How does C compute sin and other math functions

Question

I ve been poring through  NET disassemblies and the GCC source code  but can t seem to find anywhere the actual implementation of sin   and other math functions    they always seem to be referencing something else   Can anyone help me find them   I feel like it s unlikely that ALL hardware that C will run on supports trig functions in hardware  so there must be a software algorithm somewhere  right      I m aware of several ways that functions can be calculated  and have written my own routines to compute functions using taylor series for fun   I m curious about how real  production languages do it  since all of my implementations are always several orders of magnitude slower  even though I think my algorithms are pretty clever  obviously they re not

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As many people pointed out  it is implementation dependent  But as far as I understand your question  you were interested in a real software implemetnation of math functions  but just didn t manage to find one  If this is the case then here you are    Download glibc source code from http   ftp gnu org gnu glibc  Look at file dosincos c located in unpacked glibc root sysdeps ieee754 dbl-64 folder Similarly you can find implementations of the rest of the math library  just look for the file with appropriate name   You may also have a look at the files with the  tbl extension  their contents is nothing more than huge tables of precomputed values of different functions in a binary form  That is why the implementation is so fast  instead of computing all the coefficients of whatever series they use they just do a quick lookup  which is much faster  BTW  they do use Tailor series to calculate sine and cosine   I hope this helps

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Whenever such a function is evaluated  then at some level there is most likely either      A table of values which is interpolated  for fast  inaccurate applications - e g  computer graphics  The evaluation of a series that converges to the desired value --- probably not a taylor series  more likely something based on a fancy quadrature like Clenshaw-Curtis    If there is no hardware support then the compiler probably uses the latter method  emitting only assembler code  with no debug symbols   rather than using a c library --- making it tricky for you to track the actual code down in your debugger

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Don t use Taylor series  Chebyshev polynomials are both faster and more accurate  as pointed out by a couple of people above  Here is an implementation  originally from the ZX Spectrum ROM   https   albertveli wordpress com 2015 01 10 zx-sine

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It is a complex question  Intel-like CPU of the x86 family have a hardware implementation of the sin   function  but it is part of the x87 FPU and not used anymore in 64-bit mode  where SSE2 registers are used instead   In that mode  a software implementation is used   There are several such implementations out there  One is in fdlibm and is used in Java  As far as I know  the glibc implementation contains parts of fdlibm  and other parts contributed by IBM   Software implementations of transcendental functions such as sin   typically use approximations by polynomials  often obtained from Taylor series

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I ll try to answer for the case of sin   in a C program  compiled with GCC s C compiler on a current x86 processor  let s say a Intel Core 2 Duo     In the C language the Standard C Library includes common math functions  not included in the language itself  e g  pow  sin and cos for power  sine  and cosine respectively   The headers of which are included in math h   Now on a GNU Linux system  these libraries functions are provided by glibc  GNU libc or GNU C Library   But the GCC compiler wants you to link to the math library  libm so  using the -lm compiler flag to enable usage of these math functions  I m not sure why it isn t part of the standard C library  These would be a software version of the floating point functions  or  soft-float    Aside  The reason for having the math functions separate is historic  and was merely intended to reduce the size of executable programs in very old Unix systems  possibly before shared libraries were available  as far as I know   Now the compiler may optimize the standard C library function sin    provided by libm so  to be replaced with an call to a native instruction to your CPU FPU s built-in sin   function  which exists as an FPU instruction  FSIN for x86 x87  on newer processors like the Core 2 series  this is correct pretty much as far back as the i486DX   This would depend on optimization flags passed to the gcc compiler  If the compiler was told to write code that would execute on any i386 or newer processor  it would not make such an optimization  The -mcpu 486 flag would inform the compiler that it was safe to make such an optimization   Now if the program executed the software version of the sin   function  it would do so based on a CORDIC  COordinate Rotation DIgital Computer  or BKM algorithm  or more likely a table or power-series calculation which is commonly used now to calculate such transcendental functions   Src  http   en wikipedia org wiki Cordic Application   Any recent  since 2 9x approx   version of gcc also offers a built-in version of sin    builtin sin   that it will used to replace the standard call to the C library version  as an optimization    I m sure that is as clear as mud  but hopefully gives you more information than you were expecting  and lots of jumping off points to learn more yourself

