softfloat: Move sqrt_float to softfloat-parts.c.inc

Rename to parts$N_sqrt.
Reimplement float128_sqrt with FloatParts128.

Reimplement with the inverse sqrt newton-raphson algorithm from musl.
This is significantly faster than even the berkeley sqrt n-r algorithm,
because it does not use division instructions, only multiplication.

Ordinarily, changing algorithms at the same time as migrating code is
a bad idea, but this is the only way I found that didn't break one of
the routines at the same time.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2020-11-18 12:14:37 -08:00
parent 39626b0ce8
commit 9261b245f0
2 changed files with 261 additions and 152 deletions

View File

@ -597,6 +597,212 @@ static FloatPartsN *partsN(div)(FloatPartsN *a, FloatPartsN *b,
return a;
}
/*
* Square Root
*
* The base algorithm is lifted from
* https://git.musl-libc.org/cgit/musl/tree/src/math/sqrtf.c
* https://git.musl-libc.org/cgit/musl/tree/src/math/sqrt.c
* https://git.musl-libc.org/cgit/musl/tree/src/math/sqrtl.c
* and is thus MIT licenced.
*/
static void partsN(sqrt)(FloatPartsN *a, float_status *status,
const FloatFmt *fmt)
{
const uint32_t three32 = 3u << 30;
const uint64_t three64 = 3ull << 62;
uint32_t d32, m32, r32, s32, u32; /* 32-bit computation */
uint64_t d64, m64, r64, s64, u64; /* 64-bit computation */
uint64_t dh, dl, rh, rl, sh, sl, uh, ul; /* 128-bit computation */
uint64_t d0h, d0l, d1h, d1l, d2h, d2l;
uint64_t discard;
bool exp_odd;
size_t index;
if (unlikely(a->cls != float_class_normal)) {
switch (a->cls) {
case float_class_snan:
case float_class_qnan:
parts_return_nan(a, status);
return;
case float_class_zero:
return;
case float_class_inf:
if (unlikely(a->sign)) {
goto d_nan;
}
return;
default:
g_assert_not_reached();
}
}
if (unlikely(a->sign)) {
goto d_nan;
}
/*
* Argument reduction.
* x = 4^e frac; with integer e, and frac in [1, 4)
* m = frac fixed point at bit 62, since we're in base 4.
* If base-2 exponent is odd, exchange that for multiply by 2,
* which results in no shift.
*/
exp_odd = a->exp & 1;
index = extract64(a->frac_hi, 57, 6) | (!exp_odd << 6);
if (!exp_odd) {
frac_shr(a, 1);
}
/*
* Approximate r ~= 1/sqrt(m) and s ~= sqrt(m) when m in [1, 4).
*
* Initial estimate:
* 7-bit lookup table (1-bit exponent and 6-bit significand).
*
* The relative error (e = r0*sqrt(m)-1) of a linear estimate
* (r0 = a*m + b) is |e| < 0.085955 ~ 0x1.6p-4 at best;
* a table lookup is faster and needs one less iteration.
* The 7-bit table gives |e| < 0x1.fdp-9.
