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<title>Leaf Functions - GNU Compiler Collection (GCC) Internals</title>
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<a name="Leaf-Functions"></a>
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Next: <a rel="next" accesskey="n" href="Stack-Registers.html#Stack-Registers">Stack Registers</a>,
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Previous: <a rel="previous" accesskey="p" href="Values-in-Registers.html#Values-in-Registers">Values in Registers</a>,
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<h4 class="subsection">17.7.4 Handling Leaf Functions</h4>
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<p><a name="index-leaf-functions-4088"></a><a name="index-functions_002c-leaf-4089"></a>On some machines, a leaf function (i.e., one which makes no calls) can run
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more efficiently if it does not make its own register window. Often this
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means it is required to receive its arguments in the registers where they
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are passed by the caller, instead of the registers where they would
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normally arrive.
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<p>The special treatment for leaf functions generally applies only when
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other conditions are met; for example, often they may use only those
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registers for its own variables and temporaries. We use the term “leaf
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function” to mean a function that is suitable for this special
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handling, so that functions with no calls are not necessarily “leaf
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functions”.
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<p>GCC assigns register numbers before it knows whether the function is
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suitable for leaf function treatment. So it needs to renumber the
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registers in order to output a leaf function. The following macros
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accomplish this.
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<div class="defun">
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— Macro: <b>LEAF_REGISTERS</b><var><a name="index-LEAF_005fREGISTERS-4090"></a></var><br>
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<blockquote><p>Name of a char vector, indexed by hard register number, which
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contains 1 for a register that is allowable in a candidate for leaf
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function treatment.
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<p>If leaf function treatment involves renumbering the registers, then the
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registers marked here should be the ones before renumbering—those that
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GCC would ordinarily allocate. The registers which will actually be
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used in the assembler code, after renumbering, should not be marked with 1
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in this vector.
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<p>Define this macro only if the target machine offers a way to optimize
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the treatment of leaf functions.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>LEAF_REG_REMAP</b> (<var>regno</var>)<var><a name="index-LEAF_005fREG_005fREMAP-4091"></a></var><br>
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<blockquote><p>A C expression whose value is the register number to which <var>regno</var>
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should be renumbered, when a function is treated as a leaf function.
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<p>If <var>regno</var> is a register number which should not appear in a leaf
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function before renumbering, then the expression should yield −1, which
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will cause the compiler to abort.
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<p>Define this macro only if the target machine offers a way to optimize the
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treatment of leaf functions, and registers need to be renumbered to do
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this.
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</p></blockquote></div>
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<p><a name="index-current_005ffunction_005fis_005fleaf-4092"></a><a name="index-current_005ffunction_005fuses_005fonly_005fleaf_005fregs-4093"></a><code>TARGET_ASM_FUNCTION_PROLOGUE</code> and
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<code>TARGET_ASM_FUNCTION_EPILOGUE</code> must usually treat leaf functions
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specially. They can test the C variable <code>current_function_is_leaf</code>
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which is nonzero for leaf functions. <code>current_function_is_leaf</code> is
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set prior to local register allocation and is valid for the remaining
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compiler passes. They can also test the C variable
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<code>current_function_uses_only_leaf_regs</code> which is nonzero for leaf
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functions which only use leaf registers.
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<code>current_function_uses_only_leaf_regs</code> is valid after all passes
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that modify the instructions have been run and is only useful if
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<code>LEAF_REGISTERS</code> is defined.
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