Re: [RFA] [PATCH 4/4] Ignore reads of "dead" memory locations in DSE

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi Jeff,


On 7 September 2017 at 00:18, Jeff Law <law@redhat.com> wrote:
> Another old patch getting resurrected...
>
>

This patch (r253305) introduces a new FAIL on arm-none-eabi (as
opposed arm-linux-gnueabi*):
FAIL:     gcc.dg/tree-ssa/ssa-dse-26.c scan-tree-dump-times dse1
"Deleted dead store" 2

I'm not familiar with all this, but I quickly looked at the testcase,
and noticed it
uses enums.
One ABI difference between arm-non-eabi and arm-linux-gnueabi is that the
bare-metal target defaults to short-enums, while the linux one uses
no-short-enums.

Could that be the problem?

Thanks,

Christophe

> On 01/04/2017 06:50 AM, Richard Biener wrote:
>> On Thu, Dec 22, 2016 at 7:26 AM, Jeff Law <law@redhat.com> wrote:
>>> This is the final patch in the kit to improve our DSE implementation.
>>>
>>> It's based on a observation by Richi.  Namely that a read from bytes of
>>> memory that are dead can be ignored.  By ignoring such reads we can
>>> sometimes find additional stores that allow us to either eliminate or trim
>>> an earlier store more aggressively.
>>>
>>> This only hit (by hit I mean the ability to ignore resulted in finding a
>>> full or partially dead store that we didn't otherwise find) once during a
>>> bootstrap, but does hit often in the libstdc++ testsuite.  I've added a test
>>> derived from the conversation between myself and Richi last year.
>>>
>>> There's nothing in the BZ database on this issue and I can't reasonably call
>>> it a bugfix.  I wouldn't lose sleep if this deferred to gcc-8.
>>>
>>> Bootstrapped and regression tested on x86-64-linux-gnu.  OK for the trunk or
>>> defer to gcc-8?
>>>
>>>
>>>
>>>         * tree-ssa-dse.c (live_bytes_read): New function.
>>>         (dse_classify_store): Ignore reads of dead bytes.
>>>
>>>         * testsuite/gcc.dg/tree-ssa/ssa-dse-26.c: New test.
>>>         * testsuite/gcc.dg/tree-ssa/ssa-dse-26.c: Likewise.
>>>
>>>
>>>
> [ snip ]
>
>>> diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
>>> index a807d6d..f5b53fc 100644
>>> --- a/gcc/tree-ssa-dse.c
>>> +++ b/gcc/tree-ssa-dse.c
>>> @@ -475,6 +475,41 @@ maybe_trim_partially_dead_store (ao_ref *ref, sbitmap
>>> live, gimple *stmt)
>>>      }
>>>  }
>>>
>>> +/* Return TRUE if USE_REF reads bytes from LIVE where live is
>>> +   derived from REF, a write reference.
>>> +
>>> +   While this routine may modify USE_REF, it's passed by value, not
>>> +   location.  So callers do not see those modifications.  */
>>> +
>>> +static bool
>>> +live_bytes_read (ao_ref use_ref, ao_ref *ref, sbitmap live)
>>> +{
>>> +  /* We have already verified that USE_REF and REF hit the same object.
>>> +     Now verify that there's actually an overlap between USE_REF and REF.
>>> */
>>> +  if ((use_ref.offset < ref->offset
>>> +       && use_ref.offset + use_ref.size > ref->offset)
>>> +      || (use_ref.offset >= ref->offset
>>> +         && use_ref.offset < ref->offset + ref->size))
>>
>> can you use ranges_overlap_p? (tree-ssa-alias.h)
> Yes.  Didn't know about it.  Done.
>
>>
>>> +    {
>>> +      normalize_ref (&use_ref, ref);
>>> +
>>> +      /* If USE_REF covers all of REF, then it will hit one or more
>>> +        live bytes.   This avoids useless iteration over the bitmap
>>> +        below.  */
>>> +      if (use_ref.offset == ref->offset && use_ref.size == ref->size)
>>> +       return true;
>>> +
>>> +      /* Now iterate over what's left in USE_REF and see if any of
