llvm dev - Aug 2013 - [LLVMdev] structure member alignment for vector types

Hi All,
I have run into an issue when writing some LLVM code to read from a C++ 
structure.
The structure as defined in LLVM is { { <4 x float> }, { <4 x float>
},
{ <16 x float> }, { <4 x float> }, { <4 x float> } }.
On Windows, with Visual Studio 2012, the vector members of the structure 
are packed tightly together.
However, LLVM seems to be leaving 32 bytes between the second 4 x float 
vector and the 16 x float vector.  This is maybe because the 16 x float 
vector
is getting aligned to 64 bytes inside the structure.
The result is that the JIT'd LLVM code is reading from the wrong memory 
location, resulting in errors.

Is this known behaviour of LLVM?  Presumably the structure layout is 
supposed to match that of the platform / Visual Studio?
Why would it leave a 32 byte gap anyway?

I'm using LLVM 3.2, on Windows 64 bit.

Cheers,
     Nick

-- 
Nicholas Chapman

Managing Director,
Glare Technologies Limited

On Sun, Aug 18, 2013 at 3:37 PM, Nicholas Chapman
<admin at indigorenderer.com>wrote:
> Hi All,
> I have run into an issue when writing some LLVM code to read from a C++
> structure.
> The structure as defined in LLVM is { { <4 x float> }, { <4 x
float> }, {
> <16 x float> }, { <4 x float> }, { <4 x float> } }.
> On Windows, with Visual Studio 2012, the vector members of the structure
> are packed tightly together.
> However, LLVM seems to be leaving 32 bytes between the second 4 x float
> vector and the 16 x float vector.  This is maybe because the 16 x float
> vector
> is getting aligned to 64 bytes inside the structure.
> The result is that the JIT'd LLVM code is reading from the wrong memory
> location, resulting in errors.
>
> Is this known behaviour of LLVM?  Presumably the structure layout is
> supposed to match that of the platform / Visual Studio?
> Why would it leave a 32 byte gap anyway?
>
> I'm using LLVM 3.2, on Windows 64 bit.
>
>
>
The natural alignment of a 32-byte vector is 32 bytes, so LLVM will add
padding as appropriate.

It's possible the data layout string for Windows targets is incorrect,
though.  Take a look at the data layout string in your IR and
X86TargetMachine.cpp (which overrides the string specified in the IR for
code generation, IIRC).

Alternatively, you can use a packed struct and add the padding by hand.

-Eli
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