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Aligned attribute not being respected by linker

append delete Graeme Love

Hi,

SDK 16.0
Toolset: 3.52
OS: Windows 10

To fix various linker warnings that we were getting when compiling our iOS build using the MacOS command line tools, we had to add an aligned attribute to some statically declare data arrays e.g. 

%
data_type_definition VertexDataType[] __attribute__((aligned(8))) =
{
 ...
};
%

When building the same code using the iOS toolset for Win64, it seems like the linker is not respecting these alignment attributes as we're seeing warnings like: 

%
ld: warning: pointer not aligned at address 0x1075B758C
%

As you can see, the address is *not* aligned to 8 bytes.

One difference in our build processes is that we use *clang++* as our linker program on MacOS but *ld.exe* on Win64. I tried using *clang++.exe* as the linker driver on Win64 but it didn't like options like *-framework* so it seems that's a non-starter.

Is there a way to get the Win64 linker to pick up on this attribute?

Regards,

Graeme

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Replies

append delete #1. Graeme Love

It might be useful to know that the reason that we had to align the data arrays is because the types such as *VertexDataType* in the example I posted are compiled with *#pragma pack(4)* enabled.

I'm assuming that this enforces and alignment of 4 on the type which is the underlying cause of the issue.

Sadly, these types are for serialized data in a 3rd party library so it's not possible for us to change these easily.

Thanks,

Graeme

append delete #2. Pierre-Marie Baty

Hello Graeme

I'd think the problem is not with the linker here but with the compiler.

Have you tried tagging your structs with __attribute__((packed,aligned(4))) instead of #pragma pack ? The former syntax is the GCC/Clang syntax, the latter (#pragma pack) is the Microsoft one.

:: @Pierre-Marie Baty added on 15 Oct ’22 · 10:37

*edit* Sorry, I misread. You *did* use the right thing as you said in your first post. Ignore my reply.

:: @Pierre-Marie Baty added on 15 Oct ’22 · 10:45

*edit 2*

If I understand right, you have code that has VertexDataType structs that are aligned on 4-byte boundaries, compiled with #pragma pack(4).

So you'd rather want to use __attribute__((packed,aligned(4))) instead of just __attribute__((aligned(8))) in your own code, don't you ?

Note that the linker complaining that 0x1075B758C is not aligned to 8 bytes is normal on a 64-bit machine if you packed your structs on 4 byte boundaries. 0x1075B758C is DWORD-aligned (C = 12)

Unless I understand it wrong. I'm a bit confused why you'd want to force a 8-byte alignment with code that has been compiled for 4-byte alignment.

:: @Pierre-Marie Baty added on 15 Oct ’22 · 10:46

*edit 3* typo: "...is normal on a 64-bit machine" should be read as "... is normal for a 64-bit _target_"

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