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13.18 Position Independent Code
This section describes macros that help implement generation of position
independent code. Simply defining these macros is not enough to
generate valid PIC; you must also add support to the macros
GO_IF_LEGITIMATE_ADDRESS and PRINT_OPERAND_ADDRESS, as
well as LEGITIMIZE_ADDRESS. You must modify the definition of
`movsi' to do something appropriate when the source operand
contains a symbolic address. You may also need to alter the handling of
switch statements so that they use relative addresses.
The register number of the register used to address a table of static data addresses in memory. In some cases this register is defined by a processor's “application binary interface” (ABI). When this macro is defined, RTL is generated for this register once, as with the stack pointer and frame pointer registers. If this macro is not defined, it is up to the machine-dependent files to allocate such a register (if necessary). Note that this register must be fixed when in use (e.g. when
flag_picis true).
Define this macro if the register defined by
PIC_OFFSET_TABLE_REGNUMis clobbered by calls. Do not define this macro ifPIC_OFFSET_TABLE_REGNUMis not defined.
By generating position-independent code, when two different programs (A and B) share a common library (libC.a), the text of the library can be shared whether or not the library is linked at the same address for both programs. In some of these environments, position-independent code requires not only the use of different addressing modes, but also special code to enable the use of these addressing modes.
The
FINALIZE_PICmacro serves as a hook to emit these special codes once the function is being compiled into assembly code, but not before. (It is not done before, because in the case of compiling an inline function, it would lead to multiple PIC prologues being included in functions which used inline functions and were compiled to assembly language.)
A C expression that is nonzero if x is a legitimate immediate operand on the target machine when generating position independent code. You can assume that x satisfies
CONSTANT_P, so you need not check this. You can also assume flag_pic is true, so you need not check it either. You need not define this macro if all constants (includingSYMBOL_REF) can be immediate operands when generating position independent code.