The compiler-rt project is a simple library that provides an
implementation of the low-level target-specific hooks required by code
generation and other runtime components. For example, when compiling
for a 32-bit target, converting a double to a 64-bit unsigned integer
is compiling into a runtime call to the "__fixunsdfdi" function. The
compiler-rt library provides optimized implementations of this and other
low-level routines.

WWW: http://compiler-rt.llvm.org/