rngxx/src/rng/crand.c

#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <assert.h>
#include <stdio.h>

#include "crand.h"

#define internal _export(internal)

typedef unsigned short uint48_t[3];

struct jr_state
{
	union {
		long result;
		double fresult;
	};
	struct drand48_data data;
	union {
		//unsigned short xsubi[3];
		uint48_t xsubi;
		struct {
			uint64_t   xsubh : 48;
			uint16_t  _xsub  : 16;
		};
		uint64_t xsubl;
	} st;
};
typedef __typeof(((struct jr_state*)NULL)->st) jr_xsub_t;

_Static_assert( sizeof(uint48_t) == (sizeof(uint16_t) * 3), "bad uint48 (ushort[3])");
_Static_assert( sizeof(((struct jr_state*)NULL)->st) == sizeof(uint64_t), "bad uint64 (union st)");

inline static unsigned short* IFUNC_DEF(_jr_st_resolv, (jr_xsub_t* restrict state)
	__attribute__((const, nonnull, returns_nonnull, access(none, 1))));

inline static unsigned short* IFUNC_IMPL(_jr_st_resolv, low) (jr_xsub_t* restrict state)
{
	return state->xsubi;
}
inline static unsigned short* IFUNC_IMPL(_jr_st_resolv, high) (jr_xsub_t* restrict state)
{
	return state->xsubi+1;
}

inline static __attribute__((artificial, always_inline, gnu_inline, const)) int _resv_is_high()
{
	// This being `static const` initialised makes this function be seen as a proper constant expression. Nothing is leaked on to the stack of the caller and the function is replaced with a single `lea`.
	static const struct jr_state chk = {
		.st.xsubl = JR_MAX,
	};
	return chk.st._xsub 
		? 1
		: 0;
}

__attribute__((const))
inline static unsigned short* IFUNC_RESOLVER(_jr_st_resolv) (jr_xsub_t* restrict state)
{
	// This ifunc is free.
	// The optimiser recognises the return value of this function at compile time, and discards the unused function, removing the need for any runtime ifunc resolution.
	// The ifunc `_jr_st_resolv()` is essentially (almost) the same as a symbol-aliased constexpr function.
	// Ain't that neat?

#ifdef _RNGXX_JR_RESOLV_IFUNC_OLD_STACKDYN_CHECK
	// The old, dynamic stack alloc of the struct, instead of putting it in global r/o data
	struct jr_state chk = {0};
	chk.st.xsubl = JR_MAX;
#else
	// The new, static const alloc of the struct, puts the value in at compile-time. Better optimisation opportunities.
	// NOTE: Both stratergies make the ifunc resolve its target at compile-time, there is no difference in this ifunc resolver between the two. However, in `_resv_is_high()`, the old (this) stratergy prevents it being recognised as a constant-expression and stack-allocates the useless memory in the function's (macro-expanded) caller. Causing a spill of useless instructions that the new method changes to one `lea'.
	static const struct jr_state chk = {
		.st.xsubl = JR_MAX,
	};
#endif
	return chk.st._xsub 
		? & IFUNC_NAME(_jr_st_resolv, high)
		: & IFUNC_NAME(_jr_st_resolv, low);
}
#ifndef _RNGXX_JR_RESOLV_RUNTIME
#define _jr_st_resolv_f (_resv_is_high() \
				? & IFUNC_NAME(_jr_st_resolv, high) \
				: & IFUNC_NAME(_jr_st_resolv, low))
//unsigned short* (*const _jr_st_resolv_fp)(jr_xsub_t* restrict state) = _jr_st_resolv_f; // "not a constant expr ://

#define _jr_st_resolv(st) _jr_st_resolv_f (st)
#else
#define _jr_st_resolv_f _jr_st_resolv
#endif

_fspec(readonly) internal long _jr_lastl(const struct jr_state* restrict state)
{
	return state->result;
}

_fspec(readonly) internal double _jr_lastf(const struct jr_state* restrict state)
{
	return state->fresult;
}

internal void _jr_seed(struct jr_state* restrict state, unsigned long with)
{
	state->st.xsubh = with;
	seed48_r(_jr_st_resolv(&state->st), &state->data);
}

internal long _jr_proc(struct jr_state* restrict state)
{
	jrand48_r(_jr_st_resolv(&state->st), &state->data, &state->result);
	return state->result;
}

internal double _jr_procf(struct jr_state* restrict state)
{
	erand48_r(_jr_st_resolv(&state->st), &state->data, &state->fresult);
	return state->fresult;
}

__attribute__((malloc(_jr_free)))
internal struct jr_state* _jr_alloc()
{
	struct jr_state* bx = aligned_alloc(_Alignof(struct jr_state), sizeof(struct jr_state));
	if(UNLIKELY(!bx)) return NULL;

	memset(bx, 0, sizeof(struct jr_state));

	return bx;
}
__attribute__((malloc(_jr_free)))
internal struct jr_state* _jr_new(unsigned long with)
{
	struct jr_state* state = _jr_alloc();
	if(UNLIKELY(!state)) return NULL;

	_jr_seed(state, with);
	return state;
}

internal void _jr_free(struct jr_state* restrict state)
{
	if(LIKELY(state))
		free(state);
}

_fspec(pure)
internal unsigned short* _jr_state(struct jr_state* restrict state)
{
	return _jr_st_resolv(&state->st);
}

void __TEST__jr_test()
{
	struct jr_state* st = _jr_alloc();
	assert(!st->st._xsub);
	_jr_seed(st, ~0UL);
	const volatile unsigned short* res_state = _jr_st_resolv(&st->st);
	printf("seeded: %lu (full %lu, spill %u). xsubi = [%04x, %04x, %04x) %04x], resolv = [%04x, %04x, %04x) %04x]\n", (uint64_t)st->st.xsubh, st->st.xsubl, st->st._xsub,
				st->st.xsubi[0],
				st->st.xsubi[1],
				st->st.xsubi[2],
				st->st.xsubi[3],

				res_state[0],
				res_state[1],
				res_state[2],
				res_state[3]);
	assert(!st->st._xsub);
	for(int i=0;i<10;i++)
	{
		printf("res: %ld\n", _jr_proc(st));
		printf("state: %lu (full %lu, spill %u). xsubi = %p, resolv = %p\n", (uint64_t)st->st.xsubh, st->st.xsubl, st->st._xsub,
					(const void*)st->st.xsubi, (const void*)_jr_st_resolv(&st->st));
	}
	printf("ended: %lu (full %lu, spill %u). xsubi = [%04x, %04x, %04x) %04x], resolv = [%04x, %04x, %04x) %04x]\n", (uint64_t)st->st.xsubh, st->st.xsubl, st->st._xsub,
				st->st.xsubi[0],
				st->st.xsubi[1],
				st->st.xsubi[2],
				st->st.xsubi[3],

				res_state[0],
				res_state[1],
				res_state[2],
				res_state[3]);
	assert(!st->st._xsub);
	_jr_free(st);
}