Verifying a Synthesized Implementation of IEEE-754 Floating-Point ...

More documents

Recommendations

Info

16 B. Akbarpour et al.Lemma 5: D32_RTL_TO_ALGORITHM_Correct⊢ D32_RTL s1 s2 e1 e2 m1 m2 =⇒(Toint (float (BV s2,BNVAL e2,BNVAL m2)) =Toint (float (BV s1,BNVAL e1,BNVAL m1))/Int_32)FIGURE A2. Division by 32 block diagram.Verification of division by 32 block. This function is onlyrequired to be used on a specific type of numbers: multiples of32. Knowing this fact, the procedure does not need to supportall possible ranges of inputs. The operations performed can beexplained as follows: the algorithm will output zero if the inputexponent is 131 then(e2 = NBWORD 8 (BNVAL e1 − 5)) ∧(m2 = m1)else(e2 = NBWORD 8 0) ∧ (m2 = NBWORD 23 0))Then we established the correctness of the algorithmic to RTLtransition of the division by 32 block in HOL as follows:where Toint is the coercion for mapping floating-pointnumbers into machine integers, and ‘/’ denotes the divisionoperation on 2’s complement 32 bit machine integers. Thelemma is proved by rewriting on definitions of the divisionfunction in RTL (D32) and algorithmic (‘/’) level, andvaluations on floating-point numbers and machine integers.Verification of round to nearest integer block. Figure A3illustrates the round to nearest algorithm. It starts by checkingif the exponent is of the order of −2 or less. This would resultin an output of zero. The second case is to check if the exponentis −1 in which case the output would be equal to 1. These twoIF statements are for negative exponent handling since the mainalgorithm cannot deal with these cases.The basic idea is to verify the bit at the 0.5 position. If thebit is set, the decimal positions are filled with zero and we addone to the resulting integer. If the bit is reset, the bits locatedto the right of the decimal point will be reset. To accomplishthis, the input is first shifted right by a number of positionscorresponding to the exponent (so that all fraction bits are shiftedout). The number obtained should be an integer. This number isthen incremented by one if the bit at 0.5 is set else it should be leftthe same.In HOL we modeled this algorithm as follows:⊢ def ROUND1_RTL s1 s2 e1 e2 m1 m2 =∃ mbuff1 imp1 imp2 imp3 imp4 imp5 imp6 imp7 count.(if BNVAL e1 < 126 then(e2 = NBWORD 8 0) ∧ (m2 = NBWORD 23 0) ∧ (s2 = F)else(if BNVAL e1 = 126 then(e2 = NBWORD 8 127) ∧ (m2 = NBWORD 23 0) ∧ (s2 = s1)else(s2 = s1) ∧ (mbuff1 = BNVAL e1 − 127) ∧(imp1 = WCAT (WORD [F],m1)) ∧(count = 23 − mbuff1) ∧(imp2 = SHRN imp1 (count − 1)) ∧(imp3 = BIT 0 imp2) ∧(imp4 = SHRN imp2 1) ∧(imp5 = NBWORD 24 (BNVAL imp4 + BV imp3)) ∧(if mbuff1 < 23 then(if BIT (mbuff1 + 1) imp5 = T then(BNVAL e2 = mbuff1 + 1 + 127) ∧(imp6 = SHLN imp5 (count − 1))else(BNVAL e2 = mbuff1 + 127) ∧(imp6 = SHLN imp5 count)) ∧(m2 = WSEG 23 0 imp6)else(e2 = e1) ∧ (m2 = m1))))The Computer Journal, 2009
Verification of IEEE-754 Floating-Point Exponential Function 17FIGURE A3. Round to nearest integer block diagram.Then we established the correctness of the algorithmic toRTL transition of the round to nearest integer block in HOL asfollows:Lemma 6: ROUND1_RTL_TO_ALGORITHM_Correct⊢ ROUND1_RTL s1 s2 e1 e2 m1 m2 =⇒(Toint (float (BV s2,BNVAL e2,BNVAL m2)) =INTRND (float (BV s1,BNVAL e1,BNVAL m1)))where the function INTRND is the composition of theround-to-integer-value operation on floating-point numbers(ROUNDFLOAT) and the coercion function Toint. Therefore,our main task in the proof of the above-mentioned theorem wasto show that the result of the rounding operation following theRTL algorithm is the best approximation to the real value ofthe input number. This is established in HOL as the followingtheorem:Lemma 7:⊢ ROUND1_RTL s1 s2 e1 e2 m1 m2 =⇒((BV s2,BNVAL e2,BNVAL m2) =closest (valof float_format) (λa. T){a | is_integral float_format a}(valof float_format (BV s1,BNVAL e1,BNVAL m1)))where is_integral checks if a floating-point number has afinite and integer value.Verification of Modulo 32 block. Modulo 32 is an operationthat is done by simply taking the five first bits located to the leftof the decimal point. The result will then be an unsigned 5-bitinteger that will have to be converted to the single-precisionformat. For the block diagram, refer to Figure A4. The lowerright portion of the figure shows a loop that was not actuallyimplemented in the algorithm. It was drawn like that in order toreduce the complexity of the diagram. The variable ‘I’ is usedas a loop variable.The procedure is somewhat similar to that of rounding to thenearest integer. The input is first shifted right by the number ofbits corresponding to the exponent. The result is then ANDedwith the ‘11111’ bit pattern in order to isolate the five bits. Theconversion process checks where the first 1 is located startingfrom the most significant position.An exponent is then assignedaccordingly and the result is shifted left to comply with the rulesof normalization.In HOL we modeled this algorithm as follows:⊢ def MOD32_RTL s1 s2 e1 e2 m1 m2 =∃ mbuff1 imp1 imp2 imp3 imp4 imp5 imp6 count n2 n3.(s2 = s1) ∧ (BNVAL mbuff1 = BNVAL e1 − 127) ∧(imp1 = WCAT (WORD [T],m1)) ∧(if BNVAL mbuff1 > 23 then(count = BNVAL mbuff1 − 23) ∧ (imp2 = SHLN imp1 count)else(count = 23 − BNVAL mbuff1) ∧ (imp2 = SHRN imp1 count)) ∧(if BIT 7 mbuff1 = T thenn2 = NBWORD 24 0elsen2 = NBWORD 24 31 WAND imp2) ∧(if BIT 4 n2 = T then(BNVAL e2 = 131) ∧ (n3 = SHLN n2 19)else(if BIT 3 n2 = T then(BNVAL e2 = 130) ∧ (n3 = SHLN n2 20)The Computer Journal, 2009
Page 1 and 2: The Computer Journal Advance Access
Page 4 and 5: 4 B. Akbarpour et al.and − ln(2)/
Page 6 and 7: 6 B. Akbarpour et al.⊢ def Int_32
Page 8 and 9: 8 B. Akbarpour et al.The VHDL desig
Page 10: 10 B. Akbarpour et al.FIGURE 3. Mul
Page 23 and 24: Verification of IEEE-754 Floating-P

Verifying a Synthesized Implementation of IEEE-754 Floating-Point ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?