Compiler Design – Why Do Some Compilers Generate Direct Machine Code?

IIRC one of the main reasons why Fortran is said to be faster is the absence of pointer aliasing, so they can use optimizations that C compilers can't use:

In FORTRAN, function arguments may not alias each other, and the compiler assumes they do not. This enables excellent optimization, and is one major reason for FORTRAN's reputation as a fast language. (Note that aliasing may still occur within a FORTRAN function. For instance, if A is an array and i and j are indices which happen to have the same value, then A[i] and A[j] are two different names for the same memory location. Fortunately, since the base array must have the same name, index analysis can be done to determine cases where A[i] and A[j] cannot alias.)

But I agree with others here: Comparing the average number of assembler instructions generated for a line of code is complete nonsense. For instance a modern x86 core can execute two instructions in parallel if they don't access the same registers. So you can (in theory) gain a performance increase of 100% for the same set on instructions just by reordering them. Good compilers will also often generate more assembly instructions to get faster code (think loop unrolling, inlining). The total number of assembler instructions says very little about the performance of a piece of code.

Look at the instruction set documentation, and you will find entries like this one from a pic microcontroller for each instruction:

The "encoding" line tells what that instruction looks like in binary. In this case, it always starts with 5 ones, then a don't care bit (which can be either one or zero), then the "k"s stand for the literal you are adding.

The first few bits are called an "opcode," are are unique for each instruction. The CPU basically looks at the opcode to see what instruction it is, then it knows to decode the "k"s as a number to be added.

It's tedious, but not that difficult to encode and decode. I had an undergrad class where we had to do it by hand in exams.

To actually make a full executable file, you also have to do things like allocate memory, calculate branch offsets, and put it into a format like ELF, depending on your operating system.