[PATCH v7 11/12] Replace the original CRC loops with a faster CRC calculation

After the loop exit an internal function call (CRC, CRC_REV) is added, and its
result is assigned to the output CRC variable (the variable where the
calculated CRC is stored after the loop execution).  The removal of the loop is
left to CFG cleanup and DCE.

gcc/

	* gimple-crc-optimization.cc (optimize_crc_loop): New function.
	(execute): Add optimize_crc_loop function call.

diff --git a/gcc/gimple-crc-optimization.cc b/gcc/gimple-crc-optimization.cc
index fced04fc398ad4ebd921b79bf8cb5f084dbaa7fa..59fe5f656149aeeb1e11537c0edb36babd6eaab7 100644 (file)
--- a/gcc/gimple-crc-optimization.cc
+++ b/gcc/gimple-crc-optimization.cc
@@ -225,6 +225,11 @@ class crc_optimization {
   /* Returns phi statement which may hold the calculated CRC.  */
   gphi *get_output_phi ();
 
+  /* Attempts to optimize a CRC calculation loop by replacing it with a call to
+     an internal function (IFN_CRC or IFN_CRC_REV).
+     Returns true if replacement is succeeded, otherwise false.  */
+  bool optimize_crc_loop (gphi *output_crc);
+
  public:
   crc_optimization () : m_visited_stmts (BITMAP_ALLOC (NULL)),
                        m_crc_loop (nullptr), m_polynomial (0)
@@ -1207,6 +1212,73 @@ crc_optimization::get_output_phi ()
   return nullptr;
 }
 
+/* Attempts to optimize a CRC calculation loop by replacing it with a call to
+   an internal function (IFN_CRC or IFN_CRC_REV).
+   Returns true if replacement is succeeded, otherwise false.  */
+
+bool
+crc_optimization::optimize_crc_loop (gphi *output_crc)
+{
+  if (!output_crc)
+    {
+      if (dump_file)
+       fprintf (dump_file, "Couldn't determine output CRC.\n");
+      return false;
+    }
+
+  if (!m_data_arg)
+    {
+      if (dump_file && (dump_flags & TDF_DETAILS))
+       fprintf (dump_file,
+                "Data and CRC are xor-ed before for loop.  Initializing data "
+                "with 0.\n");
+      /* Create a new variable for the data.
+       Determine the data's size with the loop iteration count.  */
+      unsigned HOST_WIDE_INT
+       data_size = tree_to_uhwi (m_crc_loop->nb_iterations) + 1;
+      tree type = build_nonstandard_integer_type (data_size, 1);
+     /* For the CRC calculation, it doesn't matter CRC is calculated for the
+       (CRC^data, 0) or (CRC, data).  */
+      m_data_arg = build_int_cstu (type, 0);
+    }
+
+  /* Build tree node for the polynomial from its constant value.  */
+  tree polynomial_arg = build_int_cstu (TREE_TYPE (m_crc_arg), m_polynomial);
+  gcc_assert (polynomial_arg);
+
+  internal_fn ifn;
+  if (m_is_bit_forward)
+    ifn = IFN_CRC;
+  else
+    ifn = IFN_CRC_REV;
+
+  tree phi_result = gimple_phi_result (output_crc);
+  location_t loc;
+  loc = EXPR_LOCATION (phi_result);
+
+  /* Add IFN call and write the return value in the phi_result.  */
+  gcall *call
+      = gimple_build_call_internal (ifn, 3,
+                                   m_crc_arg,
+                                   m_data_arg,
+                                   polynomial_arg);
+
+  gimple_call_set_lhs (call, phi_result);
+  gimple_set_location (call, loc);
+  gimple_stmt_iterator si = gsi_start_bb (output_crc->bb);
+  gsi_insert_before (&si, call, GSI_SAME_STMT);
+
+  /* Remove phi statement, which was holding CRC result.  */
+  gimple_stmt_iterator tmp_gsi = gsi_for_stmt (output_crc);
+  remove_phi_node (&tmp_gsi, false);
+
+  /* Alter the exit condition of the loop to always exit.  */
+  gcond* loop_exit_cond = get_loop_exit_condition (m_crc_loop);
+  gimple_cond_make_false (loop_exit_cond);
+  update_stmt (loop_exit_cond);
+  return true;
+}
+
 unsigned int
 crc_optimization::execute (function *fun)
 {
@@ -1263,6 +1335,12 @@ crc_optimization::execute (function *fun)
          if (dump_file)
            fprintf (dump_file, "The loop with %d header BB index "
                                "calculates CRC!\n", m_crc_loop->header->index);
+
+         if (!optimize_crc_loop (output_crc))
+           {
+             if (dump_file)
+               fprintf (dump_file, "Couldn't generate faster CRC code.\n");
+           }
        }
     }
   return 0;