I am looking for a fast, integer only algorithm to find the square root (integer part thereof) of an unsigned integer. The code must have excellent performance on ARM Thumb 2 processors. It could be assembly language or C code.
Any hints welcome.
14 Answers
Integer Square Roots by Jack W. Crenshaw could be useful as another reference.
The C Snippets Archive also has an integer square root implementation. This one goes beyond just the integer result, and calculates extra fractional (fixed-point) bits of the answer. (Update: unfortunately, the C snippets archive is now defunct. The link points to the web archive of the page.) Here is the code from the C Snippets Archive:
#define BITSPERLONG 32
#define TOP2BITS(x) ((x & (3L << (BITSPERLONG-2))) >> (BITSPERLONG-2))
struct int_sqrt {
unsigned sqrt, frac;
};
/* usqrt:
ENTRY x: unsigned long
EXIT returns floor(sqrt(x) * pow(2, BITSPERLONG/2))
Since the square root never uses more than half the bits
of the input, we use the other half of the bits to contain
extra bits of precision after the binary point.
EXAMPLE
suppose BITSPERLONG = 32
then usqrt(144) = 786432 = 12 * 65536
usqrt(32) = 370727 = 5.66 * 65536
NOTES
(1) change BITSPERLONG to BITSPERLONG/2 if you do not want
the answer scaled. Indeed, if you want n bits of
precision after the binary point, use BITSPERLONG/2+n.
The code assumes that BITSPERLONG is even.
(2) This is really better off being written in assembly.
The line marked below is really a "arithmetic shift left"
on the double-long value with r in the upper half
and x in the lower half. This operation is typically
expressible in only one or two assembly instructions.
(3) Unrolling this loop is probably not a bad idea.
ALGORITHM
The calculations are the base-two analogue of the square
root algorithm we all learned in grammar school. Since we're
in base 2, there is only one nontrivial trial multiplier.
Notice that absolutely no multiplications or divisions are performed.
This means it'll be fast on a wide range of processors.
*/
void usqrt(unsigned long x, struct int_sqrt *q)
{
unsigned long a = 0L; /* accumulator */
unsigned long r = 0L; /* remainder */
unsigned long e = 0L; /* trial product */
int i;
for (i = 0; i < BITSPERLONG; i++) /* NOTE 1 */
{
r = (r << 2) + TOP2BITS(x); x <<= 2; /* NOTE 2 */
a <<= 1;
e = (a << 1) + 1;
if (r >= e)
{
r -= e;
a++;
}
}
memcpy(q, &a, sizeof(long));
}
I settled on the following code. It's essentially from the Wikipedia article on square-root computing methods. But it has been changed to use stdint.h types uint32_t etc. Strictly speaking, the return type could be changed to uint16_t.
/**
* \brief Fast Square root algorithm
*
* Fractional parts of the answer are discarded. That is:
* - SquareRoot(3) --> 1
* - SquareRoot(4) --> 2
* - SquareRoot(5) --> 2
* - SquareRoot(8) --> 2
* - SquareRoot(9) --> 3
*
* \param[in] a_nInput - unsigned integer for which to find the square root
*
* \return Integer square root of the input value.
*/
uint32_t SquareRoot(uint32_t a_nInput)
{
uint32_t op = a_nInput;
uint32_t res = 0;
uint32_t one = 1uL << 30; // The second-to-top bit is set: use 1u << 14 for uint16_t type; use 1uL<<30 for uint32_t type
// "one" starts at the highest power of four <= than the argument.
while (one > op)
{
one >>= 2;
}
while (one != 0)
{
if (op >= res + one)
{
op = op - (res + one);
res = res + 2 * one;
}
res >>= 1;
one >>= 2;
}
return res;
}
The nice thing, I discovered, is that a fairly simple modification can return the "rounded" answer. I found this useful in a certain application for greater accuracy. Note that in this case, the return type must be uint32_t because the rounded square root of 232 - 1 is 216.
/**
* \brief Fast Square root algorithm, with rounding
*
* This does arithmetic rounding of the result. That is, if the real answer
* would have a fractional part of 0.5 or greater, the result is rounded up to
* the next integer.
* - SquareRootRounded(2) --> 1
* - SquareRootRounded(3) --> 2
* - SquareRootRounded(4) --> 2
* - SquareRootRounded(6) --> 2
* - SquareRootRounded(7) --> 3
* - SquareRootRounded(8) --> 3
* - SquareRootRounded(9) --> 3
*
* \param[in] a_nInput - unsigned integer for which to find the square root
*
* \return Integer square root of the input value.
