Add gather_imm_use_stmts helper

The following adds a helper function to gather SSA use stmts without
duplicates.  It steals the only padding bit in gimple to be a
"infrastructure local flag" which should be used only temporarily
and kept cleared.  I did not add accessor functions for the flag
to not encourage (ab-)uses.

I have used an auto_vec<gimple *, 2> in the API to avoid heap
allocations for most cases (without doing statistics).  I have
verified GCC 7 performs NRV optimization on the copy but I'll
note while auto_vec<gimple *> has copy and assign deleted,
auto_vec<gimple *, N> does not.  Adding them breaks pair-fusion.cc
compile.  Without using 'auto' or range-for the API use is a bit
awkward as that exposes the number of auto-allocated elements.

The helper can be used in a range-for, see the followup for an
example.

	* gimple.h (gimple::pad): Rename to ...
	(gimple::ilf): ... this.
	* ssa-iterators.h (gather_imm_use_stmts): Declare.
	* tree-ssa-operands.cc (gather_imm_use_stmts): New function.

Author: rguenth

gcc/gimple.h

@@ -250,8 +250,8 @@ struct GTY((desc ("gimple_statement_structure (&%h)"), tag ("GSS_BASE"),
   /* Nonzero if this statement contains volatile operands.  */
   unsigned has_volatile_ops 	: 1;
 
-  /* Padding to get subcode to 16 bit alignment.  */
-  unsigned pad			: 1;
+  /* Infrastructure local flag.  Always clear.  */
+  unsigned ilf			: 1;
 
   /* The SUBCODE field can be used for tuple-specific flags for tuples
      that do not require subcodes.  Note that SUBCODE should be at

gcc/ssa-iterators.h

@@ -114,6 +114,11 @@ struct auto_end_imm_use_stmt_traverse
        (void) ((DEST) = next_imm_use_on_stmt (&(ITER))))
 
 
+/* Use this to get a vector of all gimple stmts using SSAVAR without
+   duplicates.  It's cheaper than FOR_EACH_IMM_USE_STMT and has no
+   constraints on what you are allowed to do inside an iteration
+   over the vector.  */
+extern auto_vec<gimple *, 2> gather_imm_use_stmts (tree ssavar);
 
 extern bool single_imm_use_1 (const ssa_use_operand_t *head,
 			      use_operand_p *use_p, gimple **stmt);

gcc/tree-ssa-operands.cc

@@ -1416,3 +1416,23 @@ single_imm_use_1 (const ssa_use_operand_t *head,
   return single_use;
 }
 
+/* Gather all stmts SSAVAR is used on, eliminating duplicates.  */
+
+auto_vec<gimple *, 2>
+gather_imm_use_stmts (tree ssavar)
+{
+  auto_vec<gimple *, 2> stmts;
+  imm_use_iterator iter;
+  use_operand_p use_p;
+  FOR_EACH_IMM_USE_FAST (use_p, iter, ssavar)
+    {
+      gimple *use_stmt = USE_STMT (use_p);
+      if (use_stmt->ilf)
+	continue;
+      use_stmt->ilf = 1;
+      stmts.safe_push (use_stmt);
+    }
+  for (gimple *use_stmt : stmts)
+    use_stmt->ilf = 0;
+  return stmts;
+}