Two programs with same logic
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From: karthikbalaguru on 27 Dec 2009 01:17 On Dec 23, 12:05 am, Joe Beanfish <j...(a)nospam.duh> wrote: > karthikbalaguru wrote: > > Hi, > > Two set of codes can be compared by using > > beyond compare or other equivalent software > > to determine the areas of differences and the > > areas of similarities. But, there are scenarios > > in which the same logic would be used by 2 set of > > programs/softwares but that the variable names > > might be different. So, how to determine this > > without executing the program ? Is there any tool > > that will help in identifying the existence of similar > > logics in two different programs ? Any ideas ? > > As others say, it would be difficult at best to find dups. But a > couple approaches that could help. One might be to run both codes > through an obfuscator which would replace the programmer's meaningful > variable names with generated names. Then compare. But there's still > a lot of room for programming style to mess you up. > Agreed ! > A second approach would be to compile both to assembler and compare > the assembler code. That should remove a fair amount of programmer > style problems but not all. Interesting approach ! I think, this is better than the earlier approach. But, Need to know the kind of programmer style problems that will not get covered by this second approach. Any ideas ? Thx in advans, Karthik Balaguru
From: karthikbalaguru on 27 Dec 2009 02:10 On Dec 23, 12:53 am, Kaz Kylheku <kkylh...(a)gmail.com> wrote: > On 2009-12-20, karthikbalaguru <karthikbalagur...(a)gmail.com> wrote: > > > Hi, > > Two set of codes can be compared by using > > beyond compare or other equivalent software > > to determine the areas of differences and the > > areas of similarities. But, there are scenarios > > in which the same logic would be used by 2 set of > > programs/softwares but that the variable names > > might be different. So, how to determine this > > without executing the program ? Is there any tool > > that will help in identifying the existence of similar > > logics in two different programs ? Any ideas ? > > The structural similarity you are hinting at can be easily found if you parse > the code into a nice abstract syntax tree representation. Then it's just a > matter of writing an equality function which compares two pieces of code. > > This is easiest to understand and illustrate through the Lisp language. > > The comparison is similar to a routine that compares two trees, except that it > has to understand all ``special forms'' in the language, and their semantics. > That is to say, the comparison has to behave as a ``code walker'', with respect > to two trees at the same time. It has to recognize all special syntactic > constructs, so that it can apply the right kind of comparison. > > It also must be capable of performing variable renaming. > > So for instance we might want these two to be found equivalent: > > (let ((x 1)) (+ x x)) > > (let ((y 1)) (+ y y)) > > For thaat we have to recognize that we have two matching constructs > (both are ``let''). They both bind the same number of variables, > using the same initialization values. Next, we have to normalize > the bodies. We can do that by renaming the variables on each > side: e.g. both x and y get renamed to t0 (temporary variable #0). > Each time we rename a variable we generate a new temporary. > When we rename x to t0 and substitute, we end up with: > > (let ((t0 1)) (+ t0 t0)) > > And when we do the same substitution on the variable y on the second form we > of course also end up with: > > (let ((t0 1)) (+ t0 t0)) > > So now having done this substituion over the let, we can recursively process > the body of each let and continue comparing: we now end up comparing (+ t0 t0) > to (+ t0 t0). We have a positive match on the same symbol +, and we know that > it's a function call. The two functions calls have the same number of > arguments, which we can compare one by one, finding an exact match in > ech position: the expression t0 trivially matches the expression t0. > Thus having reached bottom, we conclude that the two constructs are equivalent > (modulo variable naming). > > This applies to the comparison of C programs too because for the C language, > you can pick this kind of tree representation with symbols and atoms. > Parse the code to that, and write a comparison which applies the right > kind of rules. > Interesting. Agreed ! > The question is the semantic depth that you want in the comparison. > True ! > Should the expression !(!P || !Q) match P && Q, by the application of > De Morgan's law? > > Does your equivalence function fold constant expressions, so that > 2 + 2 matches 1 + 3? > > There is a huge range of things you can do in between the most naive > static comparison (in which even differences in variable names do matter) > to running the program and trying to identify that it does the same things > to all inputs (running into the halting problem). Yeah. True ! > Now about implementing this. Turns out, the annoying part of parsing C to > structure is already done. For instance, there is this Japanese gentleman's > project called SC: > > http://super.para.media.kyoto-u.ac.jp/~tasuku/sc/index-e.html > > With this software, you would be able to just concentrate on writing the > equivalence function. Okay, I will look into this ! Thx, Karthik Balaguru