*/
uint32_t SquareRootRounded(uint32_t a_nInput)
{
uint32_t op = a_nInput;
uint32_t res = 0;
uint32_t one = 1uL << 30; // The second-to-top bit is set: use 1u << 14 for uint16_t type; use 1uL<<30 for uint32_t type
// "one" starts at the highest power of four <= than the argument.
while (one > op)
{
one >>= 2;
}
while (one != 0)
{
if (op >= res + one)
{
op = op - (res + one);
res = res + 2 * one;
}
res >>= 1;
one >>= 2;
}
/* Do arithmetic rounding to nearest integer */
if (op > res)
{
res++;
}
return res;
}
If exact accuracy isn't required, I have a fast approximation for you, that uses 260 bytes of RAM (you could halve that, but don't).
int ftbl[33]={0,1,1,2,2,4,5,8,11,16,22,32,45,64,90,128,181,256,362,512,724,1024,1448,2048,2896,4096,5792,8192,11585,16384,23170,32768,46340};
int ftbl2[32]={ 32768,33276,33776,34269,34755,35235,35708,36174,36635,37090,37540,37984,38423,38858,39287,39712,40132,40548,40960,41367,41771,42170,42566,42959,43347,43733,44115,44493,44869,45241,45611,45977};
int fisqrt(int val)
{
int cnt=0;
int t=val;
while (t) {cnt++;t>>=1;}
if (6>=cnt) t=(val<<(6-cnt));
else t=(val>>(cnt-6));
return (ftbl[cnt]*ftbl2[t&31])>>15;
}
Here's the code to generate the tables:
ftbl[0]=0;
for (int i=0;i<32;i++) ftbl[i+1]=sqrt(pow(2.0,i));
printf("int ftbl[33]={0");
for (int i=0;i<32;i++) printf(",%d",ftbl[i+1]);
printf("};\n");
for (int i=0;i<32;i++) ftbl2[i]=sqrt(1.0+i/32.0)*32768;
printf("int ftbl2[32]={");
for (int i=0;i<32;i++) printf("%c%d",(i)?',':' ',ftbl2[i]);
printf("};\n");
Over the range 1 → 220, the maximum error is 11, and over the range 1 → 230, it's about 256. You could use larger tables and minimise this. It's worth mentioning that the error will always be negative - i.e. when it's wrong, the value will be LESS than the correct value.
You might do well to follow this with a refining stage.
The idea is simple enough: (ab)0.5 = a0.b × b0.5.
So, we take the input X = A×B where A = 2N and 1 ≤ B < 2
Then we have a lookup table for sqrt(2N), and a lookup table for sqrt(1 ≤ B < 2). We store the lookup table for sqrt(2N) as integer, which might be a mistake (testing shows no ill effects), and we store the lookup table for sqrt(1 ≤ B < 2) as 15-bit fixed-point.
We know that 1 ≤ sqrt(2N) < 65536, so that's 16-bit, and we know that we can really only multiply 16-bit × 15-bit on an ARM, without fear of reprisal, so that's what we do.
In terms of implementation, the while(t) {cnt++;t>>=1;} is effectively a count-leading-bits instruction (CLB), so if your version of the chipset has that, you're winning! Also, the shift instruction would be easy to implement with a bidirectional shifter, if you have one?
There's a Lg[N] algorithm for counting the highest set bit here.
In terms of magic numbers, for changing table sizes, THE magic number for ftbl2 is 32, though note that 6 (Lg[32]+1) is used for the shifting.
One common approach is bisection.
hi = number
lo = 0
mid = ( hi + lo ) / 2
mid2 = mid*mid
while( lo < hi-1 and mid2 != number ) {
if( mid2 < number ) {
lo = mid
else
hi = mid
mid = ( hi + lo ) / 2
mid2 = mid*mid
Something like that should work reasonably well. It makes log2(number) tests, doing
log2(number) multiplies and divides. Since the divide is a divide by 2, you can replace it with a >>.
The terminating condition may not be spot on, so be sure to test a variety of integers to be sure that the division by 2 doesn't incorrectly oscillate between two even values; they would differ by more than 1.
I have settled to something similar to the binary digit-by-digit algorithm described in this Wikipedia article.
I recently encountered the same task on ARM Cortex-M3 (STM32F103CBT6) and after searching the Internet came up with the following solution. It's not the fastest comparing with solutions offered here, but it has good accuracy (maximum error is 1, i.e. LSB on the entire UI32 input range) and relatively good speed (about 1.3M square roots per second on a 72-MHz ARM Cortex-M3 or about 55 cycles per single root including the function call).
