[LLVMdev] Create a call to function malloc using LLVM API

[Dingbao Xie]

Thanks for your reply.
I'm running the program in a 64 bit machine.
I think I get the correct result:
  %malloccall = tail call i8* @malloc(i64 ptrtoint (i32* getelementptr
(i32* null, i32 1) to i64))
  %0 = bitcast i8* %malloccall to i32*

Another problem is that I like using IRbuilder to insert instructions, but
IRbuilder has no method to create a call malloc .
 AllocaInst* arg_alloc = builder.CreateAlloca(tp); //allocate memory to the
pointer

Is there any way to insert the malloc instruction after the arg_alloc
instruction?
The only way that I can think of is not using IRbuilder.


On Mon, Jan 26, 2015 at 8:13 PM, Tim Northover <t.p.northover at gmail.com>
wrote:

> Hi,
>
> On 26 January 2015 at 17:50, Dingbao Xie <xiedingbao at gmail.com> wrote:
> > Is the second argument  type of p and the third one  type of the element
> > that p points to?
>
> Looking at http://llvm.org/docs/doxygen/html/classllvm_1_1CallInst.html:
>
> The second argument is an integer on your machine that can hold a
> pointer. Most likely i64 on a 64-bit CPU and i32 on a 32-bit CPU. It
> corresponds to the type of argument malloc expects for its size.
> You've passed i32 here, but it looks like your malloc has been
> previously declared to take an i64 so there's a good chance that's
> wrong.
>
> The third argument is the type being allocated. It's not used for size
> calculations (oddly, in my view), but just to cast the Malloc result
> back to something your program is expecting. So in C terms, if you
> were allocating space for "struct Foo", that's what you'd pass here.
>
> > The size of an integer variable is 4, but if I pass 4 as the last
> argument
> > to   CallInst::CreateMalloc, then an assertion failure will occur.
>
> I'm not *quite* sure what you're doing here, but it sounds like you
> may have passed an integer constant 4 to a function expecting a
> pointer to a Value. That's going to interpret "4" as a pointer, which
> is almost guaranteed to go very badly indeed. What you might be able
> to do is pass "ConstantInt::get(ITy, 4)" (though your getSizeOf looks
> like a better idea all round).
>
> Putting it all together, I'd expect the output (assuming tpp == i32,
> as seems to be the case here) to be:
>
>     %malloccall = call i8* @malloc(i64 ptrtoint(i32*
> getelementptr(i32* null, i32 1) to i64)
>     %res = bitcast i8* %malloccall to i32*
>
> (If you don't actually use the result, later optimisations may remove
> that final bitcast and even convert the malloc to a "tail call").
>
> Cheers.
>
> Tim.
>



-- 
Dingbao Xie
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