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| <a name="Handling-Leaf-Functions"></a> |
| <h4 class="subsection">17.7.4 Handling Leaf Functions</h4> |
| |
| <a name="index-leaf-functions"></a> |
| <a name="index-functions_002c-leaf"></a> |
| <p>On some machines, a leaf function (i.e., one which makes no calls) can run |
| more efficiently if it does not make its own register window. Often this |
| means it is required to receive its arguments in the registers where they |
| are passed by the caller, instead of the registers where they would |
| normally arrive. |
| </p> |
| <p>The special treatment for leaf functions generally applies only when |
| other conditions are met; for example, often they may use only those |
| registers for its own variables and temporaries. We use the term “leaf |
| function” to mean a function that is suitable for this special |
| handling, so that functions with no calls are not necessarily “leaf |
| functions”. |
| </p> |
| <p>GCC assigns register numbers before it knows whether the function is |
| suitable for leaf function treatment. So it needs to renumber the |
| registers in order to output a leaf function. The following macros |
| accomplish this. |
| </p> |
| <dl> |
| <dt><a name="index-LEAF_005fREGISTERS"></a>Macro: <strong>LEAF_REGISTERS</strong></dt> |
| <dd><p>Name of a char vector, indexed by hard register number, which |
| contains 1 for a register that is allowable in a candidate for leaf |
| function treatment. |
| </p> |
| <p>If leaf function treatment involves renumbering the registers, then the |
| registers marked here should be the ones before renumbering—those that |
| GCC would ordinarily allocate. The registers which will actually be |
| used in the assembler code, after renumbering, should not be marked with 1 |
| in this vector. |
| </p> |
| <p>Define this macro only if the target machine offers a way to optimize |
| the treatment of leaf functions. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-LEAF_005fREG_005fREMAP"></a>Macro: <strong>LEAF_REG_REMAP</strong> <em>(<var>regno</var>)</em></dt> |
| <dd><p>A C expression whose value is the register number to which <var>regno</var> |
| should be renumbered, when a function is treated as a leaf function. |
| </p> |
| <p>If <var>regno</var> is a register number which should not appear in a leaf |
| function before renumbering, then the expression should yield -1, which |
| will cause the compiler to abort. |
| </p> |
| <p>Define this macro only if the target machine offers a way to optimize the |
| treatment of leaf functions, and registers need to be renumbered to do |
| this. |
| </p></dd></dl> |
| |
| <a name="index-current_005ffunction_005fis_005fleaf"></a> |
| <a name="index-current_005ffunction_005fuses_005fonly_005fleaf_005fregs"></a> |
| <p><code>TARGET_ASM_FUNCTION_PROLOGUE</code> and |
| <code>TARGET_ASM_FUNCTION_EPILOGUE</code> must usually treat leaf functions |
| specially. They can test the C variable <code>current_function_is_leaf</code> |
| which is nonzero for leaf functions. <code>current_function_is_leaf</code> is |
| set prior to local register allocation and is valid for the remaining |
| compiler passes. They can also test the C variable |
| <code>current_function_uses_only_leaf_regs</code> which is nonzero for leaf |
| functions which only use leaf registers. |
| <code>current_function_uses_only_leaf_regs</code> is valid after all passes |
| that modify the instructions have been run and is only useful if |
| <code>LEAF_REGISTERS</code> is defined. |
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