Objectives. look at floating point representation in its basic form expose errors of a different form: rounding error highlight IEEE-754 standard

Transcription

1 Floating Point

2 Objectives look at floating point representation in its basic form expose errors of a different form: rounding error highlight IEEE-754 standard 1

3 Why this is important: Errors come in two forms: truncation error and rounding error we always have them case study: Intel our jobs as developers: reduce impact 2

4 Intel 3

5 Intel 4

6 Intel 5

7 Intel Timeline June 1994 Intel engineers discover the division error. Managers decide the error will not impact many people. Keep the issue internal. June 1994 Dr Nicely at Lynchburg College notices computation problems Oct 19, 1994 After months of testing, Nicely confirms that other errors are not the cause. The problem is in the Intel Processor. Oct 24, 1994 Nicely contacts Intel. Intel duplicates error. Oct 30, 1994 After no action from Intel, Nicely sends an 6

8 Intel Timeline FROM: Dr. Thomas R. Nicely Professor of Mathematics Lynchburg College 1501 Lakeside Drive Lynchburg, Virginia Phone: Fax: Internet: TO: Whom it may concern RE: Bug in the Pentium FPU DATE: 30 October 1994 It appears that there is a bug in the floating point unit (numeric coprocessor) of many, and perhaps all, Pentium processors. In short, the Pentium FPU is returning erroneous values for certain division operations. For example, 0001/ is calculated incorrectly (all digits beyond the eighth significant digit are in error). This can be verified in compiled code, an ordinary spreadsheet such as Quattro Pro or Excel, or even the Windows calculator (use the scientific mode), by computing 00( )*(1/ ), which should equal 1 exactly (within some extremely small rounding error; in general, coprocessor results should contain 19 significant decimal digits). However, the Pentiums tested return

9 Intel Timeline Nov 1, 1994 Software company Phar Lap Software receives Nicely s . Sends to colleagues at Microsoft, Borland, Watcom, etc. decide the error will not impact many people. Keep the issue internal. Nov 2, with description goes global. Nov 15, 1994 USC reverse-engineers the chip to expose the problem. Intel still denies a problem. Stock falls. Nov 22, 1994 CNN Moneyline interviews Intel. Says the problem is minor. Nov 23, 1994 The MathWorks develops a fix. Nov 24, 1994 New York Times story. Intel still sending out flawed chips. Will replace chips only if it caused a problem in an important application. Dec 12, 1994 IBM halts shipment of Pentium based PCs Dec 16, 1994 Intel stock falls again. 8

10 Numerical bugs Obvious Software has bugs Not-SO-Obvious Numerical software has two unique bugs: 1. roundoff error 2. truncation error 9

11 Numerical Errors Roundoff Roundoff occurs when digits in a decimal point ( ) are lost (0.3333) due to a limit on the memory available for storing one numerical value. Truncation Truncation error occurs when discrete values are used to approximate a mathematical expression. 10

12 Uncertainty: well- or ill-conditioned? Errors in input data can cause uncertain results input data can be experimental or rounded. leads to a certain variation in the results well-conditioned: numerical results are insensitive to small variations in the input ill-conditioned: small variations lead to drastically different numerical calculations (a.k.a. poorly conditioned) 11

13 Our Job As numerical analysts, we need to 1. solve a problem so that the calculation is not susceptible to large roundoff error 2. solve a problem so that the approximation has a tolerable truncation error How? incorporate roundoff-truncation knowledge into the mathematical model the method the algorithm the software design awareness correct interpretation of results 12

14 Wanted: Real Numbers... in a computer Computers can represent integers, using bits: 23 = = (10111) 2 How would we represent fractions, e.g ? 13

15 Wanted: Real Numbers... in a computer Computers can represent integers, using bits: 23 = = (10111) 2 How would we represent fractions, e.g ? Idea: Keep going down past zero exponent: = So: Could store a fixed number of bits with exponents 0 a fixed number of bits with exponents < 0 This is called fixed-point arithmetic. 13

16 Fixed-Point Numbers Suppose we use units of 64 bits, with 32 bits for exponents 0 and 32 bits for exponents < 0. What numbers can we represent? 14

17 Fixed-Point Numbers Suppose we use units of 64 bits, with 32 bits for exponents 0 and 32 bits for exponents < 0. What numbers can we represent?

