AW: [PATCH] Fix unaligned access when predictive commoning (PR 71083)

Bernd Edlinger bernd.edlinger@hotmail.de
Wed Aug 10 08:47:00 GMT 2016


On 08/10/16, Bernd Edlinger wrote:
>On 08/09/16 22:48, Eric Botcazou wrote:
>>> I think from Eric's comment in get_inner_ref it can be possible
>>> in Ada that the outer object is accidentally byte-aligned
>>> but the inner reference is not.  In that case I think it is
>>> better to generate a BIT_FIELD_REF instead of a COMPONENT_REF.
>>

I actually meant to say get_bit_range.

I tried to translate the c-test case to an equivalent ada test case with
my not-so-fluent Ada speak...

So here is a test case that *actually* hits the if ((boff % BITS_PER_UNIT) != 0)
code path.

Before my patch there was an unaligned SImode access, and with
the patch we have 3 QImode accesses here.

cat pr71083_pkg.ads 
package Pr71083_pkg is
  type Nibble is mod 2**4;
  type Int24  is mod 2**24;
  type StructA is record
    a : Nibble;
    b : Int24;
  end record;
  pragma Pack(StructA);
  type StructB is record
    a : Nibble;
    b : StructA;
  end record;
  pragma Pack(StructB);
  type ArrayOfStructB is array(0..100) of StructB;
  procedure Foo (X : in out ArrayOfStructB);
end Pr71083_pkg;

cat pr71083_pkg.adb
-- { dg-do compile }
-- { dg-options "-O3" }
package body Pr71083_pkg is
  procedure Foo (X : in out ArrayOfStructB) is
  begin
    for K in 0..99 loop
      X (K+1).b.b := X (K).b.b;
    end loop;
  end Foo;
end Pr71083_pkg;

cat pr71083.adb 
-- { dg-do run }
-- { dg-options "-O3" }
with Pr71083_pkg;
use Pr71083_pkg;
procedure Pr71083 is
  Test : ArrayOfStructB;
begin
  Test (0).b.b := 9999;
  Foo (Test);
  if Test (100).b.b /= 9999 then
    raise Program_Error;
  end if;
end;


I was not sure which name to choose,
so I used the same convention as in the c-torture.
How do you like that?

I would like to add that to the gnat.dg because
it seems that there is zero testing for the predictive
commoning in the gnat.dg test suite.


Bernd.

>>> The patch says get_bit_range instead...  The comment therein means that
>>> bitfields can be nested in Ada: you can have a bitfield in a record which is
>>> itself a bitfield in another record.
>>>
>>>> Eric do you agree?  Are there any Ada test cases where the
>>>> pcom optimization jumps in?
>>>
>>> According to the comment, data-ref analysis punts on bit offsets so I'm not
>>> sure how boff can be not byte-aligned.
>>>
>
> I mean in dr_analyze_innermost, we have:
>
>   base = get_inner_reference (ref, &pbitsize, &pbitpos, &poffset, &pmode,
>                               &punsignedp, &preversep, &pvolatilep);
>   gcc_assert (base != NULL_TREE);
>
>   if (pbitpos % BITS_PER_UNIT != 0)
>     {
>       if (dump_file && (dump_flags & TDF_DETAILS))
>         fprintf (dump_file, "failed: bit offset alignment.\n");
>       return false;
>     }
>
>   if (preversep)
>     {
>       if (dump_file && (dump_flags & TDF_DETAILS))
>         fprintf (dump_file, "failed: reverse storage order.\n");
>       return false;
>     }
>
>
> and that means that get_inner_reference on the outermost ref
> returns a byte-aligned bit-offset, and from there I would
> think it has the knowledge, when to punt on reversep and
> when the offset is not byte-aligned.
>
> But in get_bit_range we have a bit-field with arbitrary
> bit-offset, but surprisingly the
> get_inner_reference (TREE_OPERAND (exp, 0)) did not return
> byte-aligned bit-offset.  I doubt that data-ref ananlysis
> ever cares about the byte-alignment of intermediate
> refs.
>
> We know that the difference between the innermost ref
> and the outer ref is byte-aligned, but we do not know
> that the same is true for offset between the COMPONENT_REF
> and the outer ref.
>
> I mean boff is essentially the difference between
> bitpos of get_inner_reference(exp) and
> bitpos of get_inner_reference(TREE_OPERAND (exp, 0))
>
> This would be exposed by my patch, because previously
> we always generated BIT_FIELD_REFS, with bit-offset 0,
> but the MEM_REF at the byte-offset there is in all likelihood
> pretty much unaligned, the MEM_REF at the COMPONENT_REF
> is likely more aligned and allows better code for ARM processors,
> but only if the MEM_REF is at a byte-aligned offset at all,
> otherwise the whole transformation would be wrong.
>
>
>
> Thanks
> Bernd.



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