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<title>GNU Compiler Collection (GCC) Internals: Emulated TLS</title>
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<a name="Emulated-TLS"></a>
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<p>
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Next: <a href="MIPS-Coprocessors.html#MIPS-Coprocessors" accesskey="n" rel="next">MIPS Coprocessors</a>, Previous: <a href="Target-Attributes.html#Target-Attributes" accesskey="p" rel="prev">Target Attributes</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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="Emulating-TLS"></a>
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<h3 class="section">17.25 Emulating TLS</h3>
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<a name="index-Emulated-TLS"></a>
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<p>For targets whose psABI does not provide Thread Local Storage via
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specific relocations and instruction sequences, an emulation layer is
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used. A set of target hooks allows this emulation layer to be
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configured for the requirements of a particular target. For instance
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the psABI may in fact specify TLS support in terms of an emulation
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layer.
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</p>
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<p>The emulation layer works by creating a control object for every TLS
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object. To access the TLS object, a lookup function is provided
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which, when given the address of the control object, will return the
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address of the current thread’s instance of the TLS object.
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</p>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fGET_005fADDRESS"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_GET_ADDRESS</strong></dt>
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<dd><p>Contains the name of the helper function that uses a TLS control
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object to locate a TLS instance. The default causes libgcc’s
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emulated TLS helper function to be used.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fREGISTER_005fCOMMON"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_REGISTER_COMMON</strong></dt>
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<dd><p>Contains the name of the helper function that should be used at
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program startup to register TLS objects that are implicitly
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initialized to zero. If this is <code>NULL</code>, all TLS objects will
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have explicit initializers. The default causes libgcc’s emulated TLS
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registration function to be used.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fSECTION"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_VAR_SECTION</strong></dt>
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<dd><p>Contains the name of the section in which TLS control variables should
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be placed. The default of <code>NULL</code> allows these to be placed in
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any section.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fTMPL_005fSECTION"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_TMPL_SECTION</strong></dt>
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<dd><p>Contains the name of the section in which TLS initializers should be
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placed. The default of <code>NULL</code> allows these to be placed in any
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section.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fPREFIX"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_VAR_PREFIX</strong></dt>
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<dd><p>Contains the prefix to be prepended to TLS control variable names.
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The default of <code>NULL</code> uses a target-specific prefix.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fTMPL_005fPREFIX"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_TMPL_PREFIX</strong></dt>
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<dd><p>Contains the prefix to be prepended to TLS initializer objects. The
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default of <code>NULL</code> uses a target-specific prefix.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fFIELDS"></a>Target Hook: <em>tree</em> <strong>TARGET_EMUTLS_VAR_FIELDS</strong> <em>(tree <var>type</var>, tree *<var>name</var>)</em></dt>
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<dd><p>Specifies a function that generates the FIELD_DECLs for a TLS control
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object type. <var>type</var> is the RECORD_TYPE the fields are for and
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<var>name</var> should be filled with the structure tag, if the default of
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<code>__emutls_object</code> is unsuitable. The default creates a type suitable
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for libgcc’s emulated TLS function.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fINIT"></a>Target Hook: <em>tree</em> <strong>TARGET_EMUTLS_VAR_INIT</strong> <em>(tree <var>var</var>, tree <var>decl</var>, tree <var>tmpl_addr</var>)</em></dt>
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<dd><p>Specifies a function that generates the CONSTRUCTOR to initialize a
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TLS control object. <var>var</var> is the TLS control object, <var>decl</var>
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is the TLS object and <var>tmpl_addr</var> is the address of the
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initializer. The default initializes libgcc’s emulated TLS control object.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fALIGN_005fFIXED"></a>Target Hook: <em>bool</em> <strong>TARGET_EMUTLS_VAR_ALIGN_FIXED</strong></dt>
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<dd><p>Specifies whether the alignment of TLS control variable objects is
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fixed and should not be increased as some backends may do to optimize
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single objects. The default is false.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEMUTLS_005fDEBUG_005fFORM_005fTLS_005fADDRESS"></a>Target Hook: <em>bool</em> <strong>TARGET_EMUTLS_DEBUG_FORM_TLS_ADDRESS</strong></dt>
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<dd><p>Specifies whether a DWARF <code>DW_OP_form_tls_address</code> location descriptor
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may be used to describe emulated TLS control objects.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="MIPS-Coprocessors.html#MIPS-Coprocessors" accesskey="n" rel="next">MIPS Coprocessors</a>, Previous: <a href="Target-Attributes.html#Target-Attributes" accesskey="p" rel="prev">Target Attributes</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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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