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

[Bruce Hoult via llvm-dev]

I'm afraid I'm not yet skilled with LLVL IR, but I think you'd probably
want to do something like this:

#include <stdint.h>

typedef uint32_t elt;
typedef elt v4 __attribute__ ((vector_size (16)));

void store(v4 *p, v4 v){
  uintptr_t align = uintptr_t(p) & (sizeof(elt)-1);
  if (align == 0){
    *p = v;
  } else {
    // assert sizeof(elt) == 4
    // assert align == 2
    elt *base = (elt*)(uintptr_t(p) - 2);
    elt lomask = (1<<16)-1;
    elt himask = ~lomask;
    base[0] = (base[0] & lomask) | v[0] << 16;
    base[1] = v[0] >> 16 | v[1] << 16;
    base[2] = v[1] >> 16 | v[2] << 16;
    base[3] = v[2] >> 16 | v[3] << 16;
    base[4] = v[3] >> 16 | (base[4] & himask);
  }
}

Of course you'll want to convert one LLVM IR to another, not actually
insert C code, but you can compile this code and see what IR is produced.

Interestingly, LLVM manages to optimize this to one elt load, one elt
store, and an aligned vector store. Plus some in-registers shuffling, of
course.


On Mon, Feb 1, 2016 at 5:49 PM, jingu kang <jaykang10 at gmail.com> wrote:

> Hi Bruce,
>
> Thanks for response.
>
> I also think it is not good way. Do you have the other ways to legalize it?
>
> Thanks,
> JinGu Kang
>
> 2016-02-01 13:11 GMT+00:00 Bruce Hoult <bruce at hoult.org>:
> > 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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> >> > llvm-dev at lists.llvm.org
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> >
> >
>
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