a new value every time we see a statement with a vuse.
5. Strength reduction can be performed by anticipating expressions
we can repair later on.
+ 6. Our canonicalization of expressions during lookups don't take
+ constants into account very well. In particular, we don't fold
+ anywhere, so we can get situations where we stupidly think
+ something is a new value (a + 1 + 1 vs a + 2). This is somewhat
+ expensive to fix, but it does expose a lot more eliminations.
+ It may or not be worth it, depending on how critical you
+ consider PRE vs just plain GRE.
*/
/* For ease of terminology, "expression node" in the below refers to
/* Basic algorithm
- First we walk the statements to generate the AVAIL sets, the EXP_GEN
- sets, and the tmp_gen sets. AVAIL is a forward dataflow
- problem. EXP_GEN sets represent the generation of
- values/expressions by a given block. We use them when computing
- the ANTIC sets. The AVAIL sets consist of SSA_NAME's that
- represent values, so we know what values are available in what
- blocks. In SSA, values are never killed, so we don't need a kill
- set, or a fixpoint iteration, in order to calculate the AVAIL sets.
- In traditional parlance, AVAIL sets tell us the downsafety of the
+ First we walk the statements to generate the AVAIL sets, the
+ EXP_GEN sets, and the tmp_gen sets. EXP_GEN sets represent the
+ generation of values/expressions by a given block. We use them
+ when computing the ANTIC sets. The AVAIL sets consist of
+ SSA_NAME's that represent values, so we know what values are
+ available in what blocks. AVAIL is a forward dataflow problem. In
+ SSA, values are never killed, so we don't need a kill set, or a
+ fixpoint iteration, in order to calculate the AVAIL sets. In
+ traditional parlance, AVAIL sets tell us the downsafety of the
expressions/values.
- Next, we generate the ANTIC sets. ANTIC is a backwards dataflow
- problem. These sets represent the anticipatable expressions. An
- expression is anticipatable in a given block if it could be
- generated in that block. This means that if we had to perform an
- insertion in that block, of the value of that expression, we could.
- Calculating the ANTIC sets requires phi translation of expressions,
- because the flow goes backwards through phis. We must iterate to a
- fixpoint of the ANTIC sets, because we have a kill set.
- Even in SSA form, values are not live over the entire function,
- only from their definition point onwards. So we have to remove
- values from the ANTIC set once we go past the definition point of
- the leaders that make them up. compute_antic/compute_antic_aux
- performs this computation.
+ Next, we generate the ANTIC sets. These sets represent the
+ anticipatable expressions. ANTIC is a backwards dataflow
+ problem.An expression is anticipatable in a given block if it could
+ be generated in that block. This means that if we had to perform
+ an insertion in that block, of the value of that expression, we
+ could. Calculating the ANTIC sets requires phi translation of
+ expressions, because the flow goes backwards through phis. We must
+ iterate to a fixpoint of the ANTIC sets, because we have a kill
+ set. Even in SSA form, values are not live over the entire
+ function, only from their definition point onwards. So we have to
+ remove values from the ANTIC set once we go past the definition
+ point of the leaders that make them up.
+ compute_antic/compute_antic_aux performs this computation.
Third, we perform insertions to make partially redundant
expressions fully redundant.
Value numbers are represented using the "value handle" approach.
This means that each SSA_NAME (and for other reasons to be
- disclosed in a moment, expression nodes and constant nodes) has a
- value handle that can be retrieved through get_value_handle. This
- value handle, *is* the value number of the SSA_NAME. You can
- pointer compare the value handles for equivalence purposes.
+ disclosed in a moment, expression nodes) has a value handle that
+ can be retrieved through get_value_handle. This value handle, *is*
+ the value number of the SSA_NAME. You can pointer compare the
+ value handles for equivalence purposes.
