This file lists projects suitable for volunteers. Please see the tasks file for smaller tasks.
If you want to work on any of the projects below, please let gmp-devel@swox.com know. If you want to help with a project that already somebody else is working on, you will get in touch through gmp-devel@swox.com. (There are no email addresses of volunteers below, due to spamming problems.)
The current multiplication code uses Karatsuba, 3-way Toom, and Fermat FFT. Several new developments are desirable:
mpf_mul and by Newton approximations, a low half partial
product might be of use in a future sub-quadratic REDC. On small
sizes a partial product will be faster simply through fewer
cross-products, similar to the way squaring is faster. But work by
Thom Mulders shows that for Karatsuba and higher order algorithms the
advantage is progressively lost, so for large sizes partial products
turn out to be no faster.
Another possibility would be an optimized cube. In the basecase that should definitely be able to save cross-products in a similar fashion to squaring, but some investigation might be needed for how best to adapt the higher-order algorithms. Not sure whether cubing or further small powers have any particularly important uses though.
Write new and improve existing assembly routines. The tests/devel programs and the tune/speed.c and tune/many.pl programs are useful for testing and timing the routines you write. See the README files in those directories for more information.
Please make sure your new routines are fast for these three situations:
The most important routines are mpn_addmul_1, mpn_mul_basecase and mpn_sqr_basecase. The latter two don't exist for all machines, while mpn_addmul_1 exists for almost all machines.
Standard techniques for these routines are unrolling, software pipelining, and specialization for common operand values. For machines with poor integer multiplication, it is often possible to improve the performance using floating-point operations, or SIMD operations such as MMX or Sun's VIS.
Using floating-point operations is interesting but somewhat tricky. Since IEEE double has 53 bit of mantissa, one has to split the operands in small prices, so that no result is greater than 2^53. For 32-bit computers, splitting one operand into 16-bit pieces works. For 64-bit machines, one operand can be split into 21-bit pieces and the other into 32-bit pieces. (A 64-bit operand can be split into just three 21-bit pieces if one allows the split operands to be negative!)
Work on Schönhage GCD and GCDEXT for large numbers is in progress. Contact Niels Möller if you want to help.
Implement the functions of math.h for the GMP mpf layer! Check the book "Pi and the AGM" by Borwein and Borwein for ideas how to do this. These functions are desirable: acos, acosh, asin, asinh, atan, atanh, atan2, cos, cosh, exp, log, log10, pow, sin, sinh, tan, tanh.
The current code uses divisions, which are reasonably fast, but it'd be possible to use only multiplications by computing 1/sqrt(A) using this formula:
2
x = x (3 − A x )/2
i+1 i i
The square root can then be computed like this:
sqrt(A) = A x
n
That final multiply might be the full size of the input (though it might only need the high half of that), so there may or may not be any speedup overall.
Improve mpn_rootrem. The current code is really naive, using full precision from the first iteration. Also, calling mpn_pow_1 isn't very clever, as only 1/n of the result bits will be used; truncation after each multiplication would be better. Avoiding division might also be possible. Work mostly done, see http://swox.com/gmp/development.html.
Some work can be saved when only the quotient is required in a division, basically the necessary correction -0, -1 or -2 to the estimated quotient can almost always be determined from only a few limbs of multiply and subtract, rather than forming a complete remainder. The greatest savings are when the quotient is small compared to the dividend and divisor.
Some code along these lines can be found in the current
mpn_tdiv_qr, though perhaps calculating bigger chunks of
remainder than might be strictly necessary. That function in its current
form actually then always goes on to calculate a full remainder.
Burnikel and Zeigler describe a similar approach for the divide and
conquer case.
See also http://swox.com/gmp/development.html for some new code for divexact.
mpn_bdivmod and the redc in
mpz_powm should use some sort of divide and conquer
algorithm. This would benefit mpz_divexact, and
mpn_gcd on large unequal size operands. See "Exact Division
with Karatsuba Complexity" by Jebelean for a (brief) description.
Failing that, some sort of DIVEXACT_THRESHOLD could be added
to control whether mpz_divexact uses
mpn_bdivmod or mpn_tdiv_qr, since the latter is
faster on large divisors.
