coro_select.h

/*-
 * Copyright (c) 2026, The XTC Project
 * Use of this source code is governed by the ISC License.
 *
 * src/inc/coro_select.h
 *      Which coroutine substrate this build uses.  Defines XTC_CORO_FCTX
 *      when the hand-written fcontext substrate (coro_fctx.c) is live;
 *      otherwise ucontext (coro_uctx.c) is, or Win32 fibers on Windows.
 *      Every site that depends on the choice (coro_fctx.c, coro_uctx.c,
 *      coro_int.h's struct xtc_coro) tests this one macro, so they cannot
 *      disagree and struct xtc_coro's layout cannot drift between
 *      translation units.  Include after xtc_int.h (xtc_config.h).
 *
 *      fcontext is live when not Windows and:
 *        - the libc has no ucontext (musl), or
 *        - forced: -DXTC_CORO_FORCE_FCTX / configure --with-coro=fctx, or
 *        - by default on Linux x86-64 and aarch64 with glibc, and on
 *          Apple arm64.  ucontext's swapcontext makes a rt_sigprocmask
 *          system call on every switch (two per round trip), and on Linux
 *          that call takes the PROCESS-wide sighand lock: with many loops
 *          every fiber switch in the process serializes on one kernel
 *          spinlock.  Measured on a 64-vCPU x86-64 host: 16 loops of
 *          cross-loop request/reply 2.2M round trips/s on ucontext (56% of
 *          cycles spinning on that lock), 12.1M on fcontext; one fiber
 *          switch 550 -> 145 ns.  On a 16-vCPU aarch64 host (Neoverse-V1):
 *          one switch 1091 -> 136 ns, 16 loops 2.08M -> 8.91M round
 *          trips/s (ucontext fell from 2.45M at 8 loops).  fcontext has
 *          the same involuntary-preemption redirect on both
 *          (coro_preempt.h).
 *      ucontext is kept by -DXTC_CORO_FORCE_UCONTEXT / --with-coro=ucontext
 *      and in the single-file amalgamation (it cannot carry the .S files).
 */
 
#ifndef XTC_CORO_SELECT_H
#define XTC_CORO_SELECT_H
 
#include "xtc_int.h"   /* xtc_config.h: XTC_HAVE_UCONTEXT */
 
#if !defined(_WIN32) && \
    (!defined(XTC_HAVE_UCONTEXT) || defined(XTC_CORO_FORCE_FCTX) || \
     (!defined(XTC_CORO_FORCE_UCONTEXT) && !defined(XTC_AMALGAMATION) && \
      ((defined(__APPLE__) && defined(__aarch64__)) || \
       (defined(__linux__) && \
        (defined(__x86_64__) || defined(__aarch64__)) && \
        defined(__GLIBC__)))))
# define XTC_CORO_FCTX 1
#endif
 
#endif /* XTC_CORO_SELECT_H */