[LLVMdev] Preferred alignment of globals > 16bytes

Chris Lattner clattner at apple.com
Thu Sep 6 12:24:35 PDT 2012

On Sep 6, 2012, at 8:51 AM, Richard Osborne <richard at xmos.com> wrote:

> I recently noticed that all globals bigger than 16 bytes are being 16 byte aligned by LLVM (assuming there isn't an explicitly requested alignment). I'd really rather avoid this, at least for the XCore backend. I tracked this down to the following code in TargetData.cpp:
>  if (GV->hasInitializer() && GVAlignment == 0) {
>    if (Alignment < 16) {
>      // If the global is not external, see if it is large.  If so, give it a
>      // larger alignment.
>      if (getTypeSizeInBits(ElemType) > 128)
>        Alignment = 16;    // 16-byte alignment.
>    }
>  }
> I was a bit surprised to see these numbers hardcoded in TargetData since everything else is taken from the datalayout string. I was wondering what the logic was behind the number 16. Would it make sense to derive this number from the other alignments somehow (e.g. the maximum preferred alignment across all types). Alternatively would it make sense to make it configurable in the datalayout string?

I don't think that there is any specific logic to it.  It was just a heuristic that "made sense at the time", not based on any scientific evaluation.  If you want to raise it to some other arbitrary limit (say 32 bytes) that would be fine given some performance analysis.  If you want to design a way to express this in target data, please propose a specific way to model it.


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