// FastIntSqrt is based on Wikipedia article:
// https://en.wikipedia.org/wiki/Methods_of_computing_square_roots
// Which involves Newton's method which gives the following iterative formula:
//
// X(n+1) = (X(n) + S/X(n))/2
//
// Thanks to ARM CLZ instruction (which counts how many bits in a number are
// zeros starting from the most significant one) we can very successfully
// choose the starting value, so just three iterations are enough to achieve
// maximum possible error of 1. The algorithm uses division, but fortunately
// it is fast enough here, so square root computation takes only about 50-55
// cycles with maximum compiler optimization.
uint32_t FastIntSqrt (uint32_t value)
{
if (!value)
return 0;
uint32_t xn = 1 << ((32 - __CLZ (value))/2);
xn = (xn + value/xn)/2;
xn = (xn + value/xn)/2;
xn = (xn + value/xn)/2;
return xn;
}
I'm using IAR and it produces the following assembler code:
SECTION `.text`:CODE:NOROOT(1)
THUMB
_Z11FastIntSqrtj:
MOVS R1,R0
BNE.N ??FastIntSqrt_0
MOVS R0,#+0
BX LR
??FastIntSqrt_0:
CLZ R0,R1
RSB R0,R0,#+32
MOVS R2,#+1
LSRS R0,R0,#+1
LSL R0,R2,R0
UDIV R3,R1,R0
ADDS R0,R3,R0
LSRS R0,R0,#+1
UDIV R2,R1,R0
ADDS R0,R2,R0
LSRS R0,R0,#+1
UDIV R1,R1,R0
ADDS R0,R1,R0
LSRS R0,R0,#+1
BX LR ;; return
The most cleverly coded bit-wise integer square root implementations for ARM achieve 3 cycles per result bit, which comes out to a lower bound of 50 cycles for the square root of a 32-bit unsigned integer. An example is shown in Andrew N. Sloss, Dominic Symes, Chris Wright, "ARM System Developer's Guide", Morgan Kaufman 2004.
Since most ARM processors also have very fast integer multipliers, and most even provide a very fast implementation of the wide multiply instruction UMULL, an alternative approach that can achieve execution times on the order of 35 to 45 cycles is computation via the reciprocal square root 1/√x using fixed-point computation. For this it is necessary to normalize the input with the help of a count-leading-zeros instruction, which on most ARM processors is available as an instruction CLZ.
The computation starts with an initial low-accuracy reciprocal square root approximation from a lookup table indexed by some most significant bit of the normalized argument. The Newton-Raphson iteration to refine the reciprocal square root r of a number a with quadratic convergence is rn+1 = rn + rn* (1 - a * rn2) / 2. This can be re-arranged into algebraically equivalent forms as is convenient. In the exemplary C99 code below, an 8-bit approximation r0 is read from a 96-entry lookup table. This approximation is accurate to about 7 bits. The first Newton-Raphson iteration computes r1 = (3 * r0 - a * r03) / 2 to potentially take advantage of small operand multiplication instructions. The second Newton-Raphson iteration computes r2 = (r1 * (3 - r1 * (r1 * a))) / 2.
The normalized square root is then computed by a back multiplication s2 = a * r2 and the final approximation is achieved by denormalizing based on the count of leading zeros of the original argument a. It is important that the desired result ⌊√a⌋ is approximated with underestimation. This simplifies the checking whether the desired result has been achieved by guaranteeing that the remainder ⌊√a⌋ - s2 * s2 is positive. If the final approximation is found to be too small, the result is increased by one. Correct operation of this algorithm can easily be demonstrated by exhaustive test of all possible 232 inputs against a "golden" reference, which takes only a few minutes.
One can speed up this implementation at the expense of additional storage for the lookup table, by pre-computing 3 * r0 and r03 to simplify the first Newton-Raphson iteration. The former requires 10 bit of storage and the latter 24 bits. In order to combine each pair into a 32-bit data item, the cube is rounded to 22 bits, which introduces negligible error into the computation. This results in a lookup table of 96 * 4 = 384 bytes.
An alternative approach uses the observation that all starting approximations have the same most significant bit set, which therefore can be assumed implicitly and does not have to be stored. This allows to squeeze a 9-bit approximation r0 into an 8-bit data item, with the leading bit restored after table lookup. With a lookup table of 384 entries, all underestimates, one can achieve an accuracy of about 7.5 bits. Combining the back multiply with the Newton-Raphson iteration for the reciprocal square root, one computes s0 = a * r0, s1 = s0 + r0 * (a - s0 * s0) / 2. Because the accuracy of the starting approximation is not high enough for a very accurate final square root approximation, it can be off by up to three, and an appropriate adjustment must be made based on the magnitude of the remainder floor (sqrt (a)) - s1 * s1.
