[llvm-dev] Question about store with unaligned memory address

[Bruce Hoult via llvm-dev]

In fact this is a pretty bad legalizing/lowering because you only need to
load and edit for the first and last values in the vector. The other words
are completely replaced and don't need to be loaded at all.

I think you need to legalize differently when it is not aligned.

On Fri, Jan 29, 2016 at 10:07 PM, jingu kang via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Krzysztof,
>
> Thanks for response.
>
> The method is working almost of test cases which use load and store
> instructions connected with chain. There is other situation. Let's
> look at a example as follows:
>
> typedef unsigned short int UV __attribute__((vector_size (8)));
>
> void test (UV *x, UV *y) {
>   *x = *y / ((UV) { 4, 4, 4, 4 });
>  }
>
> The target does not support vector type so CodeGen tries to split and
> scalarize vector to legalize type. While legalizing vector type, the
> stores of each vector elements nodes are generated from
> 'DAGTypeLegalizer::SplitVecOp_STORE'. But the stores are not connected
> with chain. I guess it assumes each vector element's address is
> different. The each store is lowered to load and store nodes with high
> and low address but they are not connected with the other store's one.
> It causes problem. I am not sure how to solve this situation
> correctly.
>
> Thanks,
> JinGu Kang
>
>
> 2016-01-29 18:11 GMT+00:00 Krzysztof Parzyszek via llvm-dev
> <llvm-dev at lists.llvm.org>:
> > On 1/29/2016 10:47 AM, JinGu Kang via llvm-dev wrote:
> >>
> >>
> >> I am doing it with lowering store as follow:
> >>
> >> 1. make low and high address with alignment.
> >> 2. load 2 words from low and high address.
> >> 3. manipulate them with values to store according to alignment.
> >> 4. store 2 words modified to low and high address
> >
> >
> > Sounds ok.
> >
> >
> >> In order to keep the order between loads and stores, I have used chain
> and
> >> glue on the DAG but some passes have mixed it in machine instruction
> >> level.
> >
> >
> > Glue isn't necessary, chains are sufficient.
> >
> > I'm not sure what pass reordered dependent loads and stores, but that
> sounds
> > bad.  What matters in cases like this are the MachineMemOperands.  If
> there
> > isn't any on a load/store instruction, it should be treated
> conservatively
> > (i.e. alias everything else), if there is one, it'd better be correct.
> > Wrong MMO could certainly lead to such behavior.
> >
> > -Krzysztof
> >
> >
> > --
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by
> > The Linux Foundation
> >
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