>>> +        those bits are i LIVE.  */
>>> +      for (int i = (use_ref.offset - ref->offset) / BITS_PER_UNIT;
>>> +          i < (use_ref.offset + use_ref.size) / BITS_PER_UNIT; i++)
>>> +       if (bitmap_bit_p (live, i))
>>
>> a bitmap_bit_in_range_p () would be nice to have.  And it can be more
>> efficient than this loop...
> Yea.  That likely would help here.  I'm testing with a
> bitmap_bit_in_range_p implementation (only for sbitmaps since that's
> what we're using here).
>
> That implementation does the reasonably efficient things and is modeled
> after the sbitmap implementation of bitmap_set_range.
>
>
>>> @@ -554,6 +589,41 @@ dse_classify_store (ao_ref *ref, gimple *stmt, gimple
>>> **use_stmt,
>>>           /* If the statement is a use the store is not dead.  */
>>>           else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
>>>             {
>>> +             /* Handle common cases where we can easily build a ao_ref
>>> +                structure for USE_STMT and in doing so we find that the
>>> +                references hit non-live bytes and thus can be ignored.  */
>>> +             if (live_bytes)
>>> +               {
>>> +                 if (is_gimple_assign (use_stmt))
>>> +                   {
>>> +                     /* Other cases were noted as non-aliasing by
>>> +                        the call to ref_maybe_used_by_stmt_p.  */
>>> +                     ao_ref use_ref;
>>> +                     ao_ref_init (&use_ref, gimple_assign_rhs1 (use_stmt));
>>> +                     if (valid_ao_ref_for_dse (&use_ref)
>>> +                         && use_ref.base == ref->base
>>> +                         && use_ref.size == use_ref.max_size
>>> +                         && !live_bytes_read (use_ref, ref, live_bytes))
>>> +                       {
>>> +                         if (gimple_vdef (use_stmt))
>>> +                           {
>>> +                             /* If we have already seen a store and
>>> +                                this is also a store, then we have to
>>> +                                fail.  */
>>> +                             if (temp)
>>> +                               {
>>> +                                 fail = true;
>>> +                                 BREAK_FROM_IMM_USE_STMT (ui);
>>> +                               }
>>
>> as this case is rather cheap to test please test it together with
>> live_bytes.  Like
>>
>>   if (live_bytes && (! gimple_vdef (use_stmt) || ! temp))
> Seems reasonable.
>
>
>>
>> otherwise the patch looks reasonable for GCC 8.
> Given the sbitmap.[ch] change, posting for re-approval
>
> Bootstrapped and regression tested on x86_64.  Earlier version without
> the bitmap_bit_in_range_p improvement was also bootstrapped and
> regression tested on aarch64.
>
> Jeff
>
>
>
>         * sbitmap.c (bitmap_bit_in_range_p): New function.
>         * sbitmap.h (bitmap_bit_in_range_p): Prototype.
>         * tree-ssa-dse.c (live_bytes_read): New function.
>         (dse_classify_store): Ignore reads of dead bytes.
>
>         * testsuite/gcc.dg/tree-ssa/ssa-dse-26.c: New test.
>
> diff --git a/gcc/sbitmap.c b/gcc/sbitmap.c
> index c065d13..4bf13a1 100644
> --- a/gcc/sbitmap.c
> +++ b/gcc/sbitmap.c
> @@ -316,6 +316,59 @@ bitmap_set_range (sbitmap bmap, unsigned int start, unsigned int count)
>    bmap->elms[start_word] |= mask;
>  }
>
> +/* Return TRUE if any bit between START and END inclusive is set within
> +   the simple bitmap BMAP.  Return FALSE otherwise.  */
> +
> +bool
> +bitmap_bit_in_range_p (const_sbitmap bmap, unsigned int start, unsigned int end)
