[LLVMdev] Clang question on x86-64 class type definitions

[David Majnemer]

On Fri, May 15, 2015 at 4:48 PM, Panning, Benjamin J <
benjamin.j.panning at intel.com> wrote:

>  Hi All,
>
>
>
> I have a question on the type definitions that Clang generates when
> compiling for x86-64.  Here is the C++ code that I compile:
>
>
>
>                 class TestClass1 {
>
>                                 int X;
>
>                 public:
>
>                                 virtual int Foo() { return 1; }
>
>                 };
>
>
>
>                 class TestClass2 : public TestClass1 {
>
>                                 int X;
>
>                 public:
>
>                                 virtual int Foo() { return 1; }
>
>                 };
>
>
>
>                 class TestClass3 : public TestClass2 {
>
>                                 int X;
>
>                 public:
>
>                                 virtual int Foo() { return 1; }
>
>                 };
>
>
>
> Here is the command lines that I am using, for 32-bit and 64-bit mode
> respectively:
>
>
>
>                 clang.exe -S -emit-llvm -O0 test.cpp -o test.ll
>
>                 clang.exe -S -emit-llvm -O0 test.cpp -o test.ll
> -march=x86-64
>
>
>
> The 32-bit compile produces this, which I understand:
>
>
>
>                 %class.TestClass3 = type { %class.TestClass2, i32 }
>
>                 %class.TestClass2 = type { %class.TestClass1, i32 }
>
>                 %class.TestClass1 = type { i32 (...)**, i32 }
>
>
>
> The 64-bit compile produces this:
>
>
>
>                 %class.TestClass3 = type { %class.TestClass2, i32, [4 x
> i8] }
>
>                 %class.TestClass2 = type { [12 x i8], i32 }
>
>                 %class.TestClass1 = type { i32 (...)**, i32 }
>
>
>
> A few basic questions: (1) Why does TestClass2 have a [12 x i8] array
> instead of a reference to TestClass1?
>

What triple are you using? I cannot reproduce your %class.TestClass2 type.


> (2) What is the [4 x i8] array at the end of TestClass3?
>

The record is eight byte aligned because of the vptr at offset 0. Those
32-bytes are for padding to align the record's size to a multiple of the
alignment (8 bytes in this example).


> (3) Is it safe to use a bitcast instead of a getelementptr constant
> expression for casting a TestClass3* to a TestClass1* in this case (I see
> no other way of doing it…)?
>

Yes, it is perfectly safe to use bitcast instead of gep here.


>
>
> I’m not very knowledgeable about Clang, and I greatly appreciate your help.
>
>
>
> Thanks,
> Ben
>
>
>
>
>
>
>
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>
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