For debugging reasons, the value handle is internally more than
just a number, it is a VAR_DECL named "value.x", where x is a
Expression nodes have value handles associated with them as a
cache. Otherwise, we'd have to look them up again in the hash
table This makes significant difference (factor of two or more) on
- some test cases. They can be thrown away after the Constants have
- value handles associated with them so that they aren't special
- cased everywhere, and for consistency sake. This may be changed
- depending on memory usage vs code maintenance tradeoff. */
+ some test cases. They can be thrown away after the pass is
+ finished. */
/* Representation of expressions on value numbers:
Sets are represented as doubly linked lists kept in topological
order, with an optional supporting bitmap of values present in the
- set. The sets represent values, and the elements can be constants,
- values, or expressions. The elements can appear in different sets,
- but each element can only appear once in each set.
+ set. The sets represent values, and the elements can be values or
+ expressions. The elements can appear in different sets, but each
+ element can only appear once in each set.
Since each node in the set represents a value, we also want to be
able to map expression, set pairs to something that tells us
/* A value set element. Basically a single linked list of
- expressions/constants/values. */
+ expressions/values. */
typedef struct value_set_node
{
/* An expression. */
static value_set_t set_new (bool);
static bool is_undefined_value (tree);
static bool expressions_equal_p (tree, tree);
+static tree create_expression_by_pieces (basic_block, tree, tree);
/* We can add and remove elements and entries to and from sets
and hash tables, so we use alloc pools for them. */
{
return SSA_NAME_VALUE (expr);
}
- else if (TREE_CODE_CLASS (TREE_CODE (expr)) == 'c'
- || EXPR_P (expr))
+ else if (TREE_CODE_CLASS (TREE_CODE (expr)) == 'c')
+ return expr;
+ else if (EXPR_P (expr))
{
tree_ann_t ann = tree_ann (expr);
if (ann)
abort ();
else if (TREE_CODE (e) == SSA_NAME)
SSA_NAME_VALUE (e) = v;
- else if (TREE_CODE_CLASS (TREE_CODE (e)) == 'c'
- || EXPR_P (e))
+ else if (EXPR_P (e))
get_tree_ann (e)->common.value_handle = v;
}
{
void **slot;
struct val_expr_pair_d vep = {NULL, NULL, 0};
+ if (TREE_CODE_CLASS (TREE_CODE (e)) == 'c')
+ return e;
vep.e = e;
vep.hashcode = iterative_hash_expr (e,0);
slot = htab_find_slot_with_hash (table, &vep, vep.hashcode, NO_INSERT);
static inline void
add_to_value (tree v, tree e)
{
-#if DEBUG_VALUE_EXPRESSIONS
- var_ann_t va = var_ann (v);
-#endif
- /* For values representing numerical constants, we mark
- TREE_CONSTANT as true and set the tree chain to the actual
- constant. This is because unlike values involving expressions,
- which are only available to use where the expressions are live, a
- constant can be remade anywhere, and thus, is available
- everywhere. */
- if (TREE_CODE_CLASS (TREE_CODE (e)) == 'c')
- {
- TREE_CONSTANT (v) = true;
- TREE_CHAIN (v) = e;
- }
- else if (is_gimple_min_invariant (e))
+ var_ann_t va;
+ /* For values representing non-CST nodes, but still function
+ invariant things we mark TREE_CONSTANT as true and set the tree
+ chain to the actual constant. This is because unlike values
+ involving expressions, which are only available to use where the
+ expressions are live, a function invariant can be remade
+ anywhere, and thus, is available everywhere, just like a constant. */
+ if (TREE_CODE_CLASS (TREE_CODE (v)) == 'c')
+ return;
+ else if (is_gimple_min_invariant (v))
{
TREE_CONSTANT (v) = true;
TREE_CHAIN (v) = e;
+ return;
}
-#if DEBUG_VALUE_EXPRESSIONS
+ va = var_ann (v);
if (va->expr_set == NULL)
va->expr_set = set_new (false);
insert_into_set (va->expr_set, e);
-#endif
}
/* Insert E into TABLE with value V, and add expression E to the value
static bool
set_contains_value (value_set_t set, tree val)
{
+ /* All true constants are in every set. */
+ if (TREE_CODE_CLASS (TREE_CODE (val)) == 'c')
+ return true;
/* This is only referring to the flag above that we set on
- values referring to numerical constants, because we know that we
+ values referring to invariants, because we know that we
are dealing with one of the value handles we created. */
+
if (TREE_CONSTANT (val))
return true;
{
tree val = get_value_handle (expr);
- /* Constant values exist everywhere. */
+
+ /* Constant and invariant values exist everywhere, and thus,
+ actually keeping them in the sets is pointless. */
if (TREE_CONSTANT (val))
return;
}
}
break;
+ /* XXX: Until we have PRE of loads working, none will be ANTIC.
