re PR tree-optimization/85757 (tree optimizers fail to fully clean up fixed-size memcpy)

2018-05-17  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/85757
	* tree-ssa-dse.c (dse_classify_store): Record a PHI def and
	remove defs that only feed that PHI from further processing.

	* gcc.dg/tree-ssa/ssa-dse-34.c: New testcase.

From-SVN: r260306

gcc/ChangeLog

+2018-05-17  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/85757
+	* tree-ssa-dse.c (dse_classify_store): Record a PHI def and
+	remove defs that only feed that PHI from further processing.
+
 2018-05-16  Jim Wilson  <jimw@sifive.com>
 
 	* config/riscv/riscv.md (<optab>si3_mask, <optab>si3_mask_1): Prepend

gcc/testsuite/ChangeLog

+2018-05-17  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/85757
+	* gcc.dg/tree-ssa/ssa-dse-34.c: New testcase.
+
 2018-05-16  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/85363

gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-34.c

+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1-details" } */
+
+void f(int n, char *p0, char *p1, char *p2, char *o)
+{
+  int t0, t1;
+  __builtin_memcpy(&t0, p0, 1);
+  __builtin_memcpy(&t1, p1, 1);
+  if (n==3)
+    __builtin_memcpy(o+2, p2, 1);
+  __builtin_memcpy(o+0, &t0, 1);
+  __builtin_memcpy(o+1, &t1, 1);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 2 "dse1" } } */

gcc/tree-ssa-dse.c

@@ -577,6 +577,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
       else
 	defvar = gimple_vdef (temp);
       auto_vec<gimple *, 10> defs;
+      gimple *phi_def = NULL;
       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 	{
 	  /* Limit stmt walking.  */
@@ -600,7 +601,10 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 		 processing.  */
 	      if (!bitmap_bit_p (visited,
 				 SSA_NAME_VERSION (PHI_RESULT (use_stmt))))
-		defs.safe_push (use_stmt);
+		{
+		  defs.safe_push (use_stmt);
+		  phi_def = use_stmt;
+		}
 	    }
 	  /* If the statement is a use the store is not dead.  */
 	  else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
@@ -657,15 +661,31 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 	  return DSE_STORE_DEAD;
 	}
 
-      /* Process defs and remove paths starting with a kill from further
-         processing.  */
+      /* Process defs and remove those we need not process further.  */
       for (unsigned i = 0; i < defs.length (); ++i)
-	if (stmt_kills_ref_p (defs[i], ref))
-	  {
-	    if (by_clobber_p && !gimple_clobber_p (defs[i]))
-	      *by_clobber_p = false;
+	{
+	  gimple *def = defs[i];
+	  gimple *use_stmt;
+	  use_operand_p use_p;
+	  /* If the path to check starts with a kill we do not need to
+	     process it further.
+	     ???  With byte tracking we need only kill the bytes currently
+	     live.  */
+	  if (stmt_kills_ref_p (def, ref))
+	    {
+	      if (by_clobber_p && !gimple_clobber_p (def))
+		*by_clobber_p = false;
+	      defs.unordered_remove (i);
+	    }
+	  /* In addition to kills we can remove defs whose only use
+	     is another def in defs.  That can only ever be PHIs of which
+	     we track a single for simplicity reasons (we fail for multiple
+	     PHIs anyways).  */
+	  else if (gimple_code (def) != GIMPLE_PHI
+		   && single_imm_use (gimple_vdef (def), &use_p, &use_stmt)
+		   && use_stmt == phi_def)
 	    defs.unordered_remove (i);
-	  }
+	}
 
       /* If all defs kill the ref we are done.  */
       if (defs.is_empty ())