llvm dev - Mar 2009 - [LLVMdev] GSoC'09 question - previous mail w/o html

Hello llvm,

I am a first year PhD student at "Politehnica" University of  
Timisoara, Romania. I am a part of the LOOSE Research group
(http://www.loose.upt.ro
) and until now I have been involved in several research projects in  
reverse-engineering object oriented software.

Currently I am using llvm to statically check C/C++ programs for  
memory leaks. What I have until now is a llvm FunctionPass that tries  
to find a CFG path from each malloc or new to a corresponding free or  
delete. This is achieved by first doing a quick intra-procedural  
search for leaking paths and then inter-procedurally double checking  
the results. Further improvements would include a path viability check  
with a solver and some shape analysis.

This summer however, I plan to create an "optimization" that  
automatically fixes memory leaks in programs - obviously only those  
that can be fixed with the available information, for example:

int addAndFree(int *x, int y) {
	y += *x; free(x);
	return y;
}

int functionCalled10MTimesInSomeLoop(int index, int data[]) {
	int *x = (int*)malloc(sizeof(int));
	if(index < 3) { // do stuff with x, index and data
		*x = data[index];
		return addAndFree(x,index);
	} else
		return -1;
}

the function would not leak if that free instruction was copied in  
bb2: before ret (see bellow: -O4 version of above)

define i32 @_Z32functionCalled10MTimesInSomeLoopiPi(i32 %index, i32*  
nocapture %data) nounwind {
entry:
	%0 = malloc i32		; <i32*> [#uses=1]
	%1 = icmp sgt i32 %index, 2		; <i1> [#uses=1]
	br i1 %1, label %bb2, label %bb

bb:		; preds = %entry
	%2 = getelementptr i32* %data, i32 %index		; <i32*> [#uses=1]
	%3 = load i32* %2, align 4		; <i32> [#uses=1]
	%4 = add i32 %3, %index		; <i32> [#uses=1]
	free i32* %0
	ret i32 %4

bb2:		; preds = %entry
	; would be nice to insert: free i32* %0  ; here
	ret i32 -1
}

So seeing as this is not on the open projects list, is it even worth  
applying for SummerOfCode?

best wishes,

Mihai.

PS: sorry for including html in the previous mail

Hello, Mihai
> So seeing as this is not on the open projects list, is it even worth
> applying for SummerOfCode?
You're free to suggest your own idea for the project (if you feel that
LLVM would benefit from it, surely). However, for such sort of thing
I'd suggest you to prepare something like project proposal - to allow
others understand what are your goals, etc.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

On Mar 26, 2009, at 8:28 AM, Mihai Balint wrote:
>
> This summer however, I plan to create an "optimization" that
> automatically fixes memory leaks in programs - obviously only those
> that can be fixed with the available information, for example:
Hello,

This doesn't sound advisable.  A memory leak in a normal C/C++ program
is a bug, so this wouldn't be an optimization -- it'd be a  
transformation that
unreliably hides bugs.

Dan

On Mar 27, 2009, at 9:15 PM, Dan Gohman wrote:
>
> On Mar 26, 2009, at 8:28 AM, Mihai Balint wrote:
>>
>> This summer however, I plan to create an "optimization" that
>> automatically fixes memory leaks in programs - obviously only those
>> that can be fixed with the available information, for example:
>
> Hello,
>
> This doesn't sound advisable.  A memory leak in a normal C/C++ program
> is a bug, so this wouldn't be an optimization -- it'd be a
> transformation that
> unreliably hides bugs.
>
> Dan
>
It doesn't sound correct either.  I, for one, have a multithreaded  
code where all "created" threads allocate "request" objects,
put them
in a list and wake up the main thread.  The main thread on the other  
hand sleeps, consumes requests, frees the objects it consumed and goes  
back to sleep in a loop.  There is no path between the malloc() and  
the free(), but that's how it is supposed to be, there is no leak.

Anthony