cosmopolitan/libc/tinymath/lrint.c

/*-*- mode:c;indent-tabs-mode:t;c-basic-offset:8;tab-width:8;coding:utf-8   -*-│
│vi: set et ft=c ts=8 tw=8 fenc=utf-8                                       :vi│
╚──────────────────────────────────────────────────────────────────────────────╝
│                                                                              │
│  Musl Libc                                                                   │
│  Copyright © 2005-2014 Rich Felker, et al.                                   │
│                                                                              │
│  Permission is hereby granted, free of charge, to any person obtaining       │
│  a copy of this software and associated documentation files (the             │
│  "Software"), to deal in the Software without restriction, including         │
│  without limitation the rights to use, copy, modify, merge, publish,         │
│  distribute, sublicense, and/or sell copies of the Software, and to          │
│  permit persons to whom the Software is furnished to do so, subject to       │
│  the following conditions:                                                   │
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│  The above copyright notice and this permission notice shall be              │
│  included in all copies or substantial portions of the Software.             │
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│  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,             │
│  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF          │
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╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/limits.h"
#include "libc/math.h"
#include "libc/runtime/fenv.h"
#include "libc/tinymath/expo.internal.h"
#include "libc/tinymath/feval.internal.h"

asm(".ident\t\"\\n\\n\
Musl libc (MIT License)\\n\
Copyright 2005-2014 Rich Felker, et. al.\"");
asm(".include \"libc/disclaimer.inc\"");
// clang-format off

/*
If the result cannot be represented (overflow, nan), then
lrint raises the invalid exception.

Otherwise if the input was not an integer then the inexact
exception is raised.

C99 is a bit vague about whether inexact exception is
allowed to be raised when invalid is raised.
(F.9 explicitly allows spurious inexact exceptions, F.9.6.5
does not make it clear if that rule applies to lrint, but
IEEE 754r 7.8 seems to forbid spurious inexact exception in
the ineger conversion functions)

So we try to make sure that no spurious inexact exception is
raised in case of an overflow.

If the bit size of long > precision of double, then there
cannot be inexact rounding in case the result overflows,
otherwise LONG_MAX and LONG_MIN can be represented exactly
as a double.
*/

#if LONG_MAX < 1U<<53 && defined(FE_INEXACT)
#if FLT_EVAL_METHOD==0 || FLT_EVAL_METHOD==1
#define EPS DBL_EPSILON
#elif FLT_EVAL_METHOD==2
#define EPS LDBL_EPSILON
#endif
static dontinline long lrint_slow(double x) {
	// #pragma STDC FENV_ACCESS ON
	int e;
	e = fetestexcept(FE_INEXACT);
	x = rint(x);
	if (!e && (x > LONG_MAX || x < LONG_MIN))
		feclearexcept(FE_INEXACT);
	/* conversion */
	return x;
}
#else
#define JUST_CALL_RINT
#endif

/**
 * Rounds to nearest integer.
 */
long lrint(double x) {
#ifdef __x86_64__
	long res;
	asm("cvtsd2si\t%1,%0" : "=r"(res) : "x"(x));
	return res;
#elif defined(__aarch64__)
	long res;
	asm("frintx\t%d1,%d1\n\t"
	    "fcvtzs\t%x0,%d1"
	    : "=r"(res), "+w"(x));
	return res;
#elif defined(__powerpc64__) && defined(_ARCH_PWR5X)
	long res;
	asm("fctid\t%0,%1" : "=d"(res) : "d"(x));
	return res;
#elif defined(JUST_CALL_RINT)
	return rint(x);
#else
	uint32_t abstop = asuint64(x)>>32 & 0x7fffffff;
	uint64_t sign = asuint64(x) & (1ULL << 63);
	if (abstop < 0x41dfffff) {
		/* |x| < 0x7ffffc00, no overflow */
		double_t toint = asdouble(asuint64(1/EPS) | sign);
		double_t y = x + toint - toint;
		return (long)y;
	}
	return lrint_slow(x);
#endif /* __x86_64__ */
}