18 Fixed-Point Numbers Suppose we use units of 64 bits, with 32 bits for exponents 0 and 32 bits for exponents < 0. What numbers can we represent? Smallest: Largest:

19 Fixed-Point Numbers How many digits of relative accuracy (think relative rounding error) are available for the smallest vs. the largest number with 64 bits of fixed point?

20 Fixed-Point Numbers How many digits of relative accuracy (think relative rounding error) are available for the smallest vs. the largest number with 64 bits of fixed point? For large numbers: about 19 For small numbers: few or none Idea: Don t fix the location of the 0 exponent. Let it float. 15

21 Floating Points Normalized Floating-Point Representation Real numbers are stored as x = ±(0.d 1 d 2 d 3... d m ) β β e d 1 d 2 d 3... d m is the significand, e is the exponent e is negative, positive or zero the general normalized form requires d

22 Floating Point Example In base can be written as ( ) can be written as ( )

23 Floating Point Suppose we have only 3 bits for a (non-normalized) significand and a 1 bit exponent stored like.d 1 d 2 d 3 e 1 All possible combinations give: = ,0, = 7 So we get 0, 1 16, 2 16,..., 7 16, 0, 1 4, 2 4,..., 7 4, and 0, 1 8, 2 8,..., 7 8. On the real line: 18

24 Overflow, Underflow computations too close to zero may result in underflow computations too large may result in overflow overflow error is considered more severe underflow can just fall back to 0 19

25 Normalizing If we use the normalized form in our 4-bit case, we lose ,0,1 along with other. So we cannot represent 1 16, 1 8, and

26 Next: Floating Point Numbers We re familiar with base 10 representation of numbers: and 1234 = = we write as an integer part and a fractional part: a 3 a 2 a 1 a 0.b 1 b 2 b 3 b 4 For some (even simple) numbers, there may be an infinite number of digits to the right: π = /9 = =

27 Other bases So far, we have just base 10. What about base β? binary (β = 2), octal (β = 8), hexadecimal (β = 16), etc In the β-system we have n (a n... a 2 a 1 a 0.b 1 b 2 b 3 b 4... ) β = a k β k + b k β k k=0 k=0 22

28 Integer Conversion An algorithm to compute the base 2 representation of a base 10 integer (N) 10 = (a j a j 1... a 2 a 0 ) 2 Compute (N) 10 /2 = Q + R/2: = a j 2 j + + a a N 2 = a j 2 j a a 0 }{{}}{{} 2 =Q =R/2 Example Example: compute (11) 10 base 2 11/2 = 5R1 a 0 = 1 5/2 = 2R1 a 1 = 1 2/2 = 1R0 a 2 = 0 1/2 = 0R1 a 3 = 1 So (11) 10 = (1011) 2 23

29 The other way... Convert a base-2 number to base-10: ( ) 2 = = 1 + 2(0 + 2(1 + 2(0 + 2(0 + 2(0 + 2(1 + 2(1))))))) =

30 Converting fractions straight forward way is not easy goal: for x [0, 1] write x = 0.b 1 b 2 b 3 b 4 = β(x) = (c 1.c 2 c 3 c 4... ) β c k β k = (0.c 1 c 2 c 3... ) β k=1 multiplication by β in base-β only shifts the radix 25

31 Fraction Algorithm An algorithm to compute the binary representation of a fraction x: x = 0.b 1 b 2 b 3 b 4... = b Multiply x by 2. The integer part of 2x is b 1 2x = b b b Example Example:Compute the binary representation of = 1.25 b 1 = = 0.5 b 2 = = 1.0 b 3 = 1 So (0.625) 10 = (0.101) 2 26

32 A problem with precision r 0 = x f o r k = 1, 2,..., m i f r k 1 2 k b k = 1 r k = r k 1 2 k else b k = 0 end end k 2 k b k r k = r k 1 b k 2 k

33 Floating Point numbers Convert 13.5 = (1101.1) 2 into floating point representation. 28

34 Floating Point numbers Convert 13.5 = (1101.1) 2 into floating point representation = = (1.1011)

35 Floating Point numbers Convert 13.5 = (1101.1) 2 into floating point representation = = (1.1011) What pieces do you need to store an FP number? 28

36 Floating Point numbers Convert 13.5 = (1101.1) 2 into floating point representation = = (1.1011) What pieces do you need to store an FP number? Significand: (1.1011) 2 Exponent: 3 Idea: Notice that the leading digit (in binary) of the significand is always one. Only store

