xtc_tailcall.h

/*-
 * Copyright (c) 2026, The XTC Project
 * Use of this source code is governed by the ISC License.
 *
 * src/inc/xtc_tailcall.h
 *      Mark functions/call-sites as MUST-be-tail-called.  GCC and
 *      Clang both perform tail-call optimization (TCO) when the
 *      last operation in a function is a call to another function
 *      whose return value is also returned.  We document the
 *      intent at the call site so a script can verify that the
 *      compiler actually emitted a `jmp` (or arch-equivalent) and
 *      not a `call` followed by `ret`.
 *
 *      Usage:
 *        - At a call-site that MUST be tail-called for correctness
 *          (e.g. avoiding stack growth in a deep dispatch chain),
 *          annotate via XTC_MUSTTAIL:
 *
 *              return XTC_MUSTTAIL fn(args);
 *
 *        - GCC's `__attribute__((musttail))` (15+) and Clang's
 *          `[[clang::musttail]]` are honored.  On older toolchains
 *          we fall back to `__attribute__((always_inline))` of a
 *          helper that returns directly.
 *
 *      Validation:
 *        scripts/check-tailcalls.sh disassembles the static lib
 *        and confirms each annotated site is a `jmp` not a `call`.
 *        Run via `make check-tailcalls`.
 *
 *      When TCO matters in xtc:
 *        - State-machine dispatch: handler returning a tail-call to
 *          the next state's handler (e.g. parse trees).
 *        - Scheduler dispatch: loop's task-step jumping into the
 *          coroutine entry without growing the loop's stack.
 *        - Lock-free fast paths where one inline helper jumps to
 *          the slow path on miss.
 */
 
#ifndef XTC_TAILCALL_H
#define XTC_TAILCALL_H
 
/* musttail is a *statement* attribute, not a function attribute.
 * Both clang's `[[clang::musttail]]` and gcc-15's
 * `__attribute__((musttail))` are accepted as statement attributes,
 * but only in compilers that parse C2x bracket attributes (clang)
 * or have GCC 15's extension.  In strict C11 mode -- which xtc uses --
 * we fall back to a no-op and rely on the optimizer.  The
 * `scripts/check-tailcalls.sh` validator inspects the .o to confirm
 * `jmp` was emitted regardless. */
/* XTC_MUSTTAIL is a marker; it's prefixed to a function call inside
 * a return statement to signal intent.  In gcc 15+ and clang's C2x
 * mode the attribute can be attached to the return statement itself
 * (`__attribute__((musttail)) return foo();`) but the syntactic
 * position varies enough across toolchains that we keep the macro
 * as a no-op universally and rely on:
 *   1. the -O2 optimizer doing TCO when it can, and
 *   2. scripts/check-tailcalls.sh validating the .o emitted `jmp`.
 * When compiler enforcement matters more than convenience, callers
 * can use the explicit form directly. */
#define XTC_MUSTTAIL  /* no-op marker; see check-tailcalls.sh */
 
/* Marker macro for the validator script: annotated call sites
 * become a #pragma comment that survives into the disassembly's
 * line metadata. */
#if defined(XTC_TAILCALL_MARK_SITES)
# define XTC_TAIL_CALL(expr) \
        (__extension__ ({ \
                _Pragma("message \"xtc-tailcall-marker\""); \
                XTC_MUSTTAIL expr; \
        }))
#else
# define XTC_TAIL_CALL(expr)  (XTC_MUSTTAIL expr)
#endif
 
#endif /* XTC_TAILCALL_H */