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