/* Operand->rank hashtable. */
static struct pointer_map_t *operand_rank;
+/* Forward decls. */
+static long get_rank (tree);
+
+
+/* Bias amount for loop-carried phis. We want this to be larger than
+ the depth of any reassociation tree we can see, but not larger than
+ the rank difference between two blocks. */
+#define PHI_LOOP_BIAS (1 << 15)
+
+/* Rank assigned to a phi statement. If STMT is a loop-carried phi of
+ an innermost loop, and the phi has only a single use which is inside
+ the loop, then the rank is the block rank of the loop latch plus an
+ extra bias for the loop-carried dependence. This causes expressions
+ calculated into an accumulator variable to be independent for each
+ iteration of the loop. If STMT is some other phi, the rank is the
+ block rank of its containing block. */
+static long
+phi_rank (gimple stmt)
+{
+ basic_block bb = gimple_bb (stmt);
+ struct loop *father = bb->loop_father;
+ tree res;
+ unsigned i;
+ use_operand_p use;
+ gimple use_stmt;
+
+ /* We only care about real loops (those with a latch). */
+ if (!father->latch)
+ return bb_rank[bb->index];
+
+ /* Interesting phis must be in headers of innermost loops. */
+ if (bb != father->header
+ || father->inner)
+ return bb_rank[bb->index];
+
+ /* Ignore virtual SSA_NAMEs. */
+ res = gimple_phi_result (stmt);
+ if (!is_gimple_reg (SSA_NAME_VAR (res)))
+ return bb_rank[bb->index];
+
+ /* The phi definition must have a single use, and that use must be
+ within the loop. Otherwise this isn't an accumulator pattern. */
+ if (!single_imm_use (res, &use, &use_stmt)
+ || gimple_bb (use_stmt)->loop_father != father)
+ return bb_rank[bb->index];
+
+ /* Look for phi arguments from within the loop. If found, bias this phi. */
+ for (i = 0; i < gimple_phi_num_args (stmt); i++)
+ {
+ tree arg = gimple_phi_arg_def (stmt, i);
+ if (TREE_CODE (arg) == SSA_NAME
+ && !SSA_NAME_IS_DEFAULT_DEF (arg))
+ {
+ gimple def_stmt = SSA_NAME_DEF_STMT (arg);
+ if (gimple_bb (def_stmt)->loop_father == father)
+ return bb_rank[father->latch->index] + PHI_LOOP_BIAS;
+ }
+ }
+
+ /* Must be an uninteresting phi. */
+ return bb_rank[bb->index];
+}
+
+/* If EXP is an SSA_NAME defined by a PHI statement that represents a
+ loop-carried dependence of an innermost loop, return TRUE; else
+ return FALSE. */
+static bool
+loop_carried_phi (tree exp)
+{
+ gimple phi_stmt;
+ long block_rank;
+
+ if (TREE_CODE (exp) != SSA_NAME
+ || SSA_NAME_IS_DEFAULT_DEF (exp))
+ return false;
+
+ phi_stmt = SSA_NAME_DEF_STMT (exp);
+
+ if (gimple_code (SSA_NAME_DEF_STMT (exp)) != GIMPLE_PHI)
+ return false;
+
+ /* Non-loop-carried phis have block rank. Loop-carried phis have
+ an additional bias added in. If this phi doesn't have block rank,
+ it's biased and should not be propagated. */
+ block_rank = bb_rank[gimple_bb (phi_stmt)->index];
+
+ if (phi_rank (phi_stmt) != block_rank)
+ return true;
+
+ return false;
+}
+
+/* Return the maximum of RANK and the rank that should be propagated
+ from expression OP. For most operands, this is just the rank of OP.
+ For loop-carried phis, the value is zero to avoid undoing the bias
+ in favor of the phi. */
+static long
+propagate_rank (long rank, tree op)
+{
+ long op_rank;
+
+ if (loop_carried_phi (op))
+ return rank;
+
+ op_rank = get_rank (op);
+
+ return MAX (rank, op_rank);
+}
/* Look up the operand rank structure for expression E. */
I make no claims that this is optimal, however, it gives good
results. */
+ /* We make an exception to the normal ranking system to break
+ dependences of accumulator variables in loops. Suppose we
+ have a simple one-block loop containing:
+
+ x_1 = phi(x_0, x_2)
+ b = a + x_1
+ c = b + d
+ x_2 = c + e
+
+ As shown, each iteration of the calculation into x is fully
+ dependent upon the iteration before it. We would prefer to
+ see this in the form:
+
+ x_1 = phi(x_0, x_2)
+ b = a + d
+ c = b + e
+ x_2 = c + x_1
+
+ If the loop is unrolled, the calculations of b and c from
+ different iterations can be interleaved.
+
+ To obtain this result during reassociation, we bias the rank
+ of the phi definition x_1 upward, when it is recognized as an
+ accumulator pattern. The artificial rank causes it to be
+ added last, providing the desired independence. */
+
if (TREE_CODE (e) == SSA_NAME)
{
gimple stmt;
long rank;
int i, n;
+ tree op;
if (TREE_CODE (SSA_NAME_VAR (e)) == PARM_DECL
&& SSA_NAME_IS_DEFAULT_DEF (e))
if (gimple_bb (stmt) == NULL)
return 0;
+ if (gimple_code (stmt) == GIMPLE_PHI)
+ return phi_rank (stmt);
+
if (!is_gimple_assign (stmt)
|| gimple_vdef (stmt))
return bb_rank[gimple_bb (stmt)->index];
if (rank != -1)
return rank;
- /* Otherwise, find the maximum rank for the operands, or the bb
- rank, whichever is less. */
+ /* Otherwise, find the maximum rank for the operands. As an
+ exception, remove the bias from loop-carried phis when propagating
+ the rank so that dependent operations are not also biased. */
rank = 0;
if (gimple_assign_single_p (stmt))
{
tree rhs = gimple_assign_rhs1 (stmt);
n = TREE_OPERAND_LENGTH (rhs);
if (n == 0)
- rank = MAX (rank, get_rank (rhs));
+ rank = propagate_rank (rank, rhs);
else
{
for (i = 0; i < n; i++)
- if (TREE_OPERAND (rhs, i))
- rank = MAX(rank, get_rank (TREE_OPERAND (rhs, i)));
+ {
+ op = TREE_OPERAND (rhs, i);
+
+ if (op != NULL_TREE)
+ rank = propagate_rank (rank, op);
+ }
}
}
else
n = gimple_num_ops (stmt);
for (i = 1; i < n; i++)
{
- gcc_assert (gimple_op (stmt, i));
- rank = MAX(rank, get_rank (gimple_op (stmt, i)));
+ op = gimple_op (stmt, i);
+ gcc_assert (op);
+ rank = propagate_rank (rank, op);
}
}
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