One advantage of the alternative approach is that is halves the number of multiplies required, and in particular requires only a single wide multiplication UMULL. Especially processors where wide multiplies are fairly slow, this is an alternative worth trying.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#if defined(_MSC_VER) && defined(_WIN64)
#include <intrin.h>
#endif // defined(_MSC_VER) && defined(_WIN64)
#define CLZ_BUILTIN (1) // use compiler's built-in count-leading-zeros
#define CLZ_FPU (2) // emulate count-leading-zeros via FPU
#define CLZ_CPU (3) // emulate count-leading-zeros via CPU
#define ALT_IMPL (0) // alternative implementation with fewer multiplies
#define LARGE_TABLE (0) // ALT_IMPL=0: incorporate 1st NR-iter into table
#define CLZ_IMPL (CLZ_CPU)// choose count-leading-zeros implementation
#define GEN_TAB (0) // generate tables
uint32_t umul32_hi (uint32_t a, uint32_t b);
uint32_t float_as_uint32 (float a);
int clz32 (uint32_t a);
#if ALT_IMPL
uint8_t rsqrt_tab [384] =
{
0xfe, 0xfc, 0xfa, 0xf8, 0xf6, 0xf4, 0xf2, 0xf0, 0xee, 0xed, 0xeb, 0xe9,
0xe7, 0xe6, 0xe4, 0xe2, 0xe1, 0xdf, 0xdd, 0xdc, 0xda, 0xd8, 0xd7, 0xd5,
0xd4, 0xd2, 0xd1, 0xcf, 0xce, 0xcc, 0xcb, 0xc9, 0xc8, 0xc7, 0xc5, 0xc4,
0xc2, 0xc1, 0xc0, 0xbe, 0xbd, 0xbc, 0xba, 0xb9, 0xb8, 0xb7, 0xb5, 0xb4,
0xb3, 0xb2, 0xb0, 0xaf, 0xae, 0xad, 0xac, 0xab, 0xa9, 0xa8, 0xa7, 0xa6,
0xa5, 0xa4, 0xa3, 0xa2, 0xa0, 0x9f, 0x9e, 0x9d, 0x9c, 0x9b, 0x9a, 0x99,
0x98, 0x97, 0x96, 0x95, 0x94, 0x93, 0x92, 0x91, 0x90, 0x8f, 0x8e, 0x8d,
0x8c, 0x8b, 0x8b, 0x8a, 0x89, 0x88, 0x87, 0x86, 0x85, 0x84, 0x83, 0x83,
0x82, 0x81, 0x80, 0x7f, 0x7e, 0x7d, 0x7d, 0x7c, 0x7b, 0x7a, 0x79, 0x79,
0x78, 0x77, 0x76, 0x75, 0x75, 0x74, 0x73, 0x72, 0x72, 0x71, 0x70, 0x6f,
0x6f, 0x6e, 0x6d, 0x6c, 0x6c, 0x6b, 0x6a, 0x6a, 0x69, 0x68, 0x67, 0x67,
0x66, 0x65, 0x65, 0x64, 0x63, 0x63, 0x62, 0x61, 0x61, 0x60, 0x5f, 0x5f,
0x5e, 0x5d, 0x5d, 0x5c, 0x5c, 0x5b, 0x5a, 0x5a, 0x59, 0x58, 0x58, 0x57,
0x57, 0x56, 0x55, 0x55, 0x54, 0x54, 0x53, 0x52, 0x52, 0x51, 0x51, 0x50,
0x50, 0x4f, 0x4e, 0x4e, 0x4d, 0x4d, 0x4c, 0x4c, 0x4b, 0x4b, 0x4a, 0x4a,
0x49, 0x48, 0x48, 0x47, 0x47, 0x46, 0x46, 0x45, 0x45, 0x44, 0x44, 0x43,
0x43, 0x42, 0x42, 0x41, 0x41, 0x40, 0x40, 0x3f, 0x3f, 0x3e, 0x3e, 0x3d,
0x3d, 0x3c, 0x3c, 0x3c, 0x3b, 0x3b, 0x3a, 0x3a, 0x39, 0x39, 0x38, 0x38,
0x37, 0x37, 0x36, 0x36, 0x36, 0x35, 0x35, 0x34, 0x34, 0x33, 0x33, 0x33,
0x32, 0x32, 0x31, 0x31, 0x30, 0x30, 0x30, 0x2f, 0x2f, 0x2e, 0x2e, 0x2d,
0x2d, 0x2d, 0x2c, 0x2c, 0x2b, 0x2b, 0x2b, 0x2a, 0x2a, 0x29, 0x29, 0x29,
0x28, 0x28, 0x27, 0x27, 0x27, 0x26, 0x26, 0x26, 0x25, 0x25, 0x24, 0x24,
0x24, 0x23, 0x23, 0x23, 0x22, 0x22, 0x21, 0x21, 0x21, 0x20, 0x20, 0x20,
0x1f, 0x1f, 0x1f, 0x1e, 0x1e, 0x1e, 0x1d, 0x1d, 0x1d, 0x1c, 0x1c, 0x1c,
0x1b, 0x1b, 0x1a, 0x1a, 0x1a, 0x19, 0x19, 0x19, 0x18, 0x18, 0x18, 0x17,
0x17, 0x17, 0x17, 0x16, 0x16, 0x16, 0x15, 0x15, 0x15, 0x14, 0x14, 0x14,
0x13, 0x13, 0x13, 0x12, 0x12, 0x12, 0x11, 0x11, 0x11, 0x11, 0x10, 0x10,
