From 06170e1d314a2f2b3bc179943e45d39e50a58929 Mon Sep 17 00:00:00 2001
From: Jeff Law <law@redhat.com>
Date: Wed, 29 Dec 2004 12:21:07 -0700
Subject: [PATCH] tree-outof-ssa.c (insert_backedge_copies): New function.
* tree-outof-ssa.c (insert_backedge_copies): New function.
(rewrite_out_of_ssa): Use it.
From-SVN: r92711
---
gcc/ChangeLog | 5 +++
gcc/tree-outof-ssa.c | 97 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 102 insertions(+)
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index b01838679884..6e071508532d 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2004-12-29 Jeff Law <law@redhat.com>
+
+ * tree-outof-ssa.c (insert_backedge_copies): New function.
+ (rewrite_out_of_ssa): Use it.
+
2004-12-29 Ira Rosen <irar@il.ibm.com>
PR tree-optimization/18179
diff --git a/gcc/tree-outof-ssa.c b/gcc/tree-outof-ssa.c
index a5fc99309a5a..ecd23ede219c 100644
--- a/gcc/tree-outof-ssa.c
+++ b/gcc/tree-outof-ssa.c
@@ -2362,6 +2362,95 @@ remove_ssa_form (FILE *dump, var_map map, int flags)
dump_file = save;
}
+/* Search every PHI node for arguments associated with backedges which
+ we can trivially determine will need a copy (the argument is either
+ not an SSA_NAME or the argument has a different underlying variable
+ than the PHI result).
+
+ Insert a copy from the PHI argument to a new destination at the
+ end of the block with the backedge to the top of the loop. Update
+ the PHI argument to reference this new destination. */
+
+static void
+insert_backedge_copies (void)
+{
+ basic_block bb;
+
+ FOR_EACH_BB (bb)
+ {
+ tree phi;
+
+ for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
+ {
+ tree result = PHI_RESULT (phi);
+ tree result_var;
+ int i;
+
+ if (!is_gimple_reg (result))
+ continue;
+
+ result_var = SSA_NAME_VAR (result);
+ for (i = 0; i < PHI_NUM_ARGS (phi); i++)
+ {
+ tree arg = PHI_ARG_DEF (phi, i);
+ edge e = PHI_ARG_EDGE (phi, i);
+
+ /* If the argument is not an SSA_NAME, then we will
+ need a constant initialization. If the argument is
+ an SSA_NAME with a different underlying variable and
+ we are not combining temporaries, then we will
+ need a copy statement. */
+ if ((e->flags & EDGE_DFS_BACK)
+ && (TREE_CODE (arg) != SSA_NAME
+ || (!flag_tree_combine_temps
+ && SSA_NAME_VAR (arg) != result_var)))
+ {
+ tree stmt, name, last = NULL;
+ block_stmt_iterator bsi;
+
+ bsi = bsi_last (PHI_ARG_EDGE (phi, i)->src);
+ if (!bsi_end_p (bsi))
+ last = bsi_stmt (bsi);
+
+ /* In theory the only way we ought to get back to the
+ start of a loop should be with a COND_EXPR or GOTO_EXPR.
+ However, better safe than sorry.
+
+ If the block ends with a control statment or
+ something that might throw, then we have to
+ insert this assignment before the last
+ statement. Else insert it after the last statement. */
+ if (last && stmt_ends_bb_p (last))
+ {
+ /* If the last statement in the block is the definition
+ site of the PHI argument, then we can't insert
+ anything after it. */
+ if (TREE_CODE (arg) == SSA_NAME
+ && SSA_NAME_DEF_STMT (arg) == last)
+ continue;
+ }
+
+ /* Create a new instance of the underlying
+ variable of the PHI result. */
+ stmt = build (MODIFY_EXPR, TREE_TYPE (result_var),
+ NULL, PHI_ARG_DEF (phi, i));
+ name = make_ssa_name (result_var, stmt);
+ TREE_OPERAND (stmt, 0) = name;
+
+ /* Insert the new statement into the block and update
+ the PHI node. */
+ if (last && stmt_ends_bb_p (last))
+ bsi_insert_before (&bsi, stmt, BSI_NEW_STMT);
+ else
+ bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);
+ modify_stmt (stmt);
+ SET_PHI_ARG_DEF (phi, i, name);
+ }
+ }
+ }
+ }
+}
+
/* Take the current function out of SSA form, as described in
R. Morgan, ``Building an Optimizing Compiler'',
Butterworth-Heinemann, Boston, MA, 1998. pp 176-186. */
@@ -2373,6 +2462,14 @@ rewrite_out_of_ssa (void)
int var_flags = 0;
int ssa_flags = (SSANORM_REMOVE_ALL_PHIS | SSANORM_USE_COALESCE_LIST);
+ /* If elimination of a PHI requires inserting a copy on a backedge,
+ then we will have to split the backedge which has numerous
+ undesirable performance effects.
+
+ A significant number of such cases can be handled here by inserting
+ copies into the loop itself. */
+ insert_backedge_copies ();
+
if (!flag_tree_live_range_split)
ssa_flags |= SSANORM_COALESCE_PARTITIONS;
--
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