ctx_x86_64.c

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/* -*- C -*-
 * Serene programming language
 * Copyright (C) 2019-2026 Sameer Rahmani <[email protected]>
 *
 * This library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library.  If not, see <https://www.gnu.org/licenses/>.
 */
 
/// @file
/// x86-64 fabrication of a fresh fiber's saved context. Paired with
/// switch_x86_64.S; both are selected by CPU family in meson.build.
 
#include <stdint.h>
 
// x86-64 context fabrication. On another architecture this file compiles to
// nothing and the sibling ctx_<arch>.c supplies srn_fiber_ctx_make instead. The
// selection is by target macro, so the whole runtime source set can be handed
// to a cross compiler and only the matching architecture emits symbols.
#if defined(__x86_64__)
 
#  include "serene/rt/fiber.h"
#  include "serene/rt/fiber/arch/stack_x86_64.h"
 
// NOLINTBEGIN(performance-no-int-to-ptr)
void srn_fiber_ctx_make(
  srn_fiber_ctx_t *fiber_ctx, srn_fiber_stack_t stack, void (*fn)(void *), void *arg
) {
  // Lay srn_fiber_swap's frame (see rt/fiber/stack.h and switch_x86_64.S) at
  // the top of `stack`, rounded down to SRN_FIBER_STACK_ALIGN so the
  // return-address slot -- and thus the trampoline's entry rsp -- is correctly
  // aligned. The compound literal zero-fills the slots not named here. Register
  // slots are uintptr_t, so storing fn/trampoline is a
  // function-pointer-to-integer conversion (allowed), not the
  // function-to-object-pointer conversion that -Wpedantic forbids.
  uintptr_t top = (uintptr_t)stack.start;
  uintptr_t sp  = (top - sizeof(srn_fiber_frame_t)) & ~(uintptr_t)(SRN_FIBER_STACK_ALIGN - 1);
 
  *(srn_fiber_frame_t *)sp = (srn_fiber_frame_t){
    .mxcsr    = SRN_FIBER_MXCSR_DEFAULT,
    .fcw      = SRN_FIBER_FCW_DEFAULT,
    .r13      = (uintptr_t)fn,
    .r12      = (uintptr_t)arg,
    .ret_addr = (uintptr_t)srn_fiber_trampoline,
  };
 
  fiber_ctx->sp = (void *)sp;
}
// NOLINTEND(performance-no-int-to-ptr)
 
#endif // __x86_64__