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14.7.4 Handling Leaf Functions

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.

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”.

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.

— Macro: LEAF_REGISTERS

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.

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.

Define this macro only if the target machine offers a way to optimize the treatment of leaf functions.

— Macro: LEAF_REG_REMAP (regno)

A C expression whose value is the register number to which regno should be renumbered, when a function is treated as a leaf function.

If regno 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.

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.

TARGET_ASM_FUNCTION_PROLOGUE and TARGET_ASM_FUNCTION_EPILOGUE must usually treat leaf functions specially. They can test the C variable current_function_is_leaf which is nonzero for leaf functions. current_function_is_leaf is set prior to local register allocation and is valid for the remaining compiler passes. They can also test the C variable current_function_uses_only_leaf_regs which is nonzero for leaf functions which only use leaf registers. current_function_uses_only_leaf_regs is valid after all passes that modify the instructions have been run and is only useful if LEAF_REGISTERS is defined.