[LLVMdev] Alignment of constant loads

[Dan Gohman]

On Fri, September 12, 2008 11:33 am, Stefanus Du Toit wrote:
> Hi,
>
> We've noticed that constant loads, e.g. of v4f32 types, are not
> aligned to the preferred alignment but rather to the ABI alignment, at
> least on x86.

On x86 targets it's usually 16 for both preferred and ABI.
I guess you're using a target with a lower ABI alignment for
vectors?

> This seems to stem from SelectionDAG::getLoad() being
> called with an alignment of 0, which then does:
>
>    if (Alignment == 0)  // Ensure that codegen never sees alignment 0
>      Alignment = getMVTAlignment(VT);
>
> Inside getMVTAlignment, the ABI alignment is retrieved for the given VT.
>
> It appears that constants are already aligned to the preferred
> alignment, given this code in ScheduleDAG::AddOperand():
>
>    } else if (ConstantPoolSDNode *CP =
> dyn_cast<ConstantPoolSDNode>(Op)) {
>     [...]
>        Align = TM.getTargetData()->getPreferredTypeAlignmentShift(Type);
>
> (note there is a curious FIXME there about alignment of vector types
> -- I think this may be a relic since getPreferredTypeAlignmentShift
> should not return zero for vector types -- correct me if I'm wrong!)

I'm not sure about that FIXME offhand, but overall it looks
like the work this code is doing would be better placed in
SelectionDAG::getConstantPool.

> Do you have a suggestion as to how we can make it so that loads for
> constants are aligned to the preferred alignment, rather than the ABI
> alignment? We're not sure where this should be "fixed".

It looks like the best way to do this is to visit the handful of
places in legalize that create loads from constant pools and
add alignment parameters to the getLoad/getExtLoad calls.

If you move the handling of Alignment==0 out of ScheduleDAGEmit.cpp
and into SelectionDAG::getConstantPool, you can then have legalize
read the alignment from the node, instead of making its own
decision:
   cast<ConstantPoolSDNode>(CPIdx)->getAlignment()

Dan