+ */
+ case 'r':
+ return NULL;
+ break;
case '1':
{
tree oldop1 = TREE_OPERAND (expr, 0);
if (val == NULL)
return NULL;
-
+ /* True constants represent themselves. */
+ if (TREE_CODE_CLASS (TREE_CODE (val)) == 'c')
+ return val;
+ /* Invariants are still represented by values, since they may be
+ more than a single _CST node. */
if (TREE_CONSTANT (val))
return TREE_CHAIN (val);
return set_contains_value (set, op1);
}
break;
+ /* XXX: Until PRE of loads works, no reference nodes are ANTIC.
+ */
+ case 'r':
+ {
+ return false;
+ }
case 'x':
{
if (TREE_CODE (expr) == SSA_NAME)
fprintf (dump_file, "compute_antic required %d iterations\n", num_iterations);
}
+/* Get the expressions represented by value VAL. */
+
+static value_set_t
+get_expr_set (tree val)
+{
+ var_ann_t va = var_ann (val);
+ return va->expr_set;
+}
+
+
+/* Find a leader for an expression, or generate one using
+ create_expression_by_pieces if it's ANTIC but
+ complex.
+ BLOCK is the basic_block we are looking for leaders in.
+ EXPR is the expression to find a leader or generate for.
+ STMTS is the statement list to put the inserted expressions on.
+ Returns the SSA_NAME of the LHS of the generated expression or the
+ leader. */
+
+static tree
+find_or_generate_expression (basic_block block, tree expr, tree stmts)
+{
+ tree genop;
+ genop = find_leader (AVAIL_OUT (block), expr);
+ /* Depending on the order we process DOM branches in, the value
+ may not have propagated to all the dom children yet during
+ this iteration. In this case, the value will always be in
+ the NEW_SETS for us already, having been propogated from our
+ dominator. */
+ if (genop == NULL)
+ genop = find_leader (NEW_SETS (block), expr);
+ /* If it's still NULL, see if it is a complex expression, and if
+ so, generate it recursively, otherwise, abort, because it's
+ not really . */
+ if (genop == NULL)
+ {
+ genop = get_expr_set (expr)->head->expr;
+ if (TREE_CODE_CLASS (TREE_CODE (genop)) != '1'
+ && TREE_CODE_CLASS (TREE_CODE (genop)) != '2')
+ abort ();
+ genop = create_expression_by_pieces (block, genop, stmts);
+ }
+ return genop;
+}
+
+
+/* Create an expression in pieces, so that we can handle very complex
+ expressions that may be ANTIC, but not necessary GIMPLE.
+ BLOCK is the basic block the expression will be inserted into,
+ EXPR is the expression to insert (in value form)
+ STMTS is a statement list to append the necessary insertions into.
+
+ This function will abort if we hit some value that shouldn't be
+ ANTIC but is (IE there is no leader for it, or its components).
