[LLVMdev] Disable vectorization for unaligned data

[Francois Pichet]

Because unaligned load/store are illegal on my target.
But ExpandUnalignedStore expand to too many load/store.

It seem that ExpandUnalignedStore is called after the vectorization cost
analysis is done and not taken into account.



On Fri, Jul 19, 2013 at 4:32 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Fri, Jul 19, 2013 at 1:14 PM, Francois Pichet <pichet2000 at gmail.com>
> wrote:
> >
> > What is the proper solution to disable auto-vectorization for unaligned
> > data?
>
> Why are you trying to do this?  If auto-vectorization is making a
> given loop slower on your target, that means the cost metrics are off,
> and we should fix them.  If code size is an issue, you should tell the
> optimizer that you want to optimize for size.
>
> -Eli
>
> > I have an out of tree target and I added this:
> >
> > bool OpusTargetLowering::allowsUnalignedMemoryAccesses(EVT VT, bool
> *Fast)
> > const {
> >   if (VT.isVector())
> >     return false;
> > ....
> > }
> >
> > After that, I could see that vectorization is still done on unaligned
> data
> > except that llvm will copy the data back and forth from the source to the
> > top of the stack and work from there. This is very costly, I rather get
> > scalar operations.
> >
> > Then I tried to add:
> >   unsigned getMemoryOpCost(unsigned Opcode, Type *Src,
> >                            unsigned Alignment,
> >                            unsigned AddressSpace) const {
> >     if (Src->isVectorTy() && Alignment != 16)
> >       return 10000; // <== high number to try to avoid unaligned
> load/store.
> >     return TargetTransformInfo::getMemoryOpCost(Opcode, Src, Alignment,
> > AddressSpace);
> >   }
> >
> > Except that this doesn't work because Alignment will always be 4 even for
> > data like:
> >        int   data[16][16] __attribute__ ((aligned (16))),
> >
> > Because individual element are still 4-byte aligned.
> >
> > I am not sure what is the right way to do it?
> > Thanks.
> >
> >
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> >
>
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