[LLVMdev] Create a call to function malloc using LLVM API (original) (raw)

Jeremy Lakeman Jeremy.Lakeman at gmail.com
Mon Jan 26 22:50:33 PST 2015

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* argalloc = builder.CreateAlloca(tp); //allocate memory to the pointer Is there any way to insert the malloc instruction after the argalloc 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/classllvm11CallInst.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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