+ This function may also generate expressions that are themselves
+ partially or fully redundant. Those that are will be either made
+ fully redundant during the next iteration of insert (for partially
+ redundant ones), or eliminated by eliminate (for fully redundant
+ ones). */
+
+static tree
+create_expression_by_pieces (basic_block block, tree expr, tree stmts)
+{
+ tree name = NULL_TREE;
+ tree newexpr = NULL_TREE;
+ tree v;
+
+ switch (TREE_CODE_CLASS (TREE_CODE (expr)))
+ {
+ case '2':
+ {
+ tree_stmt_iterator tsi;
+ tree genop1, genop2;
+ tree temp;
+ tree op1 = TREE_OPERAND (expr, 0);
+ tree op2 = TREE_OPERAND (expr, 1);
+ genop1 = find_or_generate_expression (block, op1, stmts);
+ genop2 = find_or_generate_expression (block, op2, stmts);
+ temp = create_tmp_var (TREE_TYPE (expr), "pretmp");
+ add_referenced_tmp_var (temp);
+ newexpr = build (TREE_CODE (expr), TREE_TYPE (expr),
+ genop1, genop2);
+ newexpr = build (MODIFY_EXPR, TREE_TYPE (expr),
+ temp, newexpr);
+ name = make_ssa_name (temp, newexpr);
+ TREE_OPERAND (newexpr, 0) = name;
+ tsi = tsi_last (stmts);
+ tsi_link_after (&tsi, newexpr, TSI_CONTINUE_LINKING);
+ pre_stats.insertions++;
+ break;
+ }
+ case '1':
+ {
+ tree_stmt_iterator tsi;
+ tree genop1;
+ tree temp;
+ tree op1 = TREE_OPERAND (expr, 0);
+ genop1 = find_or_generate_expression (block, op1, stmts);
+ temp = create_tmp_var (TREE_TYPE (expr), "pretmp");
+ add_referenced_tmp_var (temp);
+ newexpr = build (TREE_CODE (expr), TREE_TYPE (expr),
+ genop1);
+ newexpr = build (MODIFY_EXPR, TREE_TYPE (expr),
+ temp, newexpr);
+ name = make_ssa_name (temp, newexpr);
+ TREE_OPERAND (newexpr, 0) = name;
+ tsi = tsi_last (stmts);
+ tsi_link_after (&tsi, newexpr, TSI_CONTINUE_LINKING);
+ pre_stats.insertions++;
+
+ break;
+ }
+ default:
+ abort ();
+
+ }
+ v = get_value_handle (expr);
+ add (value_table, name, v);
+ insert_into_set (NEW_SETS (block), name);
+ value_insert_into_set (AVAIL_OUT (block), name);
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, "Inserted ");
+ print_generic_expr (dump_file, newexpr, 0);
+ fprintf (dump_file, " in predecessor %d\n", block->index);
+ }
+ return name;
+}
+
/* Perform insertion of partially redundant values.
For BLOCK, do the following:
1. Propagate the NEW_SETS of the dominator into the current block.
{
tree type = TREE_TYPE (avail[block->pred->src->index]);
tree temp;
- tree v;
-
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Found partial redundancy for expression ");
pred;
pred = pred->pred_next)
{
+ tree stmts = alloc_stmt_list ();
+ tree builtexpr;
bprime = pred->src;
eprime = avail[bprime->index];
- if (TREE_CODE_CLASS (TREE_CODE (eprime)) == '2')
+ if (TREE_CODE_CLASS (TREE_CODE (eprime)) == '2'
+ || TREE_CODE_CLASS (TREE_CODE (eprime)) == '1')
{
- tree s1, s2;
- tree newexpr;
- s1 = find_leader (AVAIL_OUT (bprime),
- TREE_OPERAND (eprime, 0));
- /* Depending on the order we process
- DOM branches in, the value may
- not have propagated to all the
- dom children yet during this
- iteration. In this case, the
- value will always be in the
- NEW_SETS for *our* dominator */
- if (!s1)