From: Pascal J. Bourguignon on 27 Dec 2009 07:40 karthikbalaguru <karthikbalaguru79(a)gmail.com> writes: > On Dec 23, 12:05�am, Joe Beanfish <j...(a)nospam.duh> wrote: >> karthikbalaguru wrote: >> > Hi, >> > Two set of codes can be compared by using >> > beyond compare or other equivalent software >> > to determine the areas of differences and the >> > areas of similarities. But, there are scenarios >> > in which the same logic would be used by 2 set of >> > programs/softwares but that the variable names >> > might be different. So, how to determine this >> > without executing the program ? Is there any tool >> > that will help in identifying the existence of similar >> > logics in two different programs ? Any ideas ? >> >> As others say, it would be difficult at best to find dups. But a >> couple approaches that could help. One might be to run both codes >> through an obfuscator which would replace the programmer's meaningful >> variable names with generated names. Then compare. But there's still >> a lot of room for programming style to mess you up. >> > > Agreed ! > >> A second approach would be to compile both to assembler and compare >> the assembler code. That should remove a fair amount of programmer >> style problems but not all. > > Interesting approach ! I think, this is better than the earlier > approach. But, Need to know the kind of programmer style > problems that will not get covered by this second approach. > Any ideas ? It's not just a question or programming style. You're at the semantics of the programs. The question is to compare the semantics of two programs. So you first have to represent the semantics of a program. One way to do that would be to have a formal semantic definition of the programming language, and to translate the programs into that formalism, and then compare thetheir formal semantics. Unfortunately, not a lot of languages have a formal semantic definition, and for most of the programming languages which have been developed without such in mind, when one is developed for them, if it is complete, then it is rather complex, giving formal semantic forms that are not really more obvious than the original program (for example, in the case of C). In conclusion, for these kind of programming language, the best way to express their semantics in a formal way is to consider their compilation to some simplier processor code, and then compare these codes. Now of course when you compile these two programs: (defun s1 (v) (loop for e across v sum e)) (defun s2 (v) (do ((s 0) (i (1- (length v)) (1- i))) ((< i 0) s) (incf s (aref v i)))) you get two different listings, but you have to find a way to conclude that they do exactly the same thing: C/LISP[215]> (disassemble 's1) Disassembly of function S1 (CONST 0) = 0 1 required argument 0 optional arguments No rest parameter No keyword parameters 19 byte-code instructions: 0 (CONST&PUSH 0) ; 0 1 (NIL&PUSH) 2 (CONST&PUSH 0) ; 0 3 (JMP L19) 5 L5 5 (LOAD&PUSH 4) 6 (LOAD&PUSH 3) 7 (CALLSR&STORE 1 1 1) ; AREF 11 (LOAD&PUSH 0) 12 (LOAD&PUSH 2) 13 (CALLSR&STORE 2 53 0) ; + 17 (LOAD&INC&STORE 2) 19 L19 19 (LOAD&PUSH 2) 20 (LOAD&PUSH 5) 21 (CALLS2&PUSH 72) ; LENGTH 23 (CALLSR&JMPIFNOT 1 50 L5) ; >= 27 (POP) 28 (SKIP&RET 4) NIL C/LISP[216]> (disassemble 's2) Disassembly of function S2 (CONST 0) = 0 1 required argument 0 optional arguments No rest parameter No keyword parameters 17 byte-code instructions: 0 (CONST&PUSH 0) ; 0 1 (LOAD&PUSH 2) 2 (CALLS2&PUSH 72) ; LENGTH 4 (CALLS2&PUSH 152) ; 1- 6 (JMP L20) 8 L8 8 (LOAD&PUSH 1) 9 (LOAD&PUSH 4) 10 (LOAD&PUSH 2) 11 (CALLSR&PUSH 1 1) ; AREF 14 (CALLSR&STORE 2 53 1) ; + 18 (LOAD&DEC&STORE 0) 20 L20 20 (LOAD&PUSH 0) 21 (CALLS2&JMPIFNOT 148 L8) ; MINUSP 24 (LOAD 1) 25 (SKIP&RET 4) NIL The semantics of assembly is not more obvious than the semantics of high level programming languages. The reason why you will work with assembly is not that (otherwise we would keep programming in assembly!). The point of assembly, is that the instructions are simplier: their semantics is simplier. Therefore it is easier for programs to work with it. You want to compare semantics, then it's simplier to compare the semantics of simplier instructions. However, you still have a lot of work to do, even if each step of this work will be simplier. But it is possible to infer mechanically from both these assembly codes that each element of the vector is read, and summed, and that the sum is returned by the function. Therefore that these two functions are equivalent semantically. -- __Pascal Bourguignon__ http://www.informatimago.com/
From: Kenny McCormack on 27 Dec 2009 08:17
In article <87fx6wmvym.fsf(a)hubble.informatimago.com>, Pascal J. Bourguignon <pjb(a)informatimago.com> was apparently confused about which newsgroup he was in when he wrote: .... >Now of course when you compile these two programs: > >(defun s1 (v) > (loop for e across v sum e)) > >(defun s2 (v) > (do ((s 0) > (i (1- (length v)) (1- i))) > ((< i 0) s) > (incf s (aref v i)))) (Channelling our good friend Kiki, who is so good at pointing out the obvious) Lisp and C are two different languages. |