37 Floating Point numbers Final storage format: 13.5 = = (1.1011) Significand: 1011 a fixed number of bits Exponent: 3 a (signed!) integer allowing a certain range Exponent is most often stored as a positive offset from a certain negative number. E.g. 3 = 1023 }{{} }{{} implicit offset stored Actually stored: 1026, a positive integer. 29

38 Unrepresentable numbers? Can you think of a somewhat central number that we cannot represent as x = (1. ) 2 2 p? 30

39 Unrepresentable numbers? Can you think of a somewhat central number that we cannot represent as x = (1. ) 2 2 p? Zero. Which is somewhat embarrassing. Core problem: The implicit 1. It s a great idea, were it not for this issue. 30

40 Unrepresentable numbers? Have to break the pattern. Idea: Declare one exponent special, and turn off the leading one for that one. (say, -1023, a.k.a. stored exponent 0) For all larger exponents, the leading one remains in effect. Bonus Q: With this convention, what is the binary representation of a zero? 31

41 Web link: IEEE 754 Web link: What Every Computer Scientist Should Know About Floating-Point Arithmetic 32

42 IEEE-754: Why?!?! IEEE-754 is a widely used standard accepted by hardware/software makers defines the floating point distribution for our computation offer several rounding modes which effect accuracy Floating point arithmetic emerges in nearly every piece of code even modest mathematical operation yield loss of significant bits several pitfalls in common mathematical expressions 33

43 IEEE Floating Point (v. 754) How much storage do we actually use in practice? 32-bit word lengths are typical IEEE Single-precision floating-point numbers use 32 bits IEEE Double-precision floating-point numbers use 64 bits Goal: use the 32-bits to best represent the normalized floating point number 34

44 IEEE Single Precision (Marc-32) x = ±q 2 m 1-bit sign 8-bit exponent m 23-bit fraction q The leading one in the normalized signficand q does not need to be represented: b 1 = 1 is hidden bit IEEE 754: put x in 1.f normalized form 0 < m = c < 255 Largest exponent = 127, Smallest exponent =

45 IEEE Single Precision x = ±q 2 m Process for x = : 1. Convert both integer and fractional to binary: x = ( ) 2 2. Convert to 1.f form: x = }{{} ( } {{ } ) Convert exponent 5 = c 127 c = 132 c = (10 } 000 {{ 100 } ) 2 8 }{{} 1 10 } 000 {{ 100 } 101 } {{ }

46 IEEE Single Precision Special Cases: denormalized/subnormal numbers: use 1 extra bit in the significant: exponent is now 126 (less precision, more range), indicated by in the exponent field two zeros: +0 and 0 (0 fraction, 0 exponent) two s: + and (0 fraction, exponenet) NaN (any fraction, exponent) 37

47 IEEE Double Precision 1-bit sign 11-bit exponent 52-bit fraction single-precision: about 6 decimal digits of precision double-precision: about 15 decimal digits of precision m = c

48 Precision vs. Range type range approx range x single x x double x small numbers example 39

49 Subnormal Numbers What is the smallest representable number in an FP system with 4 stored bits in the significand and an exponent range of [ 7, 7]? 40

50 Subnormal Numbers What is the smallest representable number in an FP system with 4 stored bits in the significand and an exponent range of [ 7, 7]? First attempt: Significand as small as possible all zeros after the implicit leading one Exponent as small as possible: 7 (1.0000) Unfortunately: wrong. We can go way smaller by using the special exponent (which turns off the implicit leading one). 40

51 Subnormal Numbers We ll assume that the special exponent is 8. So: (0.0001) Numbers with the special exponent are called subnormal (or denormal) FP numbers. Technically, zero is also a subnormal. Note: It is thus quite natural to park the special exponent at the low end of the exponent range. 41

52 Subnormal Numbers Why would you want to know about subnormals? Because computing with them is often slow, because it is implemented using FP assist, i.e. not in actual hardware. Many C compilers support options to flush subnormals to zero. 42

53 Subnormal Numbers Why would you want to know about subnormals? Because computing with them is often slow, because it is implemented using FP assist, i.e. not in actual hardware. Many C compilers support options to flush subnormals to zero. FP systems without subnormals will underflow (return 0) as soon as the exponent range is exhausted. This smallest representable normal number is called the underflow level, or UFL. 42

54 Subnormal Numbers Beyond the underflow level, subnormals provide for gradual underflow by keeping going as long as there are bits in the significand, but it is important to note that subnormals don t have as many accurate digits as normal numbers. Analogously (but much more simply no supernormals ): the overflow level, OFL. 43