For the REDC, basically all that's needed is Montgomery's algorithm done in multi-limb integers. R is multiple limbs, and the inverse and operands are multi-precision.
For exact division the time to calculate a multi-limb inverse is not
amortized across many modular operations, but instead will probably
create a threshold below which the current style mpn_bdivmod
is best. There's also Krandick and Jebelean, "Bidirectional Exact
Integer Division" to basically use a low to high exact division for the
low half quotient, and a quotient-only division for the high half.
It will be noted that low-half and high-half multiplies, and a low-half square, can be used. These ought to each take as little as half the time of a full multiply, or square, though work by Thom Mulders shows the advantage is progressively lost as Karatsuba and higher algorithms are applied.
Some sort of scheme for exceptions handling would be desirable.
Presently the only thing documented is that divide by zero in GMP
functions provokes a deliberate machine divide by zero (on those systems
where such a thing exists at least). The global gmp_errno
is not actually documented, except for the old gmp_randinit
function. Being currently just a plain global means it's not
thread-safe.
The basic choices for exceptions are returning an error code or having a
handler function to be called. The disadvantage of error returns is they
have to be checked, leading to tedious and rarely executed code, and
strictly speaking such a scheme wouldn't be source or binary compatible.
The disadvantage of a handler function is that a longjmp or
similar recovery from it may be difficult. A combination would be
possible, for instance by allowing the handler to return an error code.
Divide-by-zero, sqrt-of-negative, and similar operand range errors can normally be detected at the start of functions, so exception handling would have a clean state. What's worth considering though is that the GMP function detecting the exception may have been called via some third party library or self contained application module, and hence have various bits of state to be cleaned up above it. It'd be highly desirable for an exceptions scheme to allow for such cleanups.
The C++ destructor mechanism could help with cleanups both internally and externally, but being a plain C library we don't want to depend on that.
A C++ throw might be a good optional extra exceptions
mechanism, perhaps under a build option. For GCC
-fexceptions will add the necessary frame information to
plain C code, or GMP could be compiled as C++.
Out-of-memory exceptions are expected to be handled by the
mp_set_memory_functions routines, rather than being a
prospective part of divide-by-zero etc. Some similar considerations
apply but what differs is that out-of-memory can arise deep within GMP
internals. Even fundamental routines like mpn_add_n and
mpn_addmul_1 can use temporary memory (for instance on Cray
vector systems). Allowing for an error code return would require an
awful lot of checking internally. Perhaps it'd still be worthwhile, but
it'd be a lot of changes and the extra code would probably be rather
rarely executed in normal usages.
A longjmp recovery for out-of-memory will currently, in
general, lead to memory leaks and may leave GMP variables operated on in
inconsistent states. Maybe it'd be possible to record recovery
information for use by the relevant allocate or reallocate function, but
that too would be a lot of changes.
One scheme for out-of-memory would be to note that all GMP allocations go
through the mp_set_memory_functions routines. So if the
application has an intended setjmp recovery point it can
record memory activity by GMP and abandon space allocated and variables
initialized after that point. This might be as simple as directing the
allocation functions to a separate pool, but in general would have the
disadvantage of needing application-level bookkeeping on top of the
normal system malloc. An advantage however is that it needs
nothing from GMP itself and on that basis doesn't burden applications not
needing recovery. Note that there's probably some details to be worked
out here about reallocs of existing variables, and perhaps about copying
or swapping between "permanent" and "temporary" variables.
Applications desiring a fine-grained error control, for instance a
language interpreter, would very possibly not be well served by a scheme
requiring longjmp. Wrapping every GMP function call with a
setjmp would be very inconvenient.
Another option would be to let mpz_t etc hold a sort of NaN,
a special value indicating an out-of-memory or other failure. This would
be similar to NaNs in mpfr. Unfortunately such a scheme could only be
used by programs prepared to handle such special values, since for
instance a program waiting for some condition to be satisfied could
become an infinite loop if it wasn't also watching for NaNs. The work to
implement this would be significant too, lots of checking of inputs and
intermediate results. And if mpn routines were to
participate in this (which they would have to internally) a lot of new
return values would need to be added, since of course there's no
mpz_t etc structure for them to indicate failure in.