*
* A Newton-Raphson iteration for r is
* s = m*r
* d = s*r
* u = 3 - d
* r = r*u/2
*
* Fixed point representations:
* m, s, d, u, three are all 2.30; r is 0.32
*/
m64 = a->frac_hi;
m32 = m64 >> 32;
r32 = rsqrt_tab[index] << 16;
/* |r*sqrt(m) - 1| < 0x1.FDp-9 */
s32 = ((uint64_t)m32 * r32) >> 32;
d32 = ((uint64_t)s32 * r32) >> 32;
u32 = three32 - d32;
if (N == 64) {
/* float64 or smaller */
r32 = ((uint64_t)r32 * u32) >> 31;
/* |r*sqrt(m) - 1| < 0x1.7Bp-16 */
s32 = ((uint64_t)m32 * r32) >> 32;
d32 = ((uint64_t)s32 * r32) >> 32;
u32 = three32 - d32;
if (fmt->frac_size <= 23) {
/* float32 or smaller */
s32 = ((uint64_t)s32 * u32) >> 32; /* 3.29 */
s32 = (s32 - 1) >> 6; /* 9.23 */
/* s < sqrt(m) < s + 0x1.08p-23 */
/* compute nearest rounded result to 2.23 bits */
uint32_t d0 = (m32 << 16) - s32 * s32;
uint32_t d1 = s32 - d0;
uint32_t d2 = d1 + s32 + 1;
s32 += d1 >> 31;
a->frac_hi = (uint64_t)s32 << (64 - 25);
/* increment or decrement for inexact */
if (d2 != 0) {
a->frac_hi += ((int32_t)(d1 ^ d2) < 0 ? -1 : 1);
}
goto done;
}
/* float64 */
r64 = (uint64_t)r32 * u32 * 2;
/* |r*sqrt(m) - 1| < 0x1.37-p29; convert to 64-bit arithmetic */
mul64To128(m64, r64, &s64, &discard);
mul64To128(s64, r64, &d64, &discard);
u64 = three64 - d64;
mul64To128(s64, u64, &s64, &discard); /* 3.61 */
s64 = (s64 - 2) >> 9; /* 12.52 */
/* Compute nearest rounded result */
uint64_t d0 = (m64 << 42) - s64 * s64;
uint64_t d1 = s64 - d0;
uint64_t d2 = d1 + s64 + 1;
s64 += d1 >> 63;
a->frac_hi = s64 << (64 - 54);
/* increment or decrement for inexact */
if (d2 != 0) {
a->frac_hi += ((int64_t)(d1 ^ d2) < 0 ? -1 : 1);
}
goto done;
}
r64 = (uint64_t)r32 * u32 * 2;
/* |r*sqrt(m) - 1| < 0x1.7Bp-16; convert to 64-bit arithmetic */
mul64To128(m64, r64, &s64, &discard);
mul64To128(s64, r64, &d64, &discard);
u64 = three64 - d64;
mul64To128(u64, r64, &r64, &discard);
r64 <<= 1;
/* |r*sqrt(m) - 1| < 0x1.a5p-31 */
mul64To128(m64, r64, &s64, &discard);
mul64To128(s64, r64, &d64, &discard);
u64 = three64 - d64;
mul64To128(u64, r64, &rh, &rl);
add128(rh, rl, rh, rl, &rh, &rl);
/* |r*sqrt(m) - 1| < 0x1.c001p-59; change to 128-bit arithmetic */
mul128To256(a->frac_hi, a->frac_lo, rh, rl, &sh, &sl, &discard, &discard);
mul128To256(sh, sl, rh, rl, &dh, &dl, &discard, &discard);
sub128(three64, 0, dh, dl, &uh, &ul);
mul128To256(uh, ul, sh, sl, &sh, &sl, &discard, &discard); /* 3.125 */
/* -0x1p-116 < s - sqrt(m) < 0x3.8001p-125 */
sub128(sh, sl, 0, 4, &sh, &sl);
shift128Right(sh, sl, 13, &sh, &sl); /* 16.112 */
/* s < sqrt(m) < s + 1ulp */
/* Compute nearest rounded result */
mul64To128(sl, sl, &d0h, &d0l);
d0h += 2 * sh * sl;
sub128(a->frac_lo << 34, 0, d0h, d0l, &d0h, &d0l);
sub128(sh, sl, d0h, d0l, &d1h, &d1l);
add128(sh, sl, 0, 1, &d2h, &d2l);
add128(d2h, d2l, d1h, d1l, &d2h, &d2l);
add128(sh, sl, 0, d1h >> 63, &sh, &sl);
shift128Left(sh, sl, 128 - 114, &sh, &sl);
/* increment or decrement for inexact */
if (d2h | d2l) {
if ((int64_t)(d1h ^ d2h) < 0) {