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There s nothing like hitting the source and seeing how someone has actually done it in a library in common use  let s look at one C library implementation in particular  I chose uLibC   Here s the sin function   http   git uclibc org uClibc tree libm s sin c  which looks like it handles a few special cases  and then carries out some argument reduction to map the input to the range  -pi 4 pi 4    splitting the argument into two parts  a big part and a tail  before calling  http   git uclibc org uClibc tree libm k sin c  which then operates on those two parts  If there is no tail  an approximate answer is generated using a polynomial of degree 13  If there is a tail  you get a small corrective addition based on the principle that sin x y    sin x    sin  x  y

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Computing sine cosine tangent is actually very easy to do through code using the Taylor series  Writing one yourself takes like 5 seconds   The whole process can be summed up with this equation here     Here are some routines I wrote for C   double  pow double a  double b        double c   1      for  int i 0  i lt b  i            c    a      return c     double  fact double x        double ret   1      for  int i 1  i lt  x  i             ret    i      return ret     double  sin double x        double y   x      double s   -1      for  int i 3  i lt  100  i  2            y  s   pow x i   fact i            s    -1              return y    double  cos double x        double y   1      double s   -1      for  int i 2  i lt  100  i  2            y  s   pow x i   fact i            s    -1              return y    double  tan double x         return   sin x   cos x

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Use Taylor series and try to find relation between terms of the series so you don t calculate things again and again  Here is an example for cosinus   double cosinus double x  double prec        double t  s       int p      p   0      s   1 0      t   1 0      while fabs t s   gt  prec                p            t    -t   x   x      2   p - 1     2   p            s    t            return s      using this we can get the new term of the sum using the already used one  we avoid the factorial and x2p

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They are typically implemented in software and will not use the corresponding hardware  that is  aseembly  calls in most cases  However  as Jason pointed out  these are implementation specific    Note that these software routines are not part of the compiler sources  but will rather be found in the correspoding library such as the clib  or glibc for the GNU compiler  See http   www gnu org software libc manual html mono libc html Trig-Functions  If you want greater control  you should carefully evaluate what you need exactly  Some of the typical methods are interpolation of look-up tables  the assembly call  which is often slow   or other approximation schemes such as Newton-Raphson for square roots

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if you want sin then     asm     volatile    fsin      t  vsin     0  xrads      if you want cos then     asm     volatile    fcos      t  vcos     0  xrads      if you want sqrt then     asm     volatile    fsqrt      t  vsqrt     0  value      so why use inaccurate code when the machine instructions will do

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Concerning trigonometric function like sin    cos   tan   there has been no mention  after 5 years  of an important aspect of high quality trig functions  Range reduction  An early step in any of these functions is to reduce the angle  in radians  to a range of a 2 p interval   But p is irrational so simple reductions like x   remainder x  2 M PI  introduce error as M PI  or machine pi  is an approximation of p   So  how to do x   remainder x  2 p   Early libraries used extended precision or crafted programming to give quality results but still over a limited range of double   When a large value was requested like sin pow 2 30    the results were meaningless or 0 0 and maybe with an error flag set to something like TLOSS total loss of precision or PLOSS partial loss of precision  Good range reduction of large values to an interval like -p to p is a challenging problem that rivals the challenges of the basic trig function  like sin    itself  A good report is Argument reduction for huge arguments  Good to the last bit   1992    It covers the issue well  discusses the need and how things were on various platforms  SPARC  PC  HP  30  other  and provides a solution algorithm the gives quality results for all double from -DBL MAX to DBL MAX   If the original arguments are in degrees  yet may be of a large value  use fmod   first for improved precision   A good fmod   will introduce no error and so provide excellent range reduction     sin degrees2radians x   sin degrees2radians fmod x  360 0        -360 0  lt  fmod x 360   lt   360 0  Various trig identities and remquo   offer even more improvement  Sample  sind

