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

[Jeff Law <law at redhat dot com>]

Another old patch getting resurrected...


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);
 	    }