Floating Point

References

• Slides adapted from CMU

Outline

• Background: fractional binary numbers

• IEEE floating point standard

• Example and properties

• Rounding, addition, and multiplication

• Floating point in C

• Summary

Fractional Binary Numbers

• Representation

  • Bits to the right of “binary point” represent fractional powers of 2

  • Represents rational number: \(\sum_{k=-j}^{i} b_k \cdot 2^{k}\)

  

Fractional Binary Number Examples

• Observations

  • Divide by 2 by shifting right (unsigned)

  • Multiply by 2 by shifting left

  • Numbers of form \(0.1111 \ldots_{2}\) are just below 1.0

Representable Numbers

• Limitation 1

  • Can only exactly represent numbers of the form \(\frac{x}{2^k}\)

    • Other rational numbers have repeating bit representations

  • Example

    • 1/3 = 0.01010101[01] \(\ldots_{2}\)

• Limitation 2

  • Just one setting of binary point within the \(w\) bits

    • limited range of numbers

IEEE Floating Point

• IEEE Standard 754

  • Established in 1985 as uniform standard for floating point arithmetic

  • Supported by all major CPUs

• Driven by numerical concerns

  • Nice standards for rounding, overflow, underflow

  • Difficult to make fast in hardware

Floating Point Representation

• Numerical Form: \((-1)^s \cdot M \cdot 2^E\)

  • sign bit \(s\) determines whether number is negative or positive

  • significand \(M\) normally a fractional value in range \([1.0, 2.0)\)

  • exponent \(E\) weights value by power of two

• Encoding:

  • most significant bit is sign bit \(s\)

  • exp field encodes \(E\) (but is not equal to \(E\))

  • frac field encodes \(M\) (but is not equal to \(M\))

Precision options

• Single precision: 32 bits

  • exp field is 8 bits

  • frac field is 23 bits

• Double precision: 64 bits

  • exp field is 11 bits

  • frac field is 52 bits

  

Floating Point Numbers

• Three different “kinds” of floating point numbers based on the exp field:

  • normalized: exp bits are not all ones and not all zeros
  • denormalized: exp bits are all zero
  • special: exp bits are all one

Normalized Values

• Exponent coded as a biased value: \(E = exp - bias\)

  • \(exp\): unsigned value of exp field

  • \(bias = 2^{k-1} -1\), where \(k\) is number of exponent bits

• Significand coded with implied leading 1: \(M = 1.xx \ldots x_2\)

  • \(xxx \ldots x\): bits of frac field

  • minimum when \(frac = 000 \ldots 0\) (\(M = 1.0\))

  • maximum when \(frac = 111 \ldots 1\) (\(M = 2.0 - \epsilon\))

  • get extra leading bit for “free”

Normalized Encoding Example

• Value: float F = 15213.0;

  • \(15213_{10} = 11101101101101_{2} = 1.1101101101101_{2} \times 2^{13}\)

• Significand

  • \(M = 1.1101101101101\)

  • \(frac = 11011011011010000000000\)

• Exponent

  • \(E = 13\)

  • \(bias = 127\)

  • \(exp = 140 = 10001100_{2}\)

Denormalized Values

• Exponent value: \(E = 1 - bias\) (instead of \(exp - bias\))

• Significand coded with implied leading 0: \(M = 0.xxx \ldots x_{2}\)

  • \(xxx \ldots x\): bits of frac

• Cases

  • \(exp = 000 \ldots 0, frac = 000 \ldots 0\)

    • represents zero value

    • Note distinct values: \(+0\) and \(-0\)

  • \(exp = 000 \ldots 0, frac \neq 000 \ldots 0\)

    • numbers closest to 0.0

    • equally spaced

Special Values

• Case: \(exp = 111 \ldots 1, frac = 000 \ldots 0\)

  • represents value \(\infty\) (infinity)

  • operation that overflows

  • both positive and negative

  • examples: 1.0/0.0 = -1.0/-0.0 = \(+\infty\), 1.0/-0.0 = \(-\infty\)

• Case: \(exp = 111 \ldots 1, frac \neq 000 \ldots 0\)

  • Not-a-Number (NaN)

  • represents case when no numeric value can be determined

  • examples: sqrt(-1), \(\infty = \infty\), \(\infty \times 0\)

C float Decoding Example 1

• float value = 0xC0A00000

• binary: 1100 0000 1010 0000 0000 0000 0000 0000

• \(E = exp - bias = 129 - 127 = 2_{10}\)

• \(s = 1\) negative number

• \(M = 1.010 0000 0000 0000 0000 0000 = 1 + 1/4 = 1.25_{10}\)

• \(v = (-1)^s \cdot M \cdot 2^E = (-1)^1 \cdot 1.25 \cdot 2^2 = -5_{10}\)

C float Decoding Example 2

• float value = 0x001C0000

• binary: 0000 0000 0001 1100 0000 0000 0000 0000

• \(E = exp - bias = 1 - 127 = -126_{10}\)

• \(s = 0\) positive number

• \(M = 0.001 1100 0000 0000 0000 0000 = 1/8 + 1/16 + 1/32 = 7 \cdot 2^{-5}\)

• \(v = (-1)^s \cdot M \cdot 2^E = (-1)^0 \cdot 7 \cdot 2^{-5} \cdot 2^{-126} = 7 \cdot 2^{-131}\)

Tiny Floating Point Example

• 8-bit floating point representation

  • the sign bit is the most significant bit

  • the next four bits are the \(exp\), with a bias of 7

  • the last three bits are the \(frac\)

• Same general form as IEEE format

  • normalized, denormalized

  • representation of 0, NaN, infinity

Dynamic Range (\(s = 0\))