sub128(sh, sl, 0, 1, &sh, &sl);
} else {
add128(sh, sl, 0, 1, &sh, &sl);
}
}
a->frac_lo = sl;
a->frac_hi = sh;
done:
/* Convert back from base 4 to base 2. */
a->exp >>= 1;
if (!(a->frac_hi & DECOMPOSED_IMPLICIT_BIT)) {
frac_add(a, a, a);
} else {
a->exp += 1;
}
return;
d_nan:
float_raise(float_flag_invalid, status);
parts_default_nan(a, status);
}
/*
* Rounds the floating-point value `a' to an integer, and returns the
* result as a floating-point value. The operation is performed

View File

@ -820,6 +820,12 @@ static FloatParts128 *parts128_div(FloatParts128 *a, FloatParts128 *b,
#define parts_div(A, B, S) \
PARTS_GENERIC_64_128(div, A)(A, B, S)
static void parts64_sqrt(FloatParts64 *a, float_status *s, const FloatFmt *f);
static void parts128_sqrt(FloatParts128 *a, float_status *s, const FloatFmt *f);
#define parts_sqrt(A, S, F) \
PARTS_GENERIC_64_128(sqrt, A)(A, S, F)
static bool parts64_round_to_int_normal(FloatParts64 *a, FloatRoundMode rm,
int scale, int frac_size);
static bool parts128_round_to_int_normal(FloatParts128 *a, FloatRoundMode r,
@ -1386,6 +1392,30 @@ static void frac128_widen(FloatParts256 *r, FloatParts128 *a)
#define frac_widen(A, B) FRAC_GENERIC_64_128(widen, B)(A, B)
/*
* Reciprocal sqrt table. 1 bit of exponent, 6-bits of mantessa.
* From https://git.musl-libc.org/cgit/musl/tree/src/math/sqrt_data.c
* and thus MIT licenced.
*/
static const uint16_t rsqrt_tab[128] = {
0xb451, 0xb2f0, 0xb196, 0xb044, 0xaef9, 0xadb6, 0xac79, 0xab43,
0xaa14, 0xa8eb, 0xa7c8, 0xa6aa, 0xa592, 0xa480, 0xa373, 0xa26b,
0xa168, 0xa06a, 0x9f70, 0x9e7b, 0x9d8a, 0x9c9d, 0x9bb5, 0x9ad1,
0x99f0, 0x9913, 0x983a, 0x9765, 0x9693, 0x95c4, 0x94f8, 0x9430,
0x936b, 0x92a9, 0x91ea, 0x912e, 0x9075, 0x8fbe, 0x8f0a, 0x8e59,
0x8daa, 0x8cfe, 0x8c54, 0x8bac, 0x8b07, 0x8a64, 0x89c4, 0x8925,
0x8889, 0x87ee, 0x8756, 0x86c0, 0x862b, 0x8599, 0x8508, 0x8479,
0x83ec, 0x8361, 0x82d8, 0x8250, 0x81c9, 0x8145, 0x80c2, 0x8040,
0xff02, 0xfd0e, 0xfb25, 0xf947, 0xf773, 0xf5aa, 0xf3ea, 0xf234,
0xf087, 0xeee3, 0xed47, 0xebb3, 0xea27, 0xe8a3, 0xe727, 0xe5b2,
0xe443, 0xe2dc, 0xe17a, 0xe020, 0xdecb, 0xdd7d, 0xdc34, 0xdaf1,
0xd9b3, 0xd87b, 0xd748, 0xd61a, 0xd4f1, 0xd3cd, 0xd2ad, 0xd192,
0xd07b, 0xcf69, 0xce5b, 0xcd51, 0xcc4a, 0xcb48, 0xca4a, 0xc94f,
0xc858, 0xc764, 0xc674, 0xc587, 0xc49d, 0xc3b7, 0xc2d4, 0xc1f4,
0xc116, 0xc03c, 0xbf65, 0xbe90, 0xbdbe, 0xbcef, 0xbc23, 0xbb59,
0xba91, 0xb9cc, 0xb90a, 0xb84a, 0xb78c, 0xb6d0, 0xb617, 0xb560,
};
#define partsN(NAME) glue(glue(glue(parts,N),_),NAME)
#define FloatPartsN glue(FloatParts,N)
#define FloatPartsW glue(FloatParts,W)
@ -3586,103 +3616,35 @@ float128 float128_scalbn(float128 a, int n, float_status *status)
/*
* Square Root
*
* The old softfloat code did an approximation step before zeroing in
* on the final result. However for simpleness we just compute the
* square root by iterating down from the implicit bit to enough extra
* bits to ensure we get a correctly rounded result.
