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| <title>GNU Compiler Collection (GCC) Internals: Per-Function Data</title> |
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| <a name="Defining-data-structures-for-per_002dfunction-information_002e"></a> |
| <h3 class="section">17.4 Defining data structures for per-function information.</h3> |
| <a name="index-per_002dfunction-data"></a> |
| <a name="index-data-structures"></a> |
| |
| <p>If the target needs to store information on a per-function basis, GCC |
| provides a macro and a couple of variables to allow this. Note, just |
| using statics to store the information is a bad idea, since GCC supports |
| nested functions, so you can be halfway through encoding one function |
| when another one comes along. |
| </p> |
| <p>GCC defines a data structure called <code>struct function</code> which |
| contains all of the data specific to an individual function. This |
| structure contains a field called <code>machine</code> whose type is |
| <code>struct machine_function *</code>, which can be used by targets to point |
| to their own specific data. |
| </p> |
| <p>If a target needs per-function specific data it should define the type |
| <code>struct machine_function</code> and also the macro <code>INIT_EXPANDERS</code>. |
| This macro should be used to initialize the function pointer |
| <code>init_machine_status</code>. This pointer is explained below. |
| </p> |
| <p>One typical use of per-function, target specific data is to create an |
| RTX to hold the register containing the function’s return address. This |
| RTX can then be used to implement the <code>__builtin_return_address</code> |
| function, for level 0. |
| </p> |
| <p>Note—earlier implementations of GCC used a single data area to hold |
| all of the per-function information. Thus when processing of a nested |
| function began the old per-function data had to be pushed onto a |
| stack, and when the processing was finished, it had to be popped off the |
| stack. GCC used to provide function pointers called |
| <code>save_machine_status</code> and <code>restore_machine_status</code> to handle |
| the saving and restoring of the target specific information. Since the |
| single data area approach is no longer used, these pointers are no |
| longer supported. |
| </p> |
| <dl> |
| <dt><a name="index-INIT_005fEXPANDERS"></a>Macro: <strong>INIT_EXPANDERS</strong></dt> |
| <dd><p>Macro called to initialize any target specific information. This macro |
| is called once per function, before generation of any RTL has begun. |
| The intention of this macro is to allow the initialization of the |
| function pointer <code>init_machine_status</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-init_005fmachine_005fstatus"></a>Variable: <em>void (*)(struct function *)</em> <strong>init_machine_status</strong></dt> |
| <dd><p>If this function pointer is non-<code>NULL</code> it will be called once per |
| function, before function compilation starts, in order to allow the |
| target to perform any target specific initialization of the |
| <code>struct function</code> structure. It is intended that this would be |
| used to initialize the <code>machine</code> of that structure. |
| </p> |
| <p><code>struct machine_function</code> structures are expected to be freed by GC. |
| Generally, any memory that they reference must be allocated by using |
| GC allocation, including the structure itself. |
| </p></dd></dl> |
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