above rules are guaranteed to be followed.

 </li><li> <a name="index-copy_005frtx_005fif_005fshared"></a>
 During the combiner pass, shared structure within an insn can exist
 temporarily.  However, the shared structure is copied before the
 combiner is finished with the insn.  This is done by calling
 <code>copy_rtx_if_shared</code>, which is a subroutine of
 <code>unshare_all_rtl</code>.
 </li></ul>

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	<a name="Sharing"></a>
	<div class="header">
	<p>
	Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="Calls.html#Calls" accesskey="p" rel="prev">Calls</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="Structure-Sharing-Assumptions"></a>
	<h3 class="section">13.21 Structure Sharing Assumptions</h3>
	<a name="index-sharing-of-RTL-components"></a>
	<a name="index-RTL-structure-sharing-assumptions"></a>

	<p>The compiler assumes that certain kinds of RTL expressions are unique;
	there do not exist two distinct objects representing the same value.
	In other cases, it makes an opposite assumption: that no RTL expression
	object of a certain kind appears in more than one place in the
	containing structure.
	</p>
	<p>These assumptions refer to a single function; except for the RTL
	objects that describe global variables and external functions,
	and a few standard objects such as small integer constants,
	no RTL objects are common to two functions.
	</p>
	<ul>
	<li> <a name="index-reg_002c-RTL-sharing"></a>
	Each pseudo-register has only a single <code>reg</code> object to represent it,
	and therefore only a single machine mode.

	</li><li> <a name="index-symbolic-label"></a>
	<a name="index-symbol_005fref_002c-RTL-sharing"></a>
	For any symbolic label, there is only one <code>symbol_ref</code> object
	referring to it.

	</li><li> <a name="index-const_005fint_002c-RTL-sharing"></a>
	All <code>const_int</code> expressions with equal values are shared.

	</li><li> <a name="index-pc_002c-RTL-sharing"></a>
	There is only one <code>pc</code> expression.

	</li><li> <a name="index-cc0_002c-RTL-sharing"></a>
	There is only one <code>cc0</code> expression.

	</li><li> <a name="index-const_005fdouble_002c-RTL-sharing"></a>
	There is only one <code>const_double</code> expression with value 0 for
	each floating point mode. Likewise for values 1 and 2.

	</li><li> <a name="index-const_005fvector_002c-RTL-sharing"></a>
	There is only one <code>const_vector</code> expression with value 0 for
	each vector mode, be it an integer or a double constant vector.

	</li><li> <a name="index-label_005fref_002c-RTL-sharing"></a>
	<a name="index-scratch_002c-RTL-sharing"></a>
	No <code>label_ref</code> or <code>scratch</code> appears in more than one place in
	the RTL structure; in other words, it is safe to do a tree-walk of all
	the insns in the function and assume that each time a <code>label_ref</code>
	or <code>scratch</code> is seen it is distinct from all others that are seen.

	</li><li> <a name="index-mem_002c-RTL-sharing"></a>
	Only one <code>mem</code> object is normally created for each static
	variable or stack slot, so these objects are frequently shared in all
	the places they appear. However, separate but equal objects for these
	variables are occasionally made.

	</li><li> <a name="index-asm_005foperands_002c-RTL-sharing"></a>
	When a single <code>asm</code> statement has multiple output operands, a
	distinct <code>asm_operands</code> expression is made for each output operand.
	However, these all share the vector which contains the sequence of input
	operands. This sharing is used later on to test whether two
	<code>asm_operands</code> expressions come from the same statement, so all
	optimizations must carefully preserve the sharing if they copy the
	vector at all.

	</li><li> No RTL object appears in more than one place in the RTL structure
	except as described above. Many passes of the compiler rely on this
	by assuming that they can modify RTL objects in place without unwanted
	side-effects on other insns.

	</li><li> <a name="index-unshare_005fall_005frtl"></a>
	During initial RTL generation, shared structure is freely introduced.
	After all the RTL for a function has been generated, all shared
	structure is copied by <code>unshare_all_rtl</code> in <samp>emit-rtl.c</samp>,
	after which the above rules are guaranteed to be followed.

	</li><li> <a name="index-copy_005frtx_005fif_005fshared"></a>
	During the combiner pass, shared structure within an insn can exist
	temporarily. However, the shared structure is copied before the
	combiner is finished with the insn. This is done by calling
	<code>copy_rtx_if_shared</code>, which is a subroutine of
	<code>unshare_all_rtl</code>.
	</li></ul>

	<hr>
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	<p>
	Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="Calls.html#Calls" accesskey="p" rel="prev">Calls</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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