Comment by dragontamer

There are a lot of assembly language instructions that do slightly different things than standard C++ or C, but if the programmer is aware of them they can "handle" the differences.

For example, the xchg instruction doesn't have any C equivalent. (although it has a C++ equivalent: std::swap) The programmer may see:

    A ^= B; B ^= A; A ^= B

These two are swapped. A C compiler may be smart enough to know this is an xchg instruction, or it might turn them into xors. Hard to say, really.

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Most of the low hanging fruit have been taken up for sure. Almost every "memcpy" turns into "rep stos" for example (which is the assembly-language equivalent to memcpy).

A high-level programmer may not know that "memcpy" turns into "rep stos" however, and may emit his own memory copying for-loop.

At very least, a good optimizing C / C++ programmer needs to know about these little things. They'll let the compiler turn "memcpy" into "rep stos" (for -Os) or AVX memory store instructions respectively instead of writing their own less efficient loops on the matter.