Stack overflow is another possible exception, but perhaps not one that
can be easily detected in general. On i386 GNU/Linux for instance GCC
normally doesn't generate stack probes for an alloca, but
merely adjusts %esp. A big enough alloca can
miss the stack redzone and hit arbitrary data. GMP stack usage is
normally a function of operand size, which might be enough for some
applications to know they'll be safe. Otherwise a fixed maximum usage
can probably be obtained by building with
--enable-alloca=malloc-reentrant (or
notreentrant). Arranging the default to be
alloca only on blocks up to a certain size and
malloc thereafter might be a better approach and would have
the advantage of not having calculations limited by available stack.
Actually recovering from stack overflow is of course another problem. It
might be possible to catch a SIGSEGV in the stack redzone
and do something in a sigaltstack, on systems which have
that, but recovery might otherwise not be possible. This is worth
bearing in mind because there's no point worrying about tight and careful
out-of-memory recovery if an out-of-stack is fatal.
Operand overflow is another exception to be addressed. It's easy for
instance to ask mpz_pow_ui for a result bigger than an
mpz_t can possibly represent. Currently overflows in limb
or byte count calculations will go undetected. Often they'll still end
up asking the memory functions for blocks bigger than available memory,
but that's by no means certain and results are unpredictable in general.
It'd be desirable to tighten up such size calculations. Probably only
selected routines would need checks, if it's assumed say that no input
will be more than half of all memory and hence size additions like say
mpz_mul won't overflow.
It'd be nice to have some sort of tool for getting an overview of performance. Clearly a great many things could be done, but some primary uses would be,
A combination of measuring some fundamental routines and some representative application routines might satisfy these.
The tune/time.c routines would be the easiest way to get good accurate
measurements on lots of different systems. The high level
speed_measure may or may not suit, but the basic
speed_starttime and speed_endtime would cover
lots of portability and accuracy questions.
restrict
There might be some value in judicious use of C99 style
restrict on various pointers, but this would need some
careful thought about what it implies for the various operand overlaps
permitted in GMP.
Rumour has it some pre-C99 compilers had restrict, but
expressing tighter (or perhaps looser) requirements. Might be worth
investigating that before using restrict unconditionally.
Loops are presumably where the greatest benefit would be had, by allowing
the compiler to advance reads ahead of writes, perhaps as part of loop
unrolling. However critical loops are generally coded in assembler, so
there might not be very much to gain. And on Cray systems the explicit
use of _Pragma gives an equivalent effect.
One thing to note is that Microsoft C headers (on ia64 at least) contain
__declspec(restrict), so a #define of
restrict should be avoided. It might be wisest to setup a
gmp_restrict.
The limb-by-limb dependencies in the existing Nx1 division (and remainder) code means that chips with multiple execution units or pipelined multipliers are not fully utilized.
One possibility is to follow the current preinv method but taking two
limbs at a time. That means a 2x2->4 and a 2x1->2 multiply for
each two limbs processed, and because the 2x2 and 2x1 can each be done in
parallel the latency will be not much more than 2 multiplies for two
limbs, whereas the single limb method has a 2 multiply latency for just
one limb. A version of mpn_divrem_1 doing this has been
written in C, but not yet tested on likely chips. Clearly this scheme
would extend to 3x3->9 and 3x1->3 etc, though with diminishing
returns.
For mpn_mod_1, Peter L. Montgomery proposes the following
scheme. For a limb R=2^bits_per_mp_limb, pre-calculate
values R mod N, R^2 mod N, R^3 mod N, R^4 mod N. Then take dividend
limbs and multiply them by those values, thereby reducing them (moving
them down) by the corresponding factor. The products can be added to
produce an intermediate remainder of 2 or 3 limbs to be similarly
included in the next step. The point is that such multiplies can be done
in parallel, meaning as little as 1 multiply worth of latency for 4
limbs. If the modulus N is less than R/4 (or is it R/5?) the summed
products will fit in 2 limbs, otherwise 3 will be required, but with the
high only being small. Clearly this extends to as many factors of R as a
chip can efficiently apply.