55 Summary: Translating Floating Point Numbers To summarize: To translate a stored (double precision) floating point value consisting of the stored fraction (a 52-bit integer) and the stored exponent value e stored the into its (mathematical) value, follow the following method: (1 + fraction 2 52 ) e stored e stored 0, value = (fraction 2 52 ) e stored = 0. 44

56 A problem with precision For other numbers, such as 1 5 needed. = 0.2, an infinite length is So 0.2 is stored just fine in base-10, but needs infinite number of digits in base-2!!! This is roundoff error in its basic form... 45

57 Floating Point and Rounding Error What is the relative error produced by working with floating point numbers? What is smallest floating point number > 1? Assume 4 stored bits in the significand. 46

58 Floating Point and Rounding Error What is the relative error produced by working with floating point numbers? What is smallest floating point number > 1? Assume 4 stored bits in the significand. (1.0001) = x ( ) 2 46

59 Floating Point and Rounding Error What is the relative error produced by working with floating point numbers? What is smallest floating point number > 1? Assume 4 stored bits in the significand. (1.0001) = x ( ) 2 What s the smallest FP number > 1024 in that same system? 46

60 Floating Point and Rounding Error What is the relative error produced by working with floating point numbers? What is smallest floating point number > 1? Assume 4 stored bits in the significand. (1.0001) = x ( ) 2 What s the smallest FP number > 1024 in that same system? (1.0001) = x ( ) 2 Can we give that number a name? 46

61 Floating Point and Rounding Error Unit roundoff or machine precision or machine epsilon or ε mach is a bound on the relative error in a floating point represention for a given rounding procedure. With chopping as the rounding procedure, it is the smallest number such that float(1 + ε) 1. In the above system, ε mach = (0.0001) 2. Another related quantity is ULP, or unit in the last place. It is important to note, that if chopping is used, then ε mach is the distance between 1.0 and the next largest floating point number. 47

62 ɛ m With chopping, take x = 1.0 and add 1/2, 1/4,..., 2 i : Hidden bit 52 bits e e e e e e e e e e Ooops! use fl(x) to represent the floating point machine number for the real number x fl( ) 1, but fl( ) = 1 48

63 ɛ m : Machine Epsilon Machine epsilon ɛ m is the smallest number such that fl(1 + ɛ m ) 1 With rounding-to-nearest, The double precision machine epsilon is about The single precision machine epsilon is about

64 50

65 Floating Point Errors Not all reals can be exactly represented as a machine floating point number. Then what? Round-off error IEEE options: Round to next nearest FP (preferred), Round to 0, Round up, and Round down Let x + and x be the two floating point machine numbers closest to x round to nearest: round(x) = x or x +, whichever is closest round toward 0: round(x) = x or x +, whichever is between 0 and x round toward (down): round(x) = x round toward + (up): round(x) = x + 51

66 Floating Point and Rounding Error What does this say about the relative error incurred in floating point calculations? 52

67 Floating Point and Rounding Error What does this say about the relative error incurred in floating point calculations? The factor to get from one FP number to the next larger one is (mostly) independent of magnitude: 1 + ε mach. Since we can t represent any results between x and x (1 + ε mach ), that s really the minimum error incurred. In terms of relative error: x x x = x(1 + ε mach ) x x = ε mach. At least theoretically, ε mach is the maximum relative error in any FP operations. (Practical implementations do fall short of this.) 52

68 Floating Point and Rounding Error What s that same number for double-precision floating point? (52 bits in the significand) 53

69 Floating Point and Rounding Error What s that same number for double-precision floating point? (52 bits in the significand) We can expect FP math to consistently introduce relative errors of about Working in double precision gives you about 16 (decimal) accurate digits. 53

70 Implementing Arithmetic How is floating point addition implemented? Consider adding a = (1.101) and b = (1.001) in a system with three bits in the significand. 54

71 Implementing Arithmetic How is floating point addition implemented? Consider adding a = (1.101) and b = (1.001) in a system with three bits in the significand. Rough algorithm: 1. Bring both numbers onto a common exponent 2. Do grade-school addition from the front, until you run out of digits in your system. 3. Round result. a = b = a + b

72 Problems with FP Addition What happens if you subtract two numbers of very similar magnitude? As an example, consider a = (1.1011) and b = (1.1010)

73 Problems with FP Addition What happens if you subtract two numbers of very similar magnitude? As an example, consider a = (1.1011) and b = (1.1010) a = b = a b ???? 2 1 or, once we normalize, 1.???? 2 3. There is no data to indicate what the missing digits should be. 55

