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(gmp.info)Float Internals
Float Internals
===============
Efficient calculation is the primary aim of GMP floats and the use
of whole limbs and simple rounding facilitates this.
`mpf_t' floats have a variable precision mantissa and a single
machine word signed exponent. The mantissa is represented using sign
and magnitude.
most least
significant significant
limb limb
_mp_d
|---- _mp_exp ---> |
_____ _____ _____ _____ _____
|_____|_____|_____|_____|_____|
. <------------ radix point
<-------- _mp_size --------->
The fields are as follows.
`_mp_size'
The number of limbs currently in use, or the negative of that when
representing a negative value. Zero is represented by `_mp_size'
and `_mp_exp' both set to zero, and in that case the `_mp_d' data
is unused. (In the future `_mp_exp' might be undefined when
representing zero.)
`_mp_prec'
The precision of the mantissa, in limbs. In any calculation the
aim is to produce `_mp_prec' limbs of result (the most significant
being non-zero).
`_mp_d'
A pointer to the array of limbs which is the absolute value of the
mantissa. These are stored "little endian" as per the `mpn'
functions, so `_mp_d[0]' is the least significant limb and
`_mp_d[ABS(_mp_size)-1]' the most significant.
The most significant limb is always non-zero, but there are no
other restrictions on its value, in particular the highest 1 bit
can be anywhere within the limb.
`_mp_prec+1' limbs are allocated to `_mp_d', the extra limb being
for convenience (see below). There are no reallocations during a
calculation, only in a change of precision with `mpf_set_prec'.
`_mp_exp'
The exponent, in limbs, determining the location of the implied
radix point. Zero means the radix point is just above the most
significant limb. Positive values mean a radix point offset
towards the lower limbs and hence a value >= 1, as for example in
the diagram above. Negative exponents mean a radix point further
above the highest limb.
Naturally the exponent can be any value, it doesn't have to fall
within the limbs as the diagram shows, it can be a long way above
or a long way below. Limbs other than those included in the
`{_mp_d,_mp_size}' data are treated as zero.
The following various points should be noted.
Low Zeros
The least significant limbs `_mp_d[0]' etc can be zero, though
such low zeros can always be ignored. Routines likely to produce
low zeros check and avoid them to save time in subsequent
calculations, but for most routines they're quite unlikely and
aren't checked.
Mantissa Size Range
The `_mp_size' count of limbs in use can be less than `_mp_prec' if
the value can be represented in less. This means low precision
values or small integers stored in a high precision `mpf_t' can
still be operated on efficiently.
`_mp_size' can also be greater than `_mp_prec'. Firstly a value is
allowed to use all of the `_mp_prec+1' limbs available at `_mp_d',
and secondly when `mpf_set_prec_raw' lowers `_mp_prec' it leaves
`_mp_size' unchanged and so the size can be arbitrarily bigger than
`_mp_prec'.
Rounding
All rounding is done on limb boundaries. Calculating `_mp_prec'
limbs with the high non-zero will ensure the application requested
minimum precision is obtained.
The use of simple "trunc" rounding towards zero is efficient,
since there's no need to examine extra limbs and increment or
decrement.
Bit Shifts
Since the exponent is in limbs, there are no bit shifts in basic
operations like `mpf_add' and `mpf_mul'. When differing exponents
are encountered all that's needed is to adjust pointers to line up
the relevant limbs.
Of course `mpf_mul_2exp' and `mpf_div_2exp' will require bit
shifts, but the choice is between an exponent in limbs which
requires shifts there, or one in bits which requires them almost
everywhere else.
Use of `_mp_prec+1' Limbs
The extra limb on `_mp_d' (`_mp_prec+1' rather than just
`_mp_prec') helps when an `mpf' routine might get a carry from its
operation. `mpf_add' for instance will do an `mpn_add' of
`_mp_prec' limbs. If there's no carry then that's the result, but
if there is a carry then it's stored in the extra limb of space and
`_mp_size' becomes `_mp_prec+1'.
Whenever `_mp_prec+1' limbs are held in a variable, the low limb
is not needed for the intended precision, only the `_mp_prec' high
limbs. But zeroing it out or moving the rest down is unnecessary.
Subsequent routines reading the value will simply take the high
limbs they need, and this will be `_mp_prec' if their target has
that same precision. This is no more than a pointer adjustment,
and must be checked anyway since the destination precision can be
different from the sources.
Copy functions like `mpf_set' will retain a full `_mp_prec+1' limbs
if available. This ensures that a variable which has `_mp_size'
equal to `_mp_prec+1' will get its full exact value copied.
Strictly speaking this is unnecessary since only `_mp_prec' limbs
are needed for the application's requested precision, but it's
considered that an `mpf_set' from one variable into another of the
same precision ought to produce an exact copy.
Application Precisions
`__GMPF_BITS_TO_PREC' converts an application requested precision
to an `_mp_prec'. The value in bits is rounded up to a whole limb
then an extra limb is added since the most significant limb of
`_mp_d' is only non-zero and therefore might contain only one bit.
`__GMPF_PREC_TO_BITS' does the reverse conversion, and removes the
extra limb from `_mp_prec' before converting to bits. The net
effect of reading back with `mpf_get_prec' is simply the precision
rounded up to a multiple of `mp_bits_per_limb'.
Note that the extra limb added here for the high only being
non-zero is in addition to the extra limb allocated to `_mp_d'.
For example with a 32-bit limb, an application request for 250
bits will be rounded up to 8 limbs, then an extra added for the
high being only non-zero, giving an `_mp_prec' of 9. `_mp_d' then
gets 10 limbs allocated. Reading back with `mpf_get_prec' will
take `_mp_prec' subtract 1 limb and multiply by 32, giving 256
bits.
Strictly speaking, the fact the high limb has at least one bit
means that a float with, say, 3 limbs of 32-bits each will be
holding at least 65 bits, but for the purposes of `mpf_t' it's
considered simply to be 64 bits, a nice multiple of the limb size.
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