The logical conclusion for powers R^i is a whole array "p[i] = R^i mod N" for i up to k, the size of the dividend. This could then be applied at multiplier throughput speed like an inner product. If the powers took roughly k divide steps to calculate then there'd be an advantage any time the same N was used three or more times. Suggested by Victor Shoup in connection with chinese-remainder style decompositions, but perhaps with other uses.
mpn_modexact_1_odd calculates an x in the range 0<=x<d
satisfying a = q*d + x*b^n, where b=2^bits_per_limb. The factor b^n
needed to get the true remainder r could be calculated by a powering
algorithm, allowing mpn_modexact_1_odd to be pressed into
service for an mpn_mod_1. modexact_1 is
simpler and on some chips can run noticeably faster than plain
mod_1, on Athlon for instance 11 cycles/limb instead of 17.
Such a difference could soon overcome the time to calculate b^n. The
requirement for an odd divisor in modexact can be handled by
some shifting on-the-fly, or perhaps by an extra partial-limb step at the
end.
The removal of twos in the current code could be extended to factors of 3 or 5. Taking this to its logical conclusion would be a complete decomposition into powers of primes. The power for a prime p is of course floor(n/p)+floor(n/p^2)+... Conrad Curry found this is quite fast (using simultaneous powering as per Handbook of Applied Cryptography algorithm 14.88).
A difficulty with using all primes is that quite large n can be calculated on a system with enough memory, larger than we'd probably want for a table of primes, so some sort of sieving would be wanted. Perhaps just taking out the factors of 3 and 5 would give most of the speedup that a prime decomposition can offer.
An obvious improvement to the current code would be to strip factors of 2 from each multiplier and divisor and count them separately, to be applied with a bit shift at the end. Factors of 3 and perhaps 5 could even be handled similarly.
Conrad Curry reports a big speedup for binomial coefficients using a prime powering scheme, at least for k near n/2. Of course this is only practical for moderate size n since again it requires primes up to n.
When k is small the current (n-k+1)...n/1...k will be fastest. Some sort of rule would be needed for when to use this or when to use prime powering. Such a rule will be a function of both n and k. Some investigation is needed to see what sort of shape the crossover line will have, the usual parameter tuning can of course find machine dependent constants to fill in where necessary.
An easier possibility also reported by Conrad Curry is that it may be
faster not to divide out the denominator (1...k) one-limb at a time, but
do one big division at the end. Is this because a big divisor in
mpn_bdivmod trades the latency of
mpn_divexact_1 for the throughput of
mpn_submul_1? Overheads must hurt though.
Another reason a big divisor might help is that
mpn_divexact_1 won't be getting a full limb in
mpz_bin_uiui. It's called when the n accumulator is full
but the k may be far from full. Perhaps the two could be decoupled so k
is applied when full. It'd be necessary to delay consideration of k
terms until the corresponding n terms had been applied though, since
otherwise the division won't be exact.
mpz_perfect_power_p could be improved in a number of ways,
for instance p-adic arithmetic to find possible roots.
Non-powers can be quickly identified by checking for Nth power residues
modulo small primes, like mpn_perfect_square_p does for
squares. The residues to each power N for a given remainder could be
grouped into a bit mask, the masks for the remainders to each divisor
would then be "and"ed together to hopefully leave only a few candidate
powers. Need to think about how wide to make such masks, ie. how many
powers to examine in this way.
Any zero remainders found in residue testing reveal factors which can be
divided out, with the multiplicity restricting the powers that need to be
considered, as per the current code. Further prime dividing should be
grouped into limbs like PP. Need to think about how much
dividing to do like that, probably more for bigger inputs, less for
smaller inputs.
mpn_gcd_1 would probably be better than the current private
GCD routine. The use it's put to isn't time-critical, and it might help
ensure correctness to just use the main GCD routine.
GMP is not really a number theory library and probably shouldn't have large amounts of code dedicated to sophisticated prime testing algorithms, but basic things well-implemented would suit. Tests offering certainty are probably all too big or too slow (or both!) to justify inclusion in the main library. Demo programs showing some possibilities would be good though.
The present "repetitions" argument to mpz_probab_prime_p is
rather specific to the Miller-Rabin tests of the current implementation.
