A guide to inline assembly code in GCC

The compiler generates correct code for the opcodes it uses. It knows nothing about any other opcodes which may appear in inline asm statements—to the compiler these are just opaque strings to be inserted directly into the listing passed to the assembler, which in turn only knows how to convert the opcodes into machine code, not what they actually do. Everything the compiler knows about how any given inline asm statement interacts with the rest of the program is based on the constraints specified by the programmer.

For RDTSC there is an inline intrinsic which is portable to at least GCC, Clang, and Visual C++: __rdtsc(). For GCC and Clang you need to #include <x86intrin.h> and for Visual C++ you need:

> #include <intrin.h>
> #pragma intrinsic(__rdtsc)

In all cases you simply write __rdtsc() and get a 64-bit integer, no inline asm required. Similar intrinsics exist for many other useful opcodes.