SHL ??

Pardon?
What is "the function SHL?" - it's not a standard C51 thing (not in the Library Reference).

Could you re-phrase/expand the question?

SHL? Do you mean Shift-Bit Left? If so, there is no function; it is a C operator that is implemented using the RLC assembler instruction. The 8051 has no shift instruction. To shift:

unsigned char var   = 0x01;
unsigned int bigVar = 0x8000;

char   <<= 1; // shift var left by one.
bigVar >>= 4; // shift bigVar right by 4.

A51 has a SHL operator which does a shift-left. It's not a function.

That's for the pre-processor, it is not for generating instructions. Ex:

MOV R6,#1 OR (MSK SHL 4)