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

[Jeremy Lakeman]

You can use IRBuilder to create calls to any functions. You just need a
reference to the Function*.

builder.CreateCall(module->getFunction("malloc"), AllocSize);

or

builder.CreateCall(module->getOrInsertFunction("malloc", [todo], ...),
AllocSize);


On Tue, Jan 27, 2015 at 4:49 PM, Dingbao Xie <xiedingbao at gmail.com> wrote:

> 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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