GNU Compiler Collection (GCC) Internals: Libgcc

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## 4 The GCC low-level runtime library

GCC provides a low-level runtime library, `libgcc.a` or `libgcc_s.so.1` on some platforms. GCC generates calls to routines in this library automatically, whenever it needs to perform some operation that is too complicated to emit inline code for.

Most of the routines in `libgcc` handle arithmetic operations that the target processor cannot perform directly. This includes integer multiply and divide on some machines, and all floating-point and fixed-point operations on other machines. `libgcc` also includes routines for exception handling, and a handful of miscellaneous operations.

Some of these routines can be defined in mostly machine-independent C. Others must be hand-written in assembly language for each processor that needs them.

GCC will also generate calls to C library routines, such as `memcpy` and `memset`, in some cases. The set of routines that GCC may possibly use is documented in Other Builtins in Using the GNU Compiler Collection (GCC).

These routines take arguments and return values of a specific machine mode, not a specific C type. See Machine Modes, for an explanation of this concept. For illustrative purposes, in this chapter the floating point type `float` is assumed to correspond to `SFmode`; `double` to `DFmode`; and `long double` to both `TFmode` and `XFmode`. Similarly, the integer types `int` and `unsigned int` correspond to `SImode`; `long` and `unsigned long` to `DImode`; and `long long` and `unsigned long long` to `TImode`.

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