[LLVMdev] llvm instrinsic (memcpy/memset/memmov)and ConstantExpression with cast

[John Criswell]

On 4/14/11 6:34 PM, Kodakara, Sreekumar V wrote:
>
> Hi All,
>
> I have a question on ConstantExpressions and llvm intrinsic 
> memcpy/memset/memmove. I am using llvm-2.8 release. In one of the C 
> programs that I am compiling using clang frontend, the call to memcpy 
> instrinsic looks like the following
>
> call void @llvm.memcpy.p0i8.p0i8.i64(i8* %tmp2, i8* bitcast 
> (%struct.ta* @tret to i8*), i64 4, i32 4, i1 false), !dbg !19
>
> The second argument to memcpy is of type "struct ta" and global 
> variable "tret" is defined as follows in the code.
>
> struct ta {
>
>   int a;
>
> };
>
> struct ta tret;
>
> Since memcpy takes i8* as its argument, the struct.ta* is bitcasted to 
> i8* in the call to llvm.memcpy. For a pass that I am working on, I 
> would like to know the original type of this pointer. In this example, 
> I would like to know that the second argument is a pointer to type 
> struct.ta, and hence the size of memory allocated to that pointer is 4 
> bytes.
>

You should be able to use the stripPointerCasts() method of llvm::Value 
* to strip off the bitcast.

> I understand that the second argument to memcpy intrinsic  namely i8* 
> bitcast (%struct.ta* @t1 to i8*)  is a ConstantExpression (CE). When I 
> dump CE->getArgument(0), I get the following.
>
> @t1 = global %struct.ta zeroinitializer, align 4
>
> But from here, I am not able to find the relevant class/API to extract 
> the type of CE->getArgument(0) and hence the size of it. When I tried 
> CE->getArgument(0)->getType(), I am getting the type as Pointer and 
> hence the size to be 8 bytes.
>

This is because all global variables in LLVM are pointers to the actual 
data in memory.  What you need to do is:

if (GlobalVariable * GV = dyn_cast<GlobalVariable>(CE->getArgument(0)) {
     Type * T = GV->getType()->getContainedType();
}
>
> I would like to get the type as StructTy and size to be 4 bytes. I 
> searched through the llvm code
>
> Any help is appreciated.
>

-- John T.

> Thanks
>
> Sreekumar
>
>
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