> +{
> +  unsigned int start_word = start / SBITMAP_ELT_BITS;
> +  unsigned int start_bitno = start % SBITMAP_ELT_BITS;
> +
> +  /* Testing within a word, starting at the beginning of a word.  */
> +  if (start_bitno == 0 && (end - start) < SBITMAP_ELT_BITS)
> +    {
> +      SBITMAP_ELT_TYPE mask = ((SBITMAP_ELT_TYPE)1 << (end - start)) - 1;
> +      return (bmap->elms[start_word] & mask) != 0;
> +    }
> +
> +  unsigned int end_word = end / SBITMAP_ELT_BITS;
> +  unsigned int end_bitno = end % SBITMAP_ELT_BITS;
> +
> +  /* Testing starts somewhere in the middle of a word.  Test up to the
> +     end of the word or the end of the requested region, whichever comes
> +     first.  */
> +  if (start_bitno != 0)
> +    {
> +      unsigned int nbits = ((start_word == end_word)
> +                           ? end_bitno - start_bitno
> +                           : SBITMAP_ELT_BITS - start_bitno);
> +      SBITMAP_ELT_TYPE mask = ((SBITMAP_ELT_TYPE)1 << nbits) - 1;
> +      mask <<= start_bitno;
> +      if (bmap->elms[start_word] & mask)
> +       return true;
> +      start_word++;
> +    }
> +
> +  if (start_word > end_word)
> +    return false;
> +
> +  /* Now test words at a time until we hit a partial word.  */
> +  unsigned int nwords = (end_word - start_word);
> +  while (nwords)
> +    {
> +      if (bmap->elms[start_word])
> +       return true;
> +      start_word++;
> +      nwords--;
> +    }
> +
> +  /* Now handle residuals in the last word.  */
> +  SBITMAP_ELT_TYPE mask
> +    = ((SBITMAP_ELT_TYPE)1 << (SBITMAP_ELT_BITS - end_bitno)) - 1;
> +  return (bmap->elms[start_word] & mask) != 0;
> +}
> +
>  #if GCC_VERSION < 3400
>  /* Table of number of set bits in a character, indexed by value of char.  */
>  static const unsigned char popcount_table[] =
> diff --git a/gcc/sbitmap.h b/gcc/sbitmap.h
> index ce4d27d..ff52e93 100644
> --- a/gcc/sbitmap.h
> +++ b/gcc/sbitmap.h
> @@ -51,6 +51,7 @@ along with GCC; see the file COPYING3.  If not see
>       * set_difference          : bitmap_and_compl
>       * set_disjuction          : (not implemented)
>       * set_compare             : bitmap_equal_p
> +     * bit_in_range_p          : bitmap_bit_in_range_p
>
>     Some operations on 3 sets that occur frequently in data flow problems
>     are also implemented:
> @@ -253,6 +254,7 @@ extern bool bitmap_and (sbitmap, const_sbitmap, const_sbitmap);
>  extern bool bitmap_ior (sbitmap, const_sbitmap, const_sbitmap);
>  extern bool bitmap_xor (sbitmap, const_sbitmap, const_sbitmap);
>  extern bool bitmap_subset_p (const_sbitmap, const_sbitmap);
> +extern bool bitmap_bit_in_range_p (const_sbitmap, unsigned int, unsigned int);
>
>  extern int bitmap_first_set_bit (const_sbitmap);
>  extern int bitmap_last_set_bit (const_sbitmap);
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
> new file mode 100644
> index 0000000..6605dfe
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
> @@ -0,0 +1,33 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-dse1-details" } */
> +
> +enum constraint_expr_type
> +{
> +  SCALAR, DEREF, ADDRESSOF
> +};
> +typedef struct constraint_expr
> +{
> +  enum constraint_expr_type type;
> +  unsigned int var;
> +  long offset;
> +} constraint_expr ;
> +typedef struct constraint
> +{
> +  struct constraint_expr lhs;
> +  struct constraint_expr rhs;
> +} constraint;
> +static _Bool
> +constraint_expr_equal (struct constraint_expr x, struct constraint_expr y)
> +{
> +  return x.type == y.type && x.var == y.var && x.offset == y.offset;
> +}
> +
> +_Bool