0x10, 0x0f, 0x0f, 0x0f, 0x0e, 0x0e, 0x0e, 0x0e, 0x0d, 0x0d, 0x0d, 0x0c,
0x0c, 0x0c, 0x0c, 0x0b, 0x0b, 0x0b, 0x0a, 0x0a, 0x0a, 0x0a, 0x09, 0x09,
0x09, 0x08, 0x08, 0x08, 0x08, 0x07, 0x07, 0x07, 0x07, 0x06, 0x06, 0x06,
0x05, 0x05, 0x05, 0x05, 0x04, 0x04, 0x04, 0x04, 0x03, 0x03, 0x03, 0x03,
0x02, 0x02, 0x02, 0x02, 0x01, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00,
};
/* compute floor (sqrt (a)) */
uint32_t my_isqrt32 (uint32_t a)
{
uint32_t b, r, s, scal, rem;
if (a == 0) return a;
/* Normalize argument */
scal = clz32 (a) & ~1;
b = a << scal;
/* Compute initial approximation to 1/sqrt(a) */
r = rsqrt_tab [(b >> 23) - 128] | 0x100;
/* Compute initial approximation to sqrt(a) */
s = umul32_hi (b, r << 8);
/* Refine sqrt approximation */
b = b - s * s;
s = s + ((r * (b >> 2)) >> 23);
/* Denormalize result*/
s = s >> (scal >> 1);
/* Ensure we got the floor correct */
rem = a - s * s;
if (rem < (2 * s + 1)) s = s + 0;
else if (rem < (4 * s + 4)) s = s + 1;
else if (rem < (6 * s + 9)) s = s + 2;
else s = s + 3;
return s;
}
#else // ALT_IMPL
#if LARGE_TABLE
uint32_t rsqrt_tab [96] =
{
0xfa0bfafa, 0xee6b2aee, 0xe5f02ae5, 0xdaf26ed9, 0xd2f002d0, 0xc890c2c4,
0xc1037abb, 0xb9a75ab2, 0xb4da42ac, 0xadcea2a3, 0xa6f27a9a, 0xa279c294,
0x9beb4a8b, 0x97a5ca85, 0x9163427c, 0x8d4fca76, 0x89500270, 0x8563ba6a,
0x818ac264, 0x7dc4ea5e, 0x7a120258, 0x7671da52, 0x72e4424c, 0x6f690a46,
0x6db24243, 0x6a52423d, 0x67042637, 0x6563c234, 0x62302a2e, 0x609cea2b,
0x5d836a25, 0x5bfd1a22, 0x58fd421c, 0x5783ae19, 0x560e4a16, 0x53300210,
0x51c7120d, 0x50623a0a, 0x4da4c204, 0x4c4c1601, 0x4af769fe, 0x49a6b9fb,
0x485a01f8, 0x471139f5, 0x45cc59f2, 0x448b5def, 0x4214fde9, 0x40df89e6,
0x3fade1e3, 0x3e8001e0, 0x3d55e1dd, 0x3c2f79da, 0x3c2f79da, 0x3b0cc5d7,
0x39edc1d4, 0x38d265d1, 0x37baa9ce, 0x36a689cb, 0x359601c8, 0x348909c5,
0x348909c5, 0x337f99c2, 0x3279adbf, 0x317741bc, 0x30784db9, 0x30784db9,
0x2f7cc9b6, 0x2e84b1b3, 0x2d9001b0, 0x2d9001b0, 0x2c9eb1ad, 0x2bb0b9aa,
0x2bb0b9aa, 0x2ac615a7, 0x29dec1a4, 0x29dec1a4, 0x28fab5a1, 0x2819e99e,
0x2819e99e, 0x273c599b, 0x273c599b, 0x26620198, 0x258ad995, 0x258ad995,
0x24b6d992, 0x24b6d992, 0x23e5fd8f, 0x2318418c, 0x2318418c, 0x224d9d89,
0x224d9d89, 0x21860986, 0x21860986, 0x20c18183, 0x20c18183, 0x20000180,
};
#else // LARGE_TABLE
uint8_t rsqrt_tab [96] =
{
0xfe, 0xfa, 0xf7, 0xf3, 0xf0, 0xec, 0xe9, 0xe6, 0xe4, 0xe1, 0xde, 0xdc,
0xd9, 0xd7, 0xd4, 0xd2, 0xd0, 0xce, 0xcc, 0xca, 0xc8, 0xc6, 0xc4, 0xc2,
0xc1, 0xbf, 0xbd, 0xbc, 0xba, 0xb9, 0xb7, 0xb6, 0xb4, 0xb3, 0xb2, 0xb0,
0xaf, 0xae, 0xac, 0xab, 0xaa, 0xa9, 0xa8, 0xa7, 0xa6, 0xa5, 0xa3, 0xa2,
0xa1, 0xa0, 0x9f, 0x9e, 0x9e, 0x9d, 0x9c, 0x9b, 0x9a, 0x99, 0x98, 0x97,
0x97, 0x96, 0x95, 0x94, 0x93, 0x93, 0x92, 0x91, 0x90, 0x90, 0x8f, 0x8e,
0x8e, 0x8d, 0x8c, 0x8c, 0x8b, 0x8a, 0x8a, 0x89, 0x89, 0x88, 0x87, 0x87,
0x86, 0x86, 0x85, 0x84, 0x84, 0x83, 0x83, 0x82, 0x82, 0x81, 0x81, 0x80,
};
#endif //LARGE_TABLE