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If you want to look at the actual GNU implementation of those functions in C  check out the latest trunk of glibc   See the GNU C Library

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Yes  there are software algorithms for calculating sin too  Basically  calculating these kind of stuff with a digital computer is usually done using numerical methods like approximating the Taylor series representing the function   Numerical methods can approximate functions to an arbitrary amount of accuracy and since the amount of accuracy you have in a floating number is finite  they suit these tasks pretty well

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Chebyshev polynomials  as mentioned in another answer  are the polynomials where the largest difference between the function and the polynomial is as small as possible  That is an excellent start    In some cases  the maximum error is not what you are interested in  but the maximum relative error  For example for the sine function  the error near x   0 should be much smaller than for larger values  you want a small relative error  So you would calculate the Chebyshev polynomial for sin x   x  and multiply that polynomial by x    Next you have to figure out how to evaluate the polynomial  You want to evaluate it in such a way that the intermediate values are small and therefore rounding errors are small  Otherwise the rounding errors might become a lot larger than errors in the polynomial  And with functions like the sine function  if you are careless then it may be possible that the result that you calculate for sin x is greater than the result for sin y even when x  lt  y  So careful choice of the calculation order and calculation of upper bounds for the rounding error are needed    For example  sin x   x - x 3 6   x 5   120 - x 7   5040    If you calculate naively sin x   x    1 - x 2 6   x 4 120 - x 6 5040      then that function in parentheses is decreasing  and it will happen that if y is the next larger number to x  then sometimes sin y will be smaller than sin x  Instead  calculate sin x   x - x 3    1 6 - x 2   120   x 4 5040     where this cannot happen    When calculating Chebyshev polynomials  you usually need to round the coefficients to double precision  for example  But while a Chebyshev polynomial is optimal  the Chebyshev polynomial with coefficients rounded to double precision is not the optimal polynomial with double precision coefficients    For example for sin  x   where you need coefficients for x  x 3  x 5  x 7 etc  you do the following  Calculate the best approximation of sin x with a polynomial  ax   bx 3   cx 5   dx 7  with higher than double precision  then round a to double precision  giving A  The difference between a and A would be quite large  Now calculate the best approximation of  sin x - Ax  with a polynomial  b x 3   cx 5   dx 7   You get different coefficients  because they adapt to the difference between a and A  Round b to double precision B  Then approximate  sin x - Ax - Bx 3  with a polynomial cx 5   dx 7 and so on  You will get a polynomial that is almost as good as the original Chebyshev polynomial  but much better than Chebyshev rounded to double precision    Next you should take into account the rounding errors in the choice of polynomial  You found a polynomial with minimum error in the polynomial ignoring rounding error  but you want to optimise polynomial plus rounding error  Once you have the Chebyshev polynomial  you can calculate bounds for the rounding error  Say f  x  is your function  P  x  is the polynomial  and E  x  is the rounding error  You don t want to optimise   f  x  - P  x     you want to optimise   f  x  - P  x    - E  x     You will get a slightly different polynomial that tries to keep the polynomial errors down where the rounding error is large  and relaxes the polynomial errors a bit where the rounding error is small    All this will get you easily rounding errors of at most 0 55 times the last bit  where   -     have rounding errors of at most 0 50 times the last bit

