path: root/coroutine.c

/* coroutine.c - Simple embeddable coroutine implementation
 *
 * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
 * SPDX-Licence-Identifier: AGPL-3.0-or-later
 */

#include <stdint.h> /* for uint8_t */
#include <stdio.h>  /* for fprintf(), stderr */
#include <stdlib.h> /* for calloc(), free() */
#include <assert.h>
#include <setjmp.h>

#include "coroutine.h"

/* Configuration **************************************************************/

#define COROUTINE_NUM           5
#define COROUTINE_MEASURE_STACK 1

/* Implementation *************************************************************/

/*
 * Portability notes:
 *
 * - It uses GCC `__attribute__`s, though these can likely be easily
 *   swapped for other compilers.
 *
 * - It has a small bit of CPU-specific code (assembly, and definition
 *   of a STACK_GROWS_DOWNWARD={0,1} macro) in
 *   coroutine.c:call_with_stack().  Other than this, it should be
 *   portable to other CPUs.  It currently contains implementations
 *   for __x86_64__ and __arm__, and should be fairly easy to add
 *   implementations for other CPUs.
 *
 * - It uses setjmp()/longjmp() in "unsafe" ways.  POSIX-2017
 *   longjmp(3) says
 *
 *   > If the most recent invocation of setjmp() with the
 *   > corresponding jmp_buf ... or if the function containing the
 *   > invocation of setjmp() has terminated execution in the interim,
 *   > or if the invocation of setjmp() was within the scope of an
 *   > identifier with variably modified type and execution has left
 *   > that scope in the interim, the behavior is undefined.
 *
 *   We use longjmp() both of these scenarios, but make it OK by using
 *   call_with_stack() to manage the stack ourselves, assuming the
 *   sole reason that longjmp() behavior is undefined in such cases is
 *   because the stack that its saved stack-pointer points to is no
 *   longer around.  It seems absurd that an implementation would
 *   choose to do something else, but I'm calling it out here because
 *   you never know.
 *
 *   Note that setjmp()/longjmp() are defined in 3 places: in the libc
 *   (glibc/newlib), as GCC intrinsics, and the lower-level GCC
 *   __builtin_{setjmp,newlib} which the libc and intrinsic versions
 *   likely use.  Our assumptions seem to be valid for
 *   x86_64-pc-linux-gnu/gcc-14.2.1/glibc-2.40 and
 *   arm-none-eabi/gcc-14.1.0/newlib-4.4.0.
 *
 * Why not use <ucontext.h>, the now-deprecated (was in POSIX.1-2001,
 * is gone in POSIX-2008) predecesor to <setjmp.h>?  It would let us
 * do this without any assembly or unsafe assumptions.  Simply:
 * because newlib does not provide it.
 */

/*
 * Design decisions and notes:
 *
 * - Coroutines are launched with a zeroed-out stack.  Because all
 *   stack variables should be initialized before they are read, this
 *   "shouldn't" make a difference, but: (1) Initializing it to a
 *   known value allows us to measure how much of the stack was
 *   written to, which is helpful to tune stack sizes.  (2) Leaving it
 *   uninitialized just gives me the willies.
 *
 * - Because embedded programs should be adverse to using the heap,
 *   COROUTINE_NUM is fixed, instead of having coroutine_add()
 *   dynamically grow the coroutine_table as-needed.
 *
 * - On the flip-side, coroutine stacks are allocated on the heap
 *   instead of having them be statically-allocated along with
 *   coroutine_table.  (1) This reduced the blast-area of damage for a
 *   stack-overflow; and indeed if the end of the stack alignes with a
 *   page-boundary memory-protection can even detect the overflow for
 *   us.  (2) Having different-looking addresses for stack-area vs
 *   static-area is handy for making things jump out at you when
 *   debugging.  (3) Given the above about wanting a zeroed-out stack,
 *   this allows us to take advantage of optimizations in calloc()
 *   instead of using memset, and this can likely also improve things
 *   with being page-aligned.
 *
 * - Coroutines must use cr_exit() instead of returning because if
 *   they return then they will return to call_with_stack() in
 *   coroutine_add() (not to after the longjmp() call in
 *   coroutine_main()), and besides being
 *   wrong-for-our-desired-flow-control, that's a stack location that
 *   no longer exists.
 *
 * Things to consider changing:
 *
 * - Consider having _cr_transition() go ahead and find the next
 *   coroutine to run and longjmp() direcly to it, instead of first
 *   jumping back to coroutine_main().  This could save a few cycles
 *   and a few bytes.
 */

enum coroutine_state {
	CR_NONE = 0,     /* this slot in the table is empty */
	CR_INITIALIZING, /* running, before cr_begin() */
	CR_RUNNING,      /* running, after cr_begin() */
	CR_RUNNABLE,     /* not running, but runnable */
	CR_PAUSED,       /* not running, and not runnable */ 
};

struct coroutine {
	enum coroutine_state state;
	jmp_buf              env;
	size_t               stack_size;
	void                *stack;
};

static struct coroutine coroutine_table[COROUTINE_NUM] = {0};
static cid_t            coroutine_running              = 0;
static jmp_buf          coroutine_add_env;
static jmp_buf          coroutine_main_env;

static void call_with_stack(void *stack, cr_fn_t fn, void *args) {
	static void *saved_sp = NULL;