/* compute floor (sqrt (a)) */
uint32_t my_isqrt32 (uint32_t a)
{
uint32_t b, r, s, t, scal, rem;
if (a == 0) return a;
/* Normalize argument */
scal = clz32 (a) & ~1;
b = a << scal;
/* Initial approximation to 1/sqrt(a)*/
t = rsqrt_tab [(b >> 25) - 32];
/* First NR iteration */
#if LARGE_TABLE
r = (t << 22) - umul32_hi (b, t);
#else // LARGE_TABLE
r = ((3 * t) << 22) - umul32_hi (b, (t * t * t) << 8);
#endif // LARGE_TABLE
/* Second NR iteration */
s = umul32_hi (r, b);
s = 0x30000000 - umul32_hi (r, s);
r = umul32_hi (r, s);
/* Compute sqrt(a) = a * 1/sqrt(a). Adjust to ensure it's an underestimate*/
r = umul32_hi (r, b) - 1;
/* Denormalize result */
r = r >> ((scal >> 1) + 11);
/* Make sure we got the floor correct */
rem = a - r * r;
if (rem >= (2 * r + 1)) r++;
return r;
}
#endif // ALT_IMPL
uint32_t umul32_hi (uint32_t a, uint32_t b)
{
return (uint32_t)(((uint64_t)a * b) >> 32);
}
uint32_t float_as_uint32 (float a)
{
uint32_t r;
memcpy (&r, &a, sizeof r);
return r;
}
int clz32 (uint32_t a)
{
#if (CLZ_IMPL == CLZ_FPU)
// Henry S. Warren, Jr, " Hacker's Delight 2nd ed", p. 105
int n = 158 - (float_as_uint32 ((float)(int32_t)(a & ~(a >> 1))+.5f) >> 23);
return (n < 0) ? 0 : n;
#elif (CLZ_IMPL == CLZ_CPU)
static const uint8_t clz_tab[32] = {
31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
};
a |= a >> 16;
a |= a >> 8;
a |= a >> 4;
a |= a >> 2;
a |= a >> 1;
return clz_tab [0x07c4acddu * a >> 27] + (!a);
#else // CLZ_IMPL == CLZ_BUILTIN
#if defined(_MSC_VER) && defined(_WIN64)
return __lzcnt (a);
#else // defined(_MSC_VER) && defined(_WIN64)
return __builtin_clz (a);
#endif // defined(_MSC_VER) && defined(_WIN64)
#endif // CLZ_IMPL
}
/* Henry S. Warren, Jr., "Hacker's Delight, 2nd e.d", p. 286 */
uint32_t ref_isqrt32 (uint32_t x)
{
uint32_t m, y, b;
m = 0x40000000U;
y = 0U;
while (m != 0) {
b = y | m;
y = y >> 1;
if (x >= b) {
x = x - b;
y = y | m;
}
m = m >> 2;
}
return y;
}
#if defined(_WIN32)
#if !defined(WIN32_LEAN_AND_MEAN)
#define WIN32_LEAN_AND_MEAN
#endif
#include <windows.h>
double second (void)
{
LARGE_INTEGER t;
static double oofreq;
static int checkedForHighResTimer;
static BOOL hasHighResTimer;
if (!checkedForHighResTimer) {
hasHighResTimer = QueryPerformanceFrequency (&t);
oofreq = 1.0 / (double)t.QuadPart;
checkedForHighResTimer = 1;
}
if (hasHighResTimer) {
QueryPerformanceCounter (&t);
return (double)t.QuadPart * oofreq;
} else {
return (double)GetTickCount() * 1.0e-3;
}
}
#elif defined(__linux__) || defined(__APPLE__)
#include <stddef.h>
#include <sys/time.h>
double second (void)
{
struct timeval tv;
gettimeofday(&tv, NULL);
return (double)tv.tv_sec + (double)tv.tv_usec * 1.0e-6;
}
#else
#error unsupported platform
#endif
int main (void)
{
#if ALT_IMPL
printf ("Alternate integer square root implementation\n");
#else // ALT_IMPL
#if LARGE_TABLE
printf ("Integer square root implementation w/ large table\n");
#else // LARGE_TAB