Better would be some sort of parameter asking perhaps for a maximum
chance 1/2^x of a probable prime in fact being composite. If
applications follow the advice that the present reps gives 1/4^reps
chance then perhaps such a change is unnecessary, but an explicitly
described 1/2^x would allow for changes in the implementation or even for
new proofs about the theory.
mpz_probab_prime_p always initializes a new
gmp_randstate_t for randomized tests, which unfortunately
means it's not really very random and in particular always runs the same
tests for a given input. Perhaps a new interface could accept an rstate
to use, so successive tests could increase confidence in the result.
mpn_mod_34lsub1 is an obvious and easy improvement to the
trial divisions. And since the various prime factors are constants, the
remainder can be tested with something like
#define MP_LIMB_DIVISIBLE_7_P(n) \ ((n) * MODLIMB_INVERSE_7 <= MP_LIMB_T_MAX/7)Which would help compilers that don't know how to optimize divisions by constants, and is even an improvement on current gcc 3.2 code. This technique works for any modulus, see Granlund and Montgomery "Division by Invariant Integers" section 9.
The trial divisions are done with primes generated and grouped at
runtime. This could instead be a table of data, with pre-calculated
inverses too. Storing deltas, ie. amounts to add, rather than actual
primes would save space. udiv_qrnnd_preinv style inverses
can be made to exist by adding dummy factors of 2 if necessary. Some
thought needs to be given as to how big such a table should be, based on
how much dividing would be profitable for what sort of size inputs. The
data could be shared by the perfect power testing.
Jason Moxham points out that if a sqrt(-1) mod N exists then any factor of N must be == 1 mod 4, saving half the work in trial dividing. (If x^2==-1 mod N then for a prime factor p we have x^2==-1 mod p and so the jacobi symbol (-1/p)=1. But also (-1/p)=(-1)^((p-1)/2), hence must have p==1 mod 4.) But knowing whether sqrt(-1) mod N exists is not too easy. A strong pseudoprime test can reveal one, so perhaps such a test could be inserted part way though the dividing.
Jon Grantham "Frobenius Pseudoprimes" (www.pseudoprime.com) describes a quadratic pseudoprime test taking about 3x longer than a plain test, but with only a 1/7710 chance of error (whereas 3 plain Miller-Rabin tests would offer only (1/4)^3 == 1/64). Such a test needs completely random parameters to satisfy the theory, though single-limb values would run faster. It's probably best to do at least one plain Miller-Rabin before any quadratic tests, since that can identify composites in less total time.
Some thought needs to be given to the structure of which tests (trial division, Miller-Rabin, quadratic) and how many are done, based on what sort of inputs we expect, with a view to minimizing average time.
It might be a good idea to break out subroutines for the various tests,
so that an application can combine them in ways it prefers, if sensible
defaults in mpz_probab_prime_p don't suit. In particular
this would let applications skip tests it knew would be unprofitable,
like trial dividing when an input is already known to have no small
factors.
For small inputs, combinations of theory and explicit search make it relatively easy to offer certainty. For instance numbers up to 2^32 could be handled with a strong pseudoprime test and table lookup. But it's rather doubtful whether a smallnum prime test belongs in a bignum library. Perhaps if it had other internal uses.
An mpz_nthprime might be cute, but is almost certainly
impractical for anything but small n.
On various systems, calls within libgmp still go through the PLT, TOC or other mechanism, which makes the code bigger and slower than it needs to be.
The theory would be to have all GMP intra-library calls resolved directly to the routines in the library. An application wouldn't be able to replace a routine, the way it can normally, but there seems no good reason to do that, in normal circumstances.
The visibility attribute in recent gcc is good for this,
because it lets gcc omit unnecessary GOT pointer setups or whatever if it
finds all calls are local and there's no global data references.
Documented entrypoints would be protected, and purely
internal things not wanted by test programs or anything can be
internal.
Unfortunately, on i386 it seems protected ends up causing
text segment relocations within libgmp.so, meaning the library code can't
be shared between processes, defeating the purpose of a shared library.
Perhaps this is just a gremlin in binutils (debian packaged
2.13.90.0.16-1).
The linker can be told directly (with a link script, or options) to do the same sort of thing. This doesn't change the code emitted by gcc of course, but it does mean calls are resolved directly to their targets, avoiding a PLT entry.
Keeping symbols private to libgmp.so is probably a good thing in general too, to stop anyone even attempting to access them. But some undocumented things will need or want to be kept visible, for use by mpfr, or the test and tune programs. Libtool has a standard option for selecting public symbols (used now for libmp).