fastcall

Fastcall is a calling convention used by some 32-bit x86 compilers to improve function call efficiency by passing some arguments in registers rather than on the stack. The main idea is to reduce stack traffic for the common case of functions with a few parameters, thereby speeding up calls on slower memory systems.

In typical 32-bit implementations, the first two arguments are passed in registers (commonly ECX and EDX on x86), with any remaining arguments pushed on the stack in right-to-left order. The callee is often responsible for cleaning the stack. The exact registers used and who cleans up the stack can vary by compiler and platform, so the details are not standardized across all implementations.

Fastcall originated in early C and C++ toolchains as an optimization over the more general cdecl calling convention. It was popular in some 32-bit Windows and Borland compilers but has become less common as modern 32-bit and 64-bit ABIs favor standardized conventions with more arguments passed in registers by the architecture itself. On 64-bit targets, the concept of fastcall is largely superseded by the platform’s standard calling conventions, and many compilers ignore or deprecate the fastcall keyword for 64-bit code.

Portability is a key consideration: code using fastcall is generally not portable across compilers or architectures, and name decoration or linkage can differ between environments. When cross-compiler or cross-language boundaries are involved, using standard conventions like cdecl or the platform-specific ABI is typically preferable.

In summary, fastcall is a historical 32-bit x86 optimization that passes initial arguments in registers to speed calls, with variations in practice and limited relevance on modern 64-bit systems.