printf ("Integer square root implementation w/ small table\n");
#endif
#endif // ALT_IMPL
#if GEN_TAB
printf ("Generating lookup table ...\n");
#if ALT_IMPL
for (int i = 0; i < 384; i++) {
double x = 1.0 + (i + 1) * 1.0 / 128;
double y = 1.0 / sqrt (x);
uint8_t val = (uint8_t)((y * 512) - 256);
rsqrt_tab[i] = val;
printf ("0x%02x, ", rsqrt_tab[i]);
if (i % 12 == 11) printf("\n");
}
#else // ALT_IMPL
for (int i = 0; i < 96; i++ ) {
double x1 = 1.0 + i * 1.0 / 32;
double x2 = 1.0 + (i + 1) * 1.0 / 32;
double y = (1.0 / sqrt(x1) + 1.0 / sqrt(x2)) * 0.5;
uint32_t val = (uint32_t)(y * 256 + 0.5);
#if LARGE_TABLE
uint32_t cube = val * val * val;
rsqrt_tab[i] = (((cube + 1) / 4) << 10) + (3 * val);
printf ("0x%08x, ", rsqrt_tab[i]);
if (i % 6 == 5) printf ("\n");
#else // LARGE_TABLE
rsqrt_tab[i] = val;
printf ("0x%02x, ", rsqrt_tab[i]);
if (i % 12 == 11) printf ("\n");
#endif // LARGE_TABLE
}
#endif // ALT_IMPL
#endif // GEN_TAB
printf ("Running exhaustive test ... ");
uint32_t i = 0;
do {
uint32_t ref = ref_isqrt32 (i);
uint32_t res = my_isqrt32 (i);
if (res != ref) {
printf ("error: arg=%08x res=%08x ref=%08x\n", i, res, ref);
return EXIT_FAILURE;
}
i++;
} while (i);
printf ("PASSED\n");
printf ("Running benchmark ...\n");
i = 0;
uint32_t sum[8] = {0, 0, 0, 0, 0, 0, 0, 0};
double start = second();
do {
sum [0] += my_isqrt32 (i + 0);
sum [1] += my_isqrt32 (i + 1);
sum [2] += my_isqrt32 (i + 2);
sum [3] += my_isqrt32 (i + 3);
sum [4] += my_isqrt32 (i + 4);
sum [5] += my_isqrt32 (i + 5);
sum [6] += my_isqrt32 (i + 6);
sum [7] += my_isqrt32 (i + 7);
i += 8;
} while (i);
double stop = second();
printf ("%08x \relapsed=%.5f sec\n",
sum[0]+sum[1]+sum[2]+sum[3]+sum[4]+sum[5]+sum[6]+sum[7],
stop - start);
return EXIT_SUCCESS;
}
If you need it just for ARM Thumb 2 processors, CMSIS DSP library by ARM is the best shot for you. It's made by people who designed Thumb 2 processors. Who else can beat it?
Actually you don't even need an algorithm but specialized square root hardware instructions such as VSQRT. The ARM company maintains math and DSP algorithm implementations highly optimized for Thumb 2 supported processors by trying to use its hardware like VSQRT. You can get the source code:
arm_sqrt_f32()arm_sqrt_q15.c/arm_sqrt_q31.c(q15 and q31 are the fixed-point data types specialized for ARM DSP core, which is often comes with Thum 2 compatible processors.)
Note that ARM also maintains compiled binaries of CMSIS DSP that guarantees the best possible performance for ARM Thumb architecture-specific instructions. So you should consider statically link them when you use the library. You can get the binaries here.
I've designed a 16-bit sqrt for RGB gamma compression. It dispatches into 3 different tables, based on the higher 8 bits. Disadvantages: it uses about a kilobyte for the tables, rounds unpredictable, if exact sqrt is impossible, and, in the worst case, uses single multiplication (but only for a very few input values).
