### 17.12 Implicit Calls to Library Routines

Here is an explanation of implicit calls to library routines.

— Macro: DECLARE_LIBRARY_RENAMES

> This macro, if defined, should expand to a piece of C code that will get expanded when compiling functions for libgcc.a. It can be used to provide alternate names for GCC's internal library functions if there are ABI-mandated names that the compiler should provide.

— Target Hook: void TARGET_INIT_LIBFUNCS (void)

> This hook should declare additional library routines or rename existing ones, using the functions `set_optab_libfunc` and `init_one_libfunc` defined in `optabs.c`. `init_optabs` calls this macro after initializing all the normal library routines.

> The default is to do nothing. Most ports don't need to define this hook.

— Target Hook: bool TARGET_LIBFUNC_GNU_PREFIX

> If false (the default), internal library routines start with two underscores. If set to true, these routines start with `__gnu_` instead. E.g., `__muldi3` changes to `__gnu_muldi3`. This currently only affects functions defined in `libgcc2.c`. If this is set to true, the `tm.h` file must also `#define LIBGCC2_GNU_PREFIX`.

— Macro: FLOAT_LIB_COMPARE_RETURNS_BOOL (mode, comparison)

> This macro should return `true` if the library routine that implements the floating point comparison operator comparison in mode mode will return a boolean, and false if it will return a tristate.

> GCC's own floating point libraries return tristates from the comparison operators, so the default returns false always. Most ports don't need to define this macro.

— Macro: TARGET_LIB_INT_CMP_BIASED

> This macro should evaluate to `true` if the integer comparison functions (like `__cmpdi2`) return 0 to indicate that the first operand is smaller than the second, 1 to indicate that they are equal, and 2 to indicate that the first operand is greater than the second. If this macro evaluates to `false` the comparison functions return −1, 0, and 1 instead of 0, 1, and 2. If the target uses the routines in `libgcc.a`, you do not need to define this macro.

— Macro: TARGET_HAS_NO_HW_DIVIDE

> This macro should be defined if the target has no hardware divide instructions. If this macro is defined, GCC will use an algorithm which make use of simple logical and arithmetic operations for 64-bit division. If the macro is not defined, GCC will use an algorithm which make use of a 64-bit by 32-bit divide primitive.

— Macro: TARGET_EDOM

> The value of `EDOM` on the target machine, as a C integer constant expression. If you don't define this macro, GCC does not attempt to deposit the value of `EDOM` into `errno` directly. Look in `/usr/include/errno.h` to find the value of `EDOM` on your system.

> If you do not define `TARGET_EDOM`, then compiled code reports domain errors by calling the library function and letting it report the error. If mathematical functions on your system use `matherr` when there is an error, then you should leave `TARGET_EDOM` undefined so that `matherr` is used normally.

— Macro: GEN_ERRNO_RTX

> Define this macro as a C expression to create an rtl expression that refers to the global “variable” `errno`. (On certain systems, `errno` may not actually be a variable.) If you don't define this macro, a reasonable default is used.

— Target Hook: bool TARGET_LIBC_HAS_FUNCTION (enum function_class fn_class)

> This hook determines whether a function from a class of functions fn_class is present at the runtime.

— Macro: NEXT_OBJC_RUNTIME

> Set this macro to 1 to use the "NeXT" Objective-C message sending conventions by default. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function. This is the usual setting when targeting Darwin/Mac OS X systems, which have the NeXT runtime installed.

> If the macro is set to 0, the "GNU" Objective-C message sending convention will be used by default. This convention passes just the object and the selector to the method-lookup function, which returns a pointer to the method.

> In either case, it remains possible to select code-generation for the alternate scheme, by means of compiler command line switches.