74 Problems with FP Addition Machine fills them with its best guess, which is not often good. This phenomenon is called Catastrophic Cancellation. 56

75 Floating Point Arithmetic Problem: The set of representable machine numbers is FINITE. So not all math operations are well defined! Basic algebra breaks down in floating point arithmetic Example a + (b + c) (a + b) + c 57

76 Floating Point Arithmetic Rule 1. fl(x) = x(1 + ɛ), where ɛ ɛ m Rule 2. For all operations (one of +,,, /) fl(x y) = (x y)(1 + ɛ ), where ɛ ɛ m Rule 3. For +, operations fl(a b) = fl(b a) There were many discussions on what conditions/rules should 58

77 Floating Point Arithmetic Consider the sum of 3 numbers: y = a + b + c. Done as fl(fl(a + b) + c) η = fl(a + b) = (a + b)(1 + ɛ 1 ) y 1 = fl(η + c) = (η + c)(1 + ɛ 2 ) = [(a + b)(1 + ɛ 1 ) + c] (1 + ɛ 2 ) = [(a + b + c) + (a + b)ɛ 1 )] (1 + ɛ 2 ) [ = (a + b + c) 1 + a + b ] a + b + c ɛ 1(1 + ɛ 2 ) + ɛ 2 So disregarding the high order term ɛ 1 ɛ 2 fl(fl(a+b)+c) = (a+b+c)(1+ɛ 3 ) with ɛ 3 a + b a + b + c ɛ 1 +ɛ 2 59

78 Floating Point Arithmetic If we redid the computation as y 2 = fl(a + fl(b + c)) we would find fl(a+fl(b+c)) = (a+b+c)(1+ɛ 4 ) with ɛ 4 b + c a + b + c ɛ 1 +ɛ 2 Main conclusion: The first error is amplified by the factor (a + b)/y in the first case and (b + c)/y in the second case. In order to sum n numbers more accurately, it is better to start with the small numbers first. [However, sorting before adding is usually not worth the cost!] 60

79 Floating Point Arithmetic One of the most serious problems in floating point arithmetic is that of cancellation. If two large and close-by numbers are subtracted the result (a small number) carries very few accurate digits (why?). This is fine if the result is not reused. If the result is part of another calculation, then there may be a serious problem Example Roots of the equation x 2 + 2px q = 0 Assume we want the root with smallest absolute value: 2 q 61

80 Catastrophic Cancellation Adding c = a + b will result in a large error if Let a b a b a = x.xxx 10 0 b = y.yyy 10 8 Then finite precision {}}{ x.xxx xxxx xxxx xxxx yyyy yyyy yyyy yyyy = x.xxx xxxx zzzz zzzz???? }{{????} lost precision 62

81 Catastrophic Cancellation Subtracting c = a b will result in large error if a b. For example a = x.xxxx xxxx xxx1 b = x.xxxx xxxx xxx0 lost {}}{ ssss... lost {}}{ tttt... Then finite precision {}}{ x.xxx xxxx xxx1 + x.xxx xxxx xxx0 = ???? }{{????} lost precision 63

82 Summary addition: c = a + b if a b or a b subtraction: c = a b if a b catastrophic: caused by a single operation, not by an accumulation of errors can often be fixed by mathematical rearrangement 64

83 Loss of Significance Example x = and y = What is the relative error in x y in a computer with 5 decimal digits of accuracy? x y ( x ȳ) x y =

84 Loss of Significance Loss of Precision Theorem Let x and y be (normalized) floating point machine numbers with x > y > 0. If 2 p 1 y x 2 q for positive integers p and q, the significant binary digits lost in calculating x y is between q and p. 66

85 Loss of Significance Example Consider x = and y = < 1 y x = < 2 12 So we lose 11 or 12 bits in the computation of x y. yikes! Example Back to the other example (5 digits): x = and y = < 1 y x = < 10 5 So we lose 4 or 5 bits in the computation of x y. Here, x y = which has only 1 significant digit that we can be sure about 67

86 Loss of Significance So what to do? Mainly rearrangement. f (x) = x

87 Loss of Significance So what to do? Mainly rearrangement. f (x) = x Problem at x 0. 68

88 Loss of Significance So what to do? Mainly rearrangement. f (x) = x Problem at x 0. One type of fix: ( ) ( ) x f (x) = x x no subtraction! = x 2 x