static uint8_t sqrt_50_256[] = {
114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,
133,134,135,136,137,138,139,140,141,142,143,143,144,145,146,147,148,149,150,
150,151,152,153,154,155,155,156,157,158,159,159,160,161,162,163,163,164,165,
166,167,167,168,169,170,170,171,172,173,173,174,175,175,176,177,178,178,179,
180,181,181,182,183,183,184,185,185,186,187,187,188,189,189,190,191,191,192,
193,193,194,195,195,196,197,197,198,199,199,200,201,201,202,203,203,204,204,
205,206,206,207,207,208,209,209,210,211,211,212,212,213,214,214,215,215,216,
217,217,218,218,219,219,220,221,221,222,222,223,223,224,225,225,226,226,227,
227,228,229,229,230,230,231,231,232,232,233,234,234,235,235,236,236,237,237,
238,238,239,239,240,241,241,242,242,243,243,244,244,245,245,246,246,247,247,
248,248,249,249,250,250,251,251,252,252,253,253,254,254,255,255
};
static uint8_t sqrt_0_10[] = {
1,2,3,3,4,4,5,5,5,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,9,10,10,10,10,10,11,11,11,
11,11,11,12,12,12,12,12,12,13,13,13,13,13,13,13,14,14,14,14,14,14,14,15,15,
15,15,15,15,15,15,16,16,16,16,16,16,16,16,17,17,17,17,17,17,17,17,17,18,18,
18,18,18,18,18,18,18,19,19,19,19,19,19,19,19,19,19,20,20,20,20,20,20,20,20,
20,20,21,21,21,21,21,21,21,21,21,21,21,22,22,22,22,22,22,22,22,22,22,22,23,
23,23,23,23,23,23,23,23,23,23,23,24,24,24,24,24,24,24,24,24,24,24,24,25,25,
25,25,25,25,25,25,25,25,25,25,25,26,26,26,26,26,26,26,26,26,26,26,26,26,27,
27,27,27,27,27,27,27,27,27,27,27,27,27,28,28,28,28,28,28,28,28,28,28,28,28,
28,28,29,29,29,29,29,29,29,29,29,29,29,29,29,29,29,30,30,30,30,30,30,30,30,
30,30,30,30,30,30,30,31,31,31,31,31,31,31,31,31,31,31,31,31,31,31,31,32,32,
32,32,32,32,32,32,32,32,32,32,32,32,32,32,33,33,33,33,33,33,33,33,33,33,33,
33,33,33,33,33,33,34,34,34,34,34,34,34,34,34,34,34,34,34,34,34,34,34,35,35,
35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,35,36,36,36,36,36,36,36,36,36,
36,36,36,36,36,36,36,36,36,37,37,37,37,37,37,37,37,37,37,37,37,37,37,37,37,
37,37,37,38,38,38,38,38,38,38,38,38,38,38,38,38,38,38,38,38,38,38,39,39,39,
39,39,39,39,39,39,39,39,39,39,39,39,39,39,39,39,39,40,40,40,40,40,40,40,40,
40,40,40,40,40,40,40,40,40,40,40,40,41,41,41,41,41,41,41,41,41,41,41,41,41,
41,41,41,41,41,41,41,41,42,42,42,42,42,42,42,42,42,42,42,42,42,42,42,42,42,
42,42,42,42,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,43,
43,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,44,45,45,
45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,45,46,46,46,46,
46,46,46,46,46,46,46,46,46,46,46,46,46,46,46,46,46,46,46,47,47,47,47,47,47,
47,47,47,47,47,47,47,47,47,47,47,47,47,47,47,47,47,47,48,48,48,48,48,48,48,
48,48,48,48,48,48,48,48,48,48,48,48,48,48,48,48,48,49,49,49,49,49,49,49,49,
49,49,49,49,49,49,49,49,49,49,49,49,49,49,49,49,49,50,50,50,50,50,50,50,50,
50,50,50,50,50,50,50,50,50,50,50,50,50,50,50,50,50,51,51,51,51,51,51,51,51,
51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,51,52,52,52,52,52,52,52,
52,52,52,52,52,52,52,52,52,52,52,52,52,52,52,52,52,52,52,53,53
};
static uint8_t sqrt_11_49[] = {
54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,0,76,77,78,
0,79,80,81,82,83,0,84,85,86,0,87,88,89,0,90,0,91,92,93,0,94,0,95,96,97,0,98,0,
99,100,101,0,102,0,103,0,104,105,106,0,107,0,108,0,109,0,110,0,111,112,113
};
uint16_t isqrt16(uint16_t v) {
uint16_t a, b;
uint16_t h = v>>8;
if (h <= 10) return v ? sqrt_0_10[v>>2] : 0;
if (h >= 50) return sqrt_50_256[h-50];
h = (h-11)<<1;
a = sqrt_11_49[h];
b = sqrt_11_49[h+1];
if (!a) return b;
return b*b > v ? a : b;
}
I've compared it against the log2 based sqrt, using clang's __builtin_clz (which should expand to a single assembly opcode), and the library's sqrtf, called using (int)sqrtf((float)i). And got rather strange results:
$ gcc -O3 test.c -o test && ./test
isqrt16: 6.498955
sqrtf: 6.981861
log2_sqrt: 61.755873
Clang compiled the call to sqrtf to a sqrtss instruction, which is nearly as fast as that table sqrt. Lesson learned: on x86 the compiler can provide fast enough sqrt, which is less than 10% slower than what you yourself can come up with, wasting a lot of time, or can be 10 times faster, if you use some ugly bitwise hacks. And still sqrtss is a bit slower than custom function, so if you really need these 5%, you can get them, and ARM for example has no sqrtss, so log2_sqrt shouldn't lag that bad.
On x86, where FPU is available, the old Quake hack appears to be the fastest way to calculate integer sqrt. It is 2 times faster than this table or the FPU's sqrtss.
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