*
* This does mean however the calculation is slower than before,
* especially for 64 bit floats.
*/
static FloatParts64 sqrt_float(FloatParts64 a, float_status *s, const FloatFmt *p)
{
uint64_t a_frac, r_frac, s_frac;
int bit, last_bit;
if (is_nan(a.cls)) {
parts_return_nan(&a, s);
return a;
}
if (a.cls == float_class_zero) {
return a; /* sqrt(+-0) = +-0 */
}
if (a.sign) {
float_raise(float_flag_invalid, s);
parts_default_nan(&a, s);
return a;
}
if (a.cls == float_class_inf) {
return a; /* sqrt(+inf) = +inf */
}
assert(a.cls == float_class_normal);
/* We need two overflow bits at the top. Adding room for that is a
* right shift. If the exponent is odd, we can discard the low bit
* by multiplying the fraction by 2; that's a left shift. Combine
* those and we shift right by 1 if the exponent is odd, otherwise 2.
*/
a_frac = a.frac >> (2 - (a.exp & 1));
a.exp >>= 1;
/* Bit-by-bit computation of sqrt. */
r_frac = 0;
s_frac = 0;
/* Iterate from implicit bit down to the 3 extra bits to compute a
* properly rounded result. Remember we've inserted two more bits
* at the top, so these positions are two less.
*/
bit = DECOMPOSED_BINARY_POINT - 2;
last_bit = MAX(p->frac_shift - 4, 0);
do {
uint64_t q = 1ULL << bit;
uint64_t t_frac = s_frac + q;
if (t_frac <= a_frac) {
s_frac = t_frac + q;
a_frac -= t_frac;
r_frac += q;
}
a_frac <<= 1;
} while (--bit >= last_bit);
/* Undo the right shift done above. If there is any remaining
* fraction, the result is inexact. Set the sticky bit.
*/
a.frac = (r_frac << 2) + (a_frac != 0);
return a;
}
float16 QEMU_FLATTEN float16_sqrt(float16 a, float_status *status)
{
FloatParts64 pa, pr;
FloatParts64 p;
float16_unpack_canonical(&pa, a, status);
pr = sqrt_float(pa, status, &float16_params);
return float16_round_pack_canonical(&pr, status);
float16_unpack_canonical(&p, a, status);
parts_sqrt(&p, status, &float16_params);
return float16_round_pack_canonical(&p, status);
}
static float32 QEMU_SOFTFLOAT_ATTR
soft_f32_sqrt(float32 a, float_status *status)
{
FloatParts64 pa, pr;
FloatParts64 p;
float32_unpack_canonical(&pa, a, status);
pr = sqrt_float(pa, status, &float32_params);
return float32_round_pack_canonical(&pr, status);
float32_unpack_canonical(&p, a, status);
parts_sqrt(&p, status, &float32_params);
return float32_round_pack_canonical(&p, status);
}
static float64 QEMU_SOFTFLOAT_ATTR
soft_f64_sqrt(float64 a, float_status *status)
{
FloatParts64 pa, pr;
FloatParts64 p;
float64_unpack_canonical(&pa, a, status);
pr = sqrt_float(pa, status, &float64_params);
return float64_round_pack_canonical(&pr, status);
float64_unpack_canonical(&p, a, status);
parts_sqrt(&p, status, &float64_params);
return float64_round_pack_canonical(&p, status);
}
float32 QEMU_FLATTEN float32_sqrt(float32 xa, float_status *s)
@ -3741,11 +3703,20 @@ float64 QEMU_FLATTEN float64_sqrt(float64 xa, float_status *s)
bfloat16 QEMU_FLATTEN bfloat16_sqrt(bfloat16 a, float_status *status)
{
FloatParts64 pa, pr;
FloatParts64 p;
bfloat16_unpack_canonical(&pa, a, status);
pr = sqrt_float(pa, status, &bfloat16_params);
return bfloat16_round_pack_canonical(&pr, status);
bfloat16_unpack_canonical(&p, a, status);
parts_sqrt(&p, status, &bfloat16_params);
return bfloat16_round_pack_canonical(&p, status);
}
float128 QEMU_FLATTEN float128_sqrt(float128 a, float_status *status)
{
FloatParts128 p;
float128_unpack_canonical(&p, a, status);
parts_sqrt(&p, status, &float128_params);
return float128_round_pack_canonical(&p, status);
}
/*----------------------------------------------------------------------------
@ -6473,74 +6444,6 @@ float128 float128_rem(float128 a, float128 b, float_status *status)
status);
}
/*----------------------------------------------------------------------------
| Returns the square root of the quadruple-precision floating-point value `a'.