	/* As part of sbc-harness, this only really needs to support
	 * ARM-32, but being able to run it on x86-64 is useful for
	 * debugging.  */
#if __x86_64__
#define STACK_GROWS_DOWNWARD 1
	asm volatile ("movq %%rsp , %0\n\t"    /* saved_sp = sp */
	              "movq %1    , %%rsp\n\t" /* sp = stack */
	              "movq %3    , %%rdi\n\t" /* arg0 = args */
	              "call *%2\n\t"           /* fn() */
	              "movq %0    , %%rsp"     /* sp = saved_sp */
	              :
	              : /* %0 */"m"(saved_sp),
	                /* %1 */"r"(stack),
	                /* %2 */"r"(fn),
	                /* %3 */"r"(args)
	              : "rdi"
	              );
#elif __arm__
#define STACK_GROWS_DOWNWARD 1
	/* str/ldr can only work with a "lo" register, which sp is
	 * not, so we use r0 as an intermediate because we're going to
	 * clobber it with args anyway.  */
	asm volatile ("mov r0, sp\n\t" /* [saved_sp = sp */
	              "str r0, %0\n\t" /* ] */
	              "mov sp, %1\n\t" /* [sp = stack] */
	              "mov r0, %1\n\t" /* [arg0 = args] */
	              "blx %2\n\t"     /* [fn()] */
	              "ldr r0, %0\n\t" /* [sp = staved_sp */
	              "mov sp, r0"     /* ] */
	              :
	              : /* %0 */"m"(saved_sp),
	                /* %1 */"r"(stack),
	                /* %2 */"r"(fn),
	                /* %3 */"r"(args)
	              : "r0"
	              );
#else
#error unsupported architecture
#endif
}

cid_t coroutine_add_with_stack_size(size_t stack_size, cr_fn_t fn, void *args) {
	static cid_t last_created = 0;

	assert(coroutine_running == 0 || coroutine_table[coroutine_running-1].state == CR_RUNNING);

	cid_t child;
	{
		size_t idx_base = last_created;
		for (size_t idx_shift = 0; idx_shift < COROUTINE_NUM; idx_shift++) {
			child = ((idx_base + idx_shift) % COROUTINE_NUM) + 1;
			if (coroutine_table[child-1].state == CR_NONE)
				goto found;
		}
		return 0;
	found:
	}

	last_created = child;

	coroutine_table[child-1].stack_size = stack_size;
	coroutine_table[child-1].stack = calloc(1, coroutine_table[child-1].stack_size);

	cid_t parent = coroutine_running;
	assert(parent == 0 || coroutine_table[parent-1].state == CR_RUNNING);
	coroutine_running = child; 
	coroutine_table[child-1].state = CR_INITIALIZING;
	if (!setjmp(coroutine_add_env)) { /* point=a */
		/* run until cr_begin() */
		call_with_stack(coroutine_table[child-1].stack + (STACK_GROWS_DOWNWARD ? coroutine_table[child-1].stack_size : 0), fn, args);
		assert(false); /* should cr_begin() instead of returning */
	}
	assert(coroutine_table[child-1].state == CR_RUNNABLE);
	assert(parent == 0 || coroutine_table[parent-1].state == CR_RUNNING);
	coroutine_running = parent;

	return child;
}

void coroutine_main(void) {
	bool ran;
	for (coroutine_running = 1;; coroutine_running = (coroutine_running%COROUTINE_NUM)+1) {
		if (coroutine_running == 1)
			ran = false;
		struct coroutine *cr = &coroutine_table[coroutine_running-1];
		if (cr->state == CR_RUNNABLE) {
			ran = true;
			cr->state = CR_RUNNING;
			if (!setjmp(coroutine_main_env)) { /* point=b */
				longjmp(cr->env, 1); /* jump to point=c */
				assert(false); /* should cr_exit() instead of returning */
			}
			if (cr->state == CR_NONE) {
#if COROUTINE_MEASURE_STACK
				size_t stack_used = cr->stack_size;
				while (stack_used > 0 && ((uint8_t*)cr->stack)[STACK_GROWS_DOWNWARD ? cr->stack_size - stack_used : stack_used - 1] == 0)
					stack_used--;
				printf("info: coroutine %zu exited having used %zu B stack space\n", coroutine_running, stack_used);
#endif
				free(cr->stack);
				coroutine_table[coroutine_running-1] = (struct coroutine){0};
			}
		}
		if (coroutine_running == COROUTINE_NUM && !ran) {
			fprintf(stderr, "error: no runnable coroutines\n");
			return;
		}
	}
}

bool cr_begin(void) {
	assert(coroutine_table[coroutine_running-1].state == CR_INITIALIZING);
	coroutine_table[coroutine_running-1].state = CR_RUNNABLE;
	if (!setjmp(coroutine_table[coroutine_running-1].env)) /* point=c1 */
		longjmp(coroutine_add_env, 1); /* jump to point=a */
}

static inline __attribute__ ((no_split_stack)) void _cr_transition(enum coroutine_state state) {
	assert(coroutine_running && coroutine_table[coroutine_running-1].state == CR_RUNNING);
	coroutine_table[coroutine_running-1].state = state;
	if (!setjmp(coroutine_table[coroutine_running-1].env))  /* point=c2 */
		longjmp(coroutine_main_env, 1); /* jump to point=b */
}

void cr_yield(void)           { _cr_transition(CR_RUNNABLE); }
void cr_pause_and_yield(void) { _cr_transition(CR_PAUSED); }

void cr_exit(void) {
	assert(coroutine_running && coroutine_table[coroutine_running-1].state == CR_RUNNING);
	coroutine_table[coroutine_running-1].state = CR_NONE;
	longjmp(coroutine_main_env, 1); /* jump to point=b */
}

void cr_unpause(cid_t cid) {
	assert(coroutine_table[cid-1].state == CR_PAUSED);
	coroutine_table[cid-1].state = CR_RUNNABLE;
}

cid_t cr_getcid(void) {
	return coroutine_running;
}