The 8051 instruction set is tiny, so it is highly likely that a compiler has support for using any assembler instruction (if the program needs it).

For some CISC processor architectures, there can be so many instructions and addressing modes that a lot of compilers will settle for using a subset.

An example of a instruction often not used is a BCD adjust instruction, i.e. an instruction specifically intended to use when adding two BCD-encoded numbers together, to make sure that the sum will also be a BCD number. C code does not have a way of declaring that a variable should use BCD format, so the C compiler will have a hard time figuring out that the BCD adjust instruction could be used.