[LLVMdev] Promoting malloc to alloca

[Matt Giuca]

OK thanks for the replies.

Yes, I was planning to use a garbage collector. This is for a
functional language, so there's no real way to determine when memory
needs to be freed without one.

> Firstly, the pointer has to not make it into any function call at all, since any function might in turn call free(). Then we need to do escape analysis as you pointed out, but that's not difficult. We do similar analysis to determine pointer capture already.

OK. Is there any way to access the results of this pointer capture
analysis (eg. have LLVM tell me which pointers will escape and which
won't). That way I could manually change the mallocs into allocas.

Now I can see why LLVM doesn't do this -- it would be unsafe in
general. In my language (and many other high-level garbage-collected
languages which don't generate free() calls), it would be useful, but
LLVM itself can't guarantee that it's safe.

>>
>> define i32 @dontescape(i32 %x) {
>> entry:
>>   %t = tail call i8* @malloc(i32 4)
>>   %t1 = bitcast i8* %t to i32*
>>   store i32 %x, i32* %t1
>>   %t2 = load i32* %t1
>>   ret i32 %t2
>> }
>
> When do you free it? Why not just use an alloca?

Well I can't use an alloca because the code is generated by a compiler
which just knows that it has to allocate memory, and expects it to be
freed by a garbage collector. I would like to generate an alloca
instead of a malloc in some situations, such as this, so the purpose
of this email was to work out whether I can get LLVM to promote it for
me, or whether I have to do the analysis myself.

> LLVM doesn't realize that @malloc has no other side-effects. It's treating it like a function that might perform I/O.

Right, which is why I was trying to change its annotations to show
LLVM that @malloc doesn't have I/O side-effects, so it can be removed
if its result is unused.

>> So firstly, is there anything I'm doing wrong which is preventing this
>> from working? For example, I tried declaring malloc as "readnone" (is
>> this safe?).
>
> No. If it's readnone then two calls with the same argument can be folded to one.

Good point. Is there any annotation which says LLVM can remove the
call if the result is unused (i.e., there are no side-effects), but it
can't combine two calls into one (i.e., the result might be different
each time). That seems like what "readonly" is for, but I thought I'd
check.

In other words, is it safe to annotate malloc as "readonly"?

> Doing this in general requires either that the free can't be found or that it really is right before the function exit, or else there might be a good reason the memory is on the heap. Then we have to make sure that we don't transform malloc calls inside loops (unless we also prove they're independent and Just Leaked).
>
> How large a block of memory are you willing to move from heap to stack? What if it's a variable, how hard should we work to try to determine its upper bound (we probably can't with any reasonable amount of effort). And what type is the alloca? We could take the 4 from malloc(i32 4) and create an alloca i8, 4 and let instcombine try to sort it out.
>
> Where should we look for malloc calls? Note that we only do stack to register transformation on alloca's in the first basic block if they're stacked together in a row as the very first instructions in the function.

Thanks, a useful summary of the problems.

It sounds like this analysis is best implemented in a higher-level
compiler, before generating LLVM code.