1. A method, comprising: loading an executable program comprising a sequence of machine instructions compiled according to a source instruction set architecture (source ISA) for execution by a target instruction set architecture (target ISA)-based hardware execution unit, the loading including mapping a compiler built-in helper function (“helper function”) in the executable program to a target ISA machine instruction; and executing the loaded program, the executing comprising replacing the helper function with the target ISA machine instruction to which the helper function was mapped by: retrieving the target ISA machine instruction from a mapping table entry that includes an address of a helper function that matches an address of the function call encountered in the executing program; and replacing the function call in the executing program with the retrieved target ISA machine instruction; and executing the target ISA machine instruction.