### 15.13 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.

— 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: US_SOFTWARE_GOFAST

> Define this macro if your system C library uses the US Software GOFAST library to provide floating point emulation.

> In addition to defining this macro, your architecture must set `TARGET_INIT_LIBFUNCS` to `gofast_maybe_init_libfuncs`, or else call that function from its version of that hook. It is defined in `config/gofast.h`, which must be included by your architecture's `cpu.c` file. See `sparc/sparc.c` for an example.

> If this macro is defined, the `TARGET_FLOAT_LIB_COMPARE_RETURNS_BOOL` target hook must return false for `SFmode` and `DFmode` comparisons.

— 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.

— Macro: TARGET_C99_FUNCTIONS

> When this macro is nonzero, GCC will implicitly optimize `sin` calls into `sinf` and similarly for other functions defined by C99 standard. The default is nonzero that should be proper value for most modern systems, however number of existing systems lacks support for these functions in the runtime so they needs this macro to be redefined to 0.

— Macro: NEXT_OBJC_RUNTIME

> Define this macro to generate code for Objective-C message sending using the calling convention of the NeXT system. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function.

> The default calling convention passes just the object and the selector to the lookup function, which returns a pointer to the method.