Special Properties of the IEEE Encoding

• Floating point zero same as integer zero

• Can (almost) use unsigned integer comparison

  • must first compare sign bits

  • must consider -0 = 0

  • NaNs are problematic

  • Otherwise OK

    • Denormalized vs. normalized

    • Normalized vs. infinity

Floating Point Operations: Basic Idea

• \(x +_{f} y = round(x + y)\)

• \(x \times_{f} y = round(x \times y)\)

• Basic idea

  • first compute exact result

  • make it fit into the desired precision

    • possibly overflow if exponent is too large

    • possibly round to fit into \(frac\)

Rounding

• Rounding modes (illustrate with rounding USD)

  	$1.40	$1.60	$1.50	$2.50	-$1.50
  towards zero	$1	$1	$1	$2	-$1
  round down (\(-\infty\))	$1	$1	$1	$2	-$2
  round up (\(\infty\))	$2	$2	$2	$3	-$1
  nearest even	$1	$2	$2	$2	-$2

• Nearest even rounds to the nearest, but if half-way in-between then round to nearest even

Closer Look at Round-To-Even

• Default Rounding Mode

  • Difficult to get any other kind without dropping into assembly

  • All others are statistically biased

    • sum of set of positive numbers will consistently be over- or under- estimated

  • Applying to other decimal places / bit positions

    • when exactly halfway between two possible values, round so that least significant digit is even

    • Example round to the nearest hundreth

      • 7.8950000 = 7.90 (halfway – round up)

      • 7.8850000 = 7.88 (halfway – round down)

Rounding Binary Numbers

• Binary Fractional Numbers

  • “even” when least significant bit is 0

  • “half way” when bits to right of rounding position \(= 100 \ldots_{2}\)

• Examples: round to the nearest 1/4 (2 bits right of binary point)

  value	binary	rounded	action	rounded value
  \(2 \frac{3}{32}\)	10.00011	10.00	down	\(2\)
  \(2 \frac{3}{16}\)	10.00110	10.01	up	\(2 \frac{1}{4}\)
  \(2 \frac{7}{8}\)	10.11100	11.00	up	\(3\)
  \(2 \frac{5}{8}\)	10.10100	10.10	down	\(2 \frac{1}{2}\)

Rounding

• Terminology

  • guard bit: least significant bit of result

  • round bit: the first bit removed

  • sticky bit: OR of remaining bits

• Round up conditions

  • round = 1, sticky = 1 \(\rightarrow > 0.5\)

  • guard = 1, round = 1, sticky = 0 \(\rightarrow\) round to even

Rounding Example

• Round to three bits after the binary point

  fraction	GRS	Incr?	Rounded
  1.0000000	000	N	1.000
  1.1010000	100	N	1.101
  1.0001000	010	N	1.000
  1.0011000	110	Y	1.010
  1.0001010	011	Y	1.001
  1.1111100	111	Y	10.000

Floating Point Multiplication

• \((-1)^{s1} \cdot M1 \cdot 2^{E1} \times (-1)^{s2} \cdot M2 \cdot 2^{E2}\)

• Exact result: \((-1)^{s} \cdot M \cdot 2^{E}\)

  • sign \(s\): \(s1\) ^ \(s2\)

  • significand \(M\): \(M1 \times M2\)

  • exponent \(E\): \(E1 + E2\)

• Fixing

  • If \(M \geq 2\), shift \(M\) right, increment \(E\)

  • If \(E\) out of range, overflow

  • Round \(M\) to fit \(frac\) precision

Floating Point Addition

• \((-1)^{s1} \cdot M1 \cdot 2^{E1} + (-1)^{s2} \cdot M2 \cdot 2^{E2}\), Assume \(E1 > E2\)

• Exact result: \((-1)^{s} \cdot M \cdot 2^{E}\)

  • sign \(s\), significand \(M\)

    • result of signed align and add, that is align at binary point

  • exponent \(E\): \(E1\)

• Fixing

  • If \(M \geq 2\), shift \(M\) right, increment \(E\)

  • If \(M < 1\), shift \(M\) left \(k\) positions, decrement \(E\) by \(k\)

  • If \(E\) out of range, overflow

  • Round \(M\) to fit \(frac\) precision

Properties of Floating Point Addition

• Compare to those of Abelian Group

  • Closed under addition, but may generate infinity or NaN

  • Commutative

  • Not associative

  • 0 is additive identity

  • Almost every element has an additive inverse, except for infinities and NaNs

• Monotonicity

  • \(a \geq b \rightarrow a + c \geq b + c\) except for infinities and NaNs

Properties of Floating Point Multiplication

• Compare to Commutative Ring

  • Closed under multiplication, but may generate infinity or NaN

  • Commutative

  • Not associative: possibility of overflow, inexactness of rounding

  • 1 is multiplicative identity

  • Multiplication does not distribute over addition

• Monotonicity

  • \(a \geq b \land c \geq 0 \rightarrow a * c \geq b * c\) except for infinities and NaNs

Floating Point in C

• C guarantees two levels

  • float: single precision

  • double: double precision

• Conversions / Casting

  • Casting between int, float, and double changes bit representation

  • double/float to int

    • truncates fractional part (like rounding to zero)

    • not defined when out of range or NaN

  • int to double

    • exact conversion, as long as int has \(\leq 53\) bit word size
  • int to float
    • will round according to rounding mode

Summary

• IEEE Floating Point has clear mathematical properties

• Represents numbers of form \(M \times 2^{E}\)

• One can reason about operations independent of implementation

  • as if computed with perfect precision and then rounded

• Not the same as real arithmetic

  • violates associativity and distributivity

  • makes life difficult for compilers and serious numerical applications programmers