Will Compiler optimize malloc/free or new/delete pair into alloca

Is there any mature C/C++ compiler, capable of optimizing malloc/free (or new/delete) pairs info alloca? In other words, convert from heap-based memory to stack-based (ONLY for some limited cases).

This optimization may be allowed only for pair of malloc/free when both functions are in the same function (or even in the same block of {}), and free is called every time when malloc is called. Also, lets consider that pointer to malloced memory is not saved in some global variable.

So, will GCC/LLVM+clang/Intel Compiler convert such block of code:

{ char *carray; carray = malloc(100); // or malloc(N) // some string-like work with carray free(carray); }

into

{ char*carray; carray = alloca(100); // or if(N<const1) carray=alloca(N);else carray=malloc(N) // the same work // nothing // or if(N>=const1) free(carray) }

This conversion may be not very useful for every program, but I think, there may be some special compiler option.

PS (update1) We can limit our discussion only to cases when compiler Knows that malloc and free is from libc (stdlib)

--------------Solutions-------------

In general, no compiler performs this optimization. That's good, because this thing can be potentially very harmful: bear in mind that the stack is usually very limited in its size. If a compiler optimized malloc+free into an alloca, the observable behavior of the code would change: for some inputs, it wouldn't crash with malloc+free, but it would with alloca (because the stack space got exhausted). Therefore, this optimization is unsafe (and illegal according to the standard, because it changes the observable behavior) and compilers don't even try to perform it.

That said, in some very specific circumstances, a compiler could perform it, but no compiler I'm aware of does.

The optimization performed by LLVM and mentioned in the comments is a different thing, it only optimizes out mallocs that are only compared to null and then freed.

There's an off-shoot of LLVM called poolalloc that does this optimization. It's being maintained as part of SAFECode, and isn't in the mainline LLVM distribution.

It's described in Chris Lattner's PhD thesis and in this PLDI paper. The code is here.

Technically, the compilers can optimize anything as long as they follow the As-If rule.
So, optimizing heap allocations to stack allocations would be possible but do to the compiler needs to be intelligent enough to probe the usage and determine that changing the allocation to stack won't affect the observable behavior of the program.

I am not aware of any compiler which does this.

Category:c# Time:2012-04-28 Views:0

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