[llvm-dev] [SDAG] Recovering pointer types

[Nemanja Ivanovic via llvm-dev]

David, thank you for the prompt response.
I originally implemented this with a check of getAlignment(). However, both
getAlignment() and getOriginalAlignment() on the <4 x i32> load return the
stronger alignment (16) rather than the alignment that the i32* parameter
would have (4).

So I am a bit stuck in terms of how to verify the original alignment.

On Mon, Dec 26, 2016 at 3:26 PM, David Chisnall <David.Chisnall at cl.cam.ac.uk
> wrote:

> On 26 Dec 2016, at 14:58, Nemanja Ivanovic via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > I am wondering if there is a good/easy way to recover the original type
> of a pointer parameter in the SDAG. Here's the problem that I am dealing
> with:
> >
> > define <4 x i32> @test(i32* nocapture readonly %a) local_unnamed_addr #0
> {
> > entry:
> >   %0 = bitcast i32* %a to <4 x i32>*
> >   %1 = load <4 x i32>, <4 x i32>* %0, align 16, !tbaa !2
> >   ret <4 x i32> %1
> > }
> >
> > The problem is that the alignment requirements on my target for a load
> of an i32* are different from those on a <4 x i32>*. I don't see a way to
> specify that with the DataLayout and when the SelectionDAG is built, the
> bitcast goes away because both the source and destination types are the
> same (i64 according to the DataLayout).
> >
> > So I end up with this as the initial SDAG:
> > Initial selection DAG: BB#0 'test:entry'
> > SelectionDAG has 9 nodes:
> >   t0: ch = EntryToken
> >   t3: i64 = Constant<0>
> >       t2: i64,ch = CopyFromReg t0, Register:i64 %vreg0
> >     t5: v4i32,ch = load<LD16[%0](tbaa=<0x10038f18a98>)> t0, t2,
> undef:i64
> >   t7: ch,glue = CopyToReg t0, Register:v4i32 %V2, t5
> >   t8: ch = PPCISD::RET_FLAG t7, Register:v4i32 %V2, t7:1
> >
> > What I would like to do is emit efficient code for cases where the
> parameter pointer has the same alignment requirements as the load and emit
> the conservative but less efficient code in other cases.
>
> Do you actually need to know the original type for this?  Isn’t it enough
> to know the alignment?  The getAlignment() / getOriginalAlignment() methods
> on the LoadSDNode should give you this.
>
> David
>
>
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