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| <h5 class="subsubsection">9.7.4.2 Symbols in position-independent code</h5> |
| |
| <p><a name="index-Symbols-in-position_002dindependent-code_002c-CRIS-728"></a><a name="index-CRIS-symbols-in-position_002dindependent-code-729"></a><a name="index-Position_002dindependent-code_002c-symbols-in_002c-CRIS-730"></a> |
| When generating <a name="crispic"></a>position-independent code (SVR4 |
| PIC) for use in cris-axis-linux-gnu or crisv32-axis-linux-gnu |
| shared libraries, symbol |
| suffixes are used to specify what kind of run-time symbol lookup |
| will be used, expressed in the object as different |
| <em>relocation types</em>. Usually, all absolute symbol values |
| must be located in a table, the <em>global offset table</em>, |
| leaving the code position-independent; independent of values of |
| global symbols and independent of the address of the code. The |
| suffix modifies the value of the symbol, into for example an |
| index into the global offset table where the real symbol value |
| is entered, or a PC-relative value, or a value relative to the |
| start of the global offset table. All symbol suffixes start |
| with the character ‘<samp><span class="samp">:</span></samp>’ (omitted in the list below). Every |
| symbol use in code or a read-only section must therefore have a |
| PIC suffix to enable a useful shared library to be created. |
| Usually, these constructs must not be used with an additive |
| constant offset as is usually allowed, i.e. no 4 as in |
| <code>symbol + 4</code> is allowed. This restriction is checked at |
| link-time, not at assembly-time. |
| |
| <dl> |
| <dt><code>GOT</code><dd> |
| Attaching this suffix to a symbol in an instruction causes the |
| symbol to be entered into the global offset table. The value is |
| a 32-bit index for that symbol into the global offset table. |
| The name of the corresponding relocation is |
| ‘<samp><span class="samp">R_CRIS_32_GOT</span></samp>’. Example: <code>move.d |
| [$r0+extsym:GOT],$r9</code> |
| |
| <br><dt><code>GOT16</code><dd> |
| Same as for ‘<samp><span class="samp">GOT</span></samp>’, but the value is a 16-bit index into the |
| global offset table. The corresponding relocation is |
| ‘<samp><span class="samp">R_CRIS_16_GOT</span></samp>’. Example: <code>move.d |
| [$r0+asymbol:GOT16],$r10</code> |
| |
| <br><dt><code>PLT</code><dd> |
| This suffix is used for function symbols. It causes a |
| <em>procedure linkage table</em>, an array of code stubs, to be |
| created at the time the shared object is created or linked |
| against, together with a global offset table entry. The value |
| is a pc-relative offset to the corresponding stub code in the |
| procedure linkage table. This arrangement causes the run-time |
| symbol resolver to be called to look up and set the value of the |
| symbol the first time the function is called (at latest; |
| depending environment variables). It is only safe to leave the |
| symbol unresolved this way if all references are function calls. |
| The name of the relocation is ‘<samp><span class="samp">R_CRIS_32_PLT_PCREL</span></samp>’. |
| Example: <code>add.d fnname:PLT,$pc</code> |
| |
| <br><dt><code>PLTG</code><dd> |
| Like PLT, but the value is relative to the beginning of the |
| global offset table. The relocation is |
| ‘<samp><span class="samp">R_CRIS_32_PLT_GOTREL</span></samp>’. Example: <code>move.d |
| fnname:PLTG,$r3</code> |
| |
| <br><dt><code>GOTPLT</code><dd> |
| Similar to ‘<samp><span class="samp">PLT</span></samp>’, but the value of the symbol is a 32-bit |
| index into the global offset table. This is somewhat of a mix |
| between the effect of the ‘<samp><span class="samp">GOT</span></samp>’ and the ‘<samp><span class="samp">PLT</span></samp>’ suffix; |
| the difference to ‘<samp><span class="samp">GOT</span></samp>’ is that there will be a procedure |
| linkage table entry created, and that the symbol is assumed to |
| be a function entry and will be resolved by the run-time |
| resolver as with ‘<samp><span class="samp">PLT</span></samp>’. The relocation is |
| ‘<samp><span class="samp">R_CRIS_32_GOTPLT</span></samp>’. Example: <code>jsr |
| [$r0+fnname:GOTPLT]</code> |
| |
| <br><dt><code>GOTPLT16</code><dd> |
| A variant of ‘<samp><span class="samp">GOTPLT</span></samp>’ giving a 16-bit value. Its |
| relocation name is ‘<samp><span class="samp">R_CRIS_16_GOTPLT</span></samp>’. Example: <code>jsr |
| [$r0+fnname:GOTPLT16]</code> |
| |
| <br><dt><code>GOTOFF</code><dd> |
| This suffix must only be attached to a local symbol, but may be |
| used in an expression adding an offset. The value is the |
| address of the symbol relative to the start of the global offset |
| table. The relocation name is ‘<samp><span class="samp">R_CRIS_32_GOTREL</span></samp>’. |
| Example: <code>move.d [$r0+localsym:GOTOFF],r3</code> |
| </dl> |
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