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For sin specifically  using Taylor expansion would give you   sin x     x - x 3 3    x 5 5  - x 7 7           1   you would keep adding terms until either the difference between them is lower than an accepted tolerance level or just for a finite amount of steps  faster  but less precise   An example would be something like   float sin float x      float res 0  pow x  fact 1    for int i 0  i lt 5    i          res  pow fact      pow  -1 x x      fact   2  i 1    2  i 1  1          return res      Note   1  works because of the aproximation sin x  x for small angles  For bigger angles you need to calculate more and more terms to get acceptable results  You can use a while argument and continue for a certain accuracy   double sin  double x       int i   1      double cur   x      double acc   1      double fact  1      double pow   x      while  fabs acc   gt   00000001  amp  amp    i  lt  100           fact      2 i   2 i 1            pow    -1   x x           acc    pow   fact          cur    acc          i              return cur

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In GNU libm  the implementation of sin is system-dependent  Therefore you can find the implementation  for each platform  somewhere in the appropriate subdirectory of sysdeps   One directory includes an implementation in C  contributed by IBM  Since October 2011  this is the code that actually runs when you call sin   on a typical x86-64 Linux system  It is apparently faster than the fsin assembly instruction  Source code  sysdeps ieee754 dbl-64 s sin c  look for   sin  double x    This code is very complex  No one software algorithm is as fast as possible and also accurate over the whole range of x values  so the library implements several different algorithms  and its first job is to look at x and decide which algorithm to use    When x is very very close to 0  sin x     x is the right answer  A bit further out  sin x  uses the familiar Taylor series  However  this is only accurate near 0  so    When the angle is more than about 7    a different algorithm is used  computing Taylor-series approximations for both sin x  and cos x   then using values from a precomputed table to refine the approximation  When  x    2  none of the above algorithms would work  so the code starts by computing some value closer to 0 that can be fed to sin or cos instead  There s yet another branch to deal with x being a NaN or infinity    This code uses some numerical hacks I ve never seen before  though for all I know they might be well-known among floating-point experts  Sometimes a few lines of code would take several paragraphs to explain  For example  these two lines  double t    x   hpinv   toint   double xn   t - toint    are used  sometimes  in reducing x to a value close to 0 that differs from x by a multiple of  pi  2  specifically xn  times   pi  2  The way this is done without division or branching is rather clever  But there s no comment at all     Older 32-bit versions of GCC glibc used the fsin instruction  which is surprisingly inaccurate for some inputs  There s a fascinating blog post illustrating this with just 2 lines of code   fdlibm s implementation of sin in pure C is much simpler than glibc s and is nicely commented  Source code  fdlibm s sin c and fdlibm k sin c

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The actual implementation of library functions is up to the specific compiler and or library provider   Whether it s done in hardware or software  whether it s a Taylor expansion or not  etc   will vary     I realize that s absolutely no help

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Functions like sine and cosine are implemented in microcode inside microprocessors   Intel chips  for example  have assembly instructions for these  A C compiler will generate code that calls these assembly instructions   By contrast  a Java compiler will not   Java evaluates trig functions in software rather than hardware  and so it runs much slower    Chips do not use Taylor series to compute trig functions  at least not entirely   First of all they use CORDIC  but they may also use a short Taylor series to polish up the result of CORDIC or for special cases such as computing sine with high relative accuracy for very small angles   For more explanation  see this StackOverflow answer