> +constraint_equal (struct constraint a, struct constraint b)
> +{
> +  return constraint_expr_equal (a.lhs, b.lhs)
> +    && constraint_expr_equal (a.rhs, b.rhs);
> +}
> +
> +/* { dg-final { scan-tree-dump-times "Deleted dead store" 2 "dse1" } } */
> +
> diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
> index 70c8b07..1eca740 100644
> --- a/gcc/tree-ssa-dse.c
> +++ b/gcc/tree-ssa-dse.c
> @@ -468,6 +468,36 @@ maybe_trim_partially_dead_store (ao_ref *ref, sbitmap live, gimple *stmt)
>      }
>  }
>
> +/* Return TRUE if USE_REF reads bytes from LIVE where live is
> +   derived from REF, a write reference.
> +
> +   While this routine may modify USE_REF, it's passed by value, not
> +   location.  So callers do not see those modifications.  */
> +
> +static bool
> +live_bytes_read (ao_ref use_ref, ao_ref *ref, sbitmap live)
> +{
> +  /* We have already verified that USE_REF and REF hit the same object.
> +     Now verify that there's actually an overlap between USE_REF and REF.  */
> +  if (ranges_overlap_p (use_ref.offset, use_ref.size, ref->offset, ref->size))
> +    {
> +      normalize_ref (&use_ref, ref);
> +
> +      /* If USE_REF covers all of REF, then it will hit one or more
> +        live bytes.   This avoids useless iteration over the bitmap
> +        below.  */
> +      if (use_ref.offset <= ref->offset
> +         && use_ref.offset + use_ref.size >= ref->offset + ref->size)
> +       return true;
> +
> +      /* Now check if any of the remaining bits in use_ref are set in LIVE.  */
> +      unsigned int start = (use_ref.offset - ref->offset) / BITS_PER_UNIT;
> +      unsigned int end  = (use_ref.offset + use_ref.size) / BITS_PER_UNIT;
> +      return bitmap_bit_in_range_p (live, start, end);
> +    }
> +  return true;
> +}
> +
>  /* A helper of dse_optimize_stmt.
>     Given a GIMPLE_ASSIGN in STMT that writes to REF, find a candidate
>     statement *USE_STMT that may prove STMT to be dead.
> @@ -547,6 +577,31 @@ dse_classify_store (ao_ref *ref, gimple *stmt, gimple **use_stmt,
>           /* If the statement is a use the store is not dead.  */
>           else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
>             {
> +             /* Handle common cases where we can easily build a ao_ref
> +                structure for USE_STMT and in doing so we find that the
> +                references hit non-live bytes and thus can be ignored.  */
> +             if (live_bytes && (!gimple_vdef (use_stmt) || !temp))
> +               {
> +                 if (is_gimple_assign (use_stmt))
> +                   {
> +                     /* Other cases were noted as non-aliasing by
> +                        the call to ref_maybe_used_by_stmt_p.  */
> +                     ao_ref use_ref;
> +                     ao_ref_init (&use_ref, gimple_assign_rhs1 (use_stmt));
> +                     if (valid_ao_ref_for_dse (&use_ref)
> +                         && use_ref.base == ref->base
> +                         && use_ref.size == use_ref.max_size
> +                         && !live_bytes_read (use_ref, ref, live_bytes))
> +                       {
> +                         /* If this statement has a VDEF, then it is the
> +                            first store we have seen, so walk through it.  */
> +                         if (gimple_vdef (use_stmt))
> +                           temp = use_stmt;
> +                         continue;
> +                       }
> +                   }
> +               }
> +
>               fail = true;
>               BREAK_FROM_IMM_USE_STMT (ui);
>             }
>