| The operation is performed according to the IEC/IEEE Standard for Binary
| Floating-Point Arithmetic.
*----------------------------------------------------------------------------*/
float128 float128_sqrt(float128 a, float_status *status)
{
bool aSign;
int32_t aExp, zExp;
uint64_t aSig0, aSig1, zSig0, zSig1, zSig2, doubleZSig0;
uint64_t rem0, rem1, rem2, rem3, term0, term1, term2, term3;
aSig1 = extractFloat128Frac1( a );
aSig0 = extractFloat128Frac0( a );
aExp = extractFloat128Exp( a );
aSign = extractFloat128Sign( a );
if ( aExp == 0x7FFF ) {
if (aSig0 | aSig1) {
return propagateFloat128NaN(a, a, status);
}
if ( ! aSign ) return a;
goto invalid;
}
if ( aSign ) {
if ( ( aExp | aSig0 | aSig1 ) == 0 ) return a;
invalid:
float_raise(float_flag_invalid, status);
return float128_default_nan(status);
}
if ( aExp == 0 ) {
if ( ( aSig0 | aSig1 ) == 0 ) return packFloat128( 0, 0, 0, 0 );
normalizeFloat128Subnormal( aSig0, aSig1, &aExp, &aSig0, &aSig1 );
}
zExp = ( ( aExp - 0x3FFF )>>1 ) + 0x3FFE;
aSig0 |= UINT64_C(0x0001000000000000);
zSig0 = estimateSqrt32( aExp, aSig0>>17 );
shortShift128Left( aSig0, aSig1, 13 - ( aExp & 1 ), &aSig0, &aSig1 );
zSig0 = estimateDiv128To64( aSig0, aSig1, zSig0<<32 ) + ( zSig0<<30 );
doubleZSig0 = zSig0<<1;
mul64To128( zSig0, zSig0, &term0, &term1 );
sub128( aSig0, aSig1, term0, term1, &rem0, &rem1 );
while ( (int64_t) rem0 < 0 ) {
--zSig0;
doubleZSig0 -= 2;
add128( rem0, rem1, zSig0>>63, doubleZSig0 | 1, &rem0, &rem1 );
}
zSig1 = estimateDiv128To64( rem1, 0, doubleZSig0 );
if ( ( zSig1 & 0x1FFF ) <= 5 ) {
if ( zSig1 == 0 ) zSig1 = 1;
mul64To128( doubleZSig0, zSig1, &term1, &term2 );
sub128( rem1, 0, term1, term2, &rem1, &rem2 );
mul64To128( zSig1, zSig1, &term2, &term3 );
sub192( rem1, rem2, 0, 0, term2, term3, &rem1, &rem2, &rem3 );
while ( (int64_t) rem1 < 0 ) {
--zSig1;
shortShift128Left( 0, zSig1, 1, &term2, &term3 );
term3 |= 1;
term2 |= doubleZSig0;
add192( rem1, rem2, rem3, 0, term2, term3, &rem1, &rem2, &rem3 );
}
zSig1 |= ( ( rem1 | rem2 | rem3 ) != 0 );
}
shift128ExtraRightJamming( zSig0, zSig1, 0, 14, &zSig0, &zSig1, &zSig2 );
return roundAndPackFloat128(0, zExp, zSig0, zSig1, zSig2, status);
}
static inline FloatRelation
floatx80_compare_internal(floatx80 a, floatx80 b, bool is_quiet,
float_status *status)