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

[Richard Biener <richard.guenther@gmail.com>]

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.
>
>
>
> 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/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c
> new file mode 100644
> index 0000000..48dc92e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c
> @@ -0,0 +1,21 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-dse1-details -fno-tree-fre -fno-tree-sra"
> } */
> +
> +struct S { struct R { int x; int y; } r; int z; } s;
> +
> +extern void blah (struct S);
> +
> +void
> +foo ()
> +{
> +  struct S s = { {1, 2}, 3 };
> +  s.r.x = 1;
> +   s.r.y = 2;
> +   struct R r = s.r;
> +  s.z = 3;
> +  blah (s);
> +}
> +
> +
> +/* { dg-final { scan-tree-dump-times "Deleted dead store" 4 "dse1" } } */
> +
> 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)

> +    {
> +      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...

> +         return true;
> +      return false;
> +    }
> +  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.
> @@ -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))

otherwise the patch looks reasonable for GCC 8.

Richard.

> +
> +                             /* Otherwise walk through this store.  */
> +                             temp = use_stmt;
> +                           }
> +                         continue;
> +                       }
> +                   }
> +               }
> +
>               fail = true;
>               BREAK_FROM_IMM_USE_STMT (ui);
>             }
>