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OK kiddies  time for the pros     This is one of my biggest complaints with inexperienced software engineers  They come in calculating transcendental functions from scratch  using Taylor s series  as if nobody had ever done these calculations before in their lives  Not true  This is a well defined problem and has been approached thousands of times by very clever software and hardware engineers and has a well defined solution  Basically  most of the transcendental functions use Chebyshev Polynomials to calculate them  As to which polynomials are used depends on the circumstances  First  the bible on this matter is a book called  Computer Approximations  by Hart and Cheney  In that book  you can decide if you have a hardware adder  multiplier  divider  etc  and decide which operations are fastest  e g  If you had a really fast divider  the fastest way to calculate sine might be P1 x  P2 x  where P1  P2 are Chebyshev polynomials  Without the fast divider  it might be just P x   where P has much more terms than P1 or P2    so it d be slower  So  first step is to determine your hardware and what it can do  Then you choose the appropriate combination of Chebyshev polynomials  is usually of the form cos ax    aP x  for cosine for example  again where P is a Chebyshev polynomial   Then you decide what decimal precision you want  e g  if you want 7 digits precision  you look that up in the appropriate table in the book I mentioned  and it will give you  for precision   7 33  a number N   4 and a polynomial number 3502  N is the order of the polynomial  so it s p4 x 4   p3 x 3   p2 x 2   p1 x   p0   because N 4  Then you look up the actual value of the p4 p3 p2 p1 p0 values in the back of the book under 3502  they ll be in floating point   Then you implement your algorithm in software in the form     p4 x   p3  x   p2  x   p1  x   p0     and this is how you d calculate cosine to 7 decimal places on that hardware   Note that most hardware implementations of transcendental operations in an FPU usually involve some microcode and operations like this  depends on the hardware   Chebyshev polynomials are used for most transcendentals but not all  e g  Square root is faster to use a double iteration of Newton raphson method using a lookup table first  Again  that book  Computer Approximations  will tell you that   If you plan on implmementing these functions  I d recommend to anyone that they get a copy of that book  It really is the bible for these kinds of algorithms  Note that there are bunches of alternative means for calculating these values like cordics  etc  but these tend to be best for specific algorithms where you only need low precision  To guarantee the precision every time  the chebyshev polynomials are the way to go  Like I said  well defined problem  Has been solved for 50 years now     and thats how it s done   Now  that being said  there are techniques whereby the Chebyshev polynomials can be used to get a single precision result with a low degree polynomial  like the example for cosine above   Then  there are other techniques to interpolate between values to increase the accuracy without having to go to a much larger polynomial  such as  Gal s Accurate Tables Method   This latter technique is what the post referring to the ACM literature is referring to  But ultimately  the Chebyshev Polynomials are what are used to get 90  of the way there   Enjoy

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The essence of how it does this lies in this excerpt from Applied Numerical Analysis by Gerald Wheatley      When your software program asks the computer to get a value of    or   have you wondered how it can get the   values if the most powerful functions it can compute are polynomials    It doesnt look these up in tables and interpolate  Rather  the   computer approximates every function other than polynomials from some   polynomial that is tailored to give the values very accurately    A few points to mention on the above is that some algorithms do infact interpolate from a table  albeit only for the first few iterations  Also note how it mentions that computers utilise approximating polynomials without specifying which type of approximating polynomial  As others in the thread have pointed out  Chebyshev polynomials are more efficient than Taylor polynomials in this case

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If you want an implementation in software  not hardware  the place to look for a definitive answer to this question is Chapter 5 of Numerical Recipes   My copy is in a box  so I can t give details  but the short version  if I remember this right  is that you take tan theta 2  as your primitive operation and compute the others from there   The computation is done with a series approximation  but it s something that converges much more quickly than a Taylor series   Sorry I can t rembember more without getting my hand on the book

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Improved version of code from Blindy s answer   define EPSILON  0000000000001    this is smallest effective threshold  at least on my OS  WSL ubuntu 18     possibly because factorial part turns 0 at some point    and it happens faster then series element turns 0     validation was made against sin   from  lt math h gt  double ft sin double x        int k   2      double r   x      double acc   1      double den   1      double num   x       precision drops rapidly when x is not close to 0     so move x to 0 as close as possible     while  x  gt  PI          x -  PI      while  x  lt  -PI          x    PI      if  x  gt  PI   2          return  ft sin PI - x        if  x  lt  -PI   2          return  ft sin -PI - x        not using fabs for performance reasons     while  acc  gt  EPSILON    acc  lt  -EPSILON                num    -x   x          den    k    k   1           acc   num   den          r    acc          k    2            return  r

[c] How does C compute sin() and other math functions?

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