According to the RFC [0], this review contains the compiler-rt parts of large integer divison for _BitInt.
It adds the functions
/// Computes the unsigned division of a / b.
/// Writes the quotient to quo and the remainder to rem.
///
/// words denotes the number of elements in a and b.
COMPILER_RT_ABI void __udivmodei5(su_int *quo, su_int *rem,
const su_int *a, const su_int *b,
unsigned int words);
/// Computes the signed division of a / b.
/// Writes the quotient to quo and the remainder to rem.
///
/// words denotes the number of elements in a and b.
/// Warning: Might modify a and b.
COMPILER_RT_ABI void __divmodei5(su_int *quo, su_int *rem,
su_int *a, su_int *b,
unsigned int words);into builtins.
In addition it introduces a new "bitint" library containing only those new functions,
which is meant as a way to provide those when using libgcc as runtime.
If lib/bitint is specific to lib/builtins, it should probably be placed in lib/builtins/