- s1 = find_leader (NEW_SETS (dom),
- TREE_OPERAND (eprime, 0));
- if (!s1)
- abort ();
-
- s2 = find_leader (AVAIL_OUT (bprime),
- TREE_OPERAND (eprime, 1));
- if (!s2)
- s2 = find_leader (NEW_SETS (dom),
- TREE_OPERAND (eprime, 1));
- if (!s2)
- abort ();
-
- temp = create_tmp_var (TREE_TYPE (eprime),
- "pretmp");
- add_referenced_tmp_var (temp);
- newexpr = build (TREE_CODE (eprime),
- TREE_TYPE (eprime),
- s1, s2);
- newexpr = build (MODIFY_EXPR,
- TREE_TYPE (eprime),
- temp, newexpr);
- temp = make_ssa_name (temp, newexpr);
- TREE_OPERAND (newexpr, 0) = temp;
- bsi_insert_on_edge (pred, newexpr);
+ builtexpr = create_expression_by_pieces (bprime,
+ eprime,
+ stmts);
+ bsi_insert_on_edge (pred, stmts);
bsi_commit_edge_inserts (NULL);
-
- if (dump_file && (dump_flags & TDF_DETAILS))
- {
- fprintf (dump_file, "Inserted ");
- print_generic_expr (dump_file, newexpr, 0);
- fprintf (dump_file, " in predecessor %d\n", pred->src->index);
- }
- pre_stats.insertions++;
- v = lookup_or_add (value_table, eprime);
- add (value_table, temp, v);
- insert_into_set (NEW_SETS (bprime), temp);
- value_insert_into_set (AVAIL_OUT (bprime),
- temp);
- avail[bprime->index] = temp;
- }
- else if (TREE_CODE_CLASS (TREE_CODE (eprime)) == '1')
- {
- tree s1;
- tree newexpr;
- s1 = find_leader (AVAIL_OUT (bprime),
- TREE_OPERAND (eprime, 0));
- /* Depending on the order we process
- DOM branches in, the value may not have
- propagated to all the dom
- children yet in the current
- iteration, but it will be in
- NEW_SETS if it is not yet
- propagated. */
-
- if (!s1)
- s1 = find_leader (NEW_SETS (dom),
- TREE_OPERAND (eprime, 0));
- if (!s1)
- abort ();
-
- temp = create_tmp_var (TREE_TYPE (eprime),
- "pretmp");
- add_referenced_tmp_var (temp);
- newexpr = build (TREE_CODE (eprime),
- TREE_TYPE (eprime),
- s1);
- newexpr = build (MODIFY_EXPR,
- TREE_TYPE (eprime),
- temp, newexpr);
- temp = make_ssa_name (temp, newexpr);
- TREE_OPERAND (newexpr, 0) = temp;
- bsi_insert_on_edge (pred, newexpr);
- bsi_commit_edge_inserts (NULL);
-
- if (dump_file && (dump_flags & TDF_DETAILS))
- {
- fprintf (dump_file, "Inserted ");
- print_generic_expr (dump_file, newexpr, 0);
- fprintf (dump_file, " in predecessor %d\n", pred->src->index);
- }
- pre_stats.insertions++;
- v = lookup_or_add (value_table, eprime);
- add (value_table, temp, v);
- insert_into_set (NEW_SETS (bprime), temp);
- value_insert_into_set (AVAIL_OUT (bprime),
- temp);
- avail[bprime->index] = temp;
- }
- }
+ avail[bprime->index] = builtexpr;
+ }
+ }
/* Now build a phi for the new variable. */
temp = create_tmp_var (type, "prephitmp");
add_referenced_tmp_var (temp);
}
continue;
}
- else if (TREE_CODE (stmt) == RETURN_EXPR
- && TREE_OPERAND (stmt, 0)
- && TREE_CODE (TREE_OPERAND (stmt, 0)) == MODIFY_EXPR)
- stmt = TREE_OPERAND (stmt, 0);
-
+
if (TREE_CODE (stmt) == MODIFY_EXPR)
{
op0 = TREE_OPERAND (stmt, 0);
continue;
op1 = TREE_OPERAND (stmt, 1);
STRIP_USELESS_TYPE_CONVERSION (op1);
- if (TREE_CODE_CLASS (TREE_CODE (op1)) == 'c')
+ if (is_gimple_min_invariant (op1))
{
add (value_table, op0, lookup_or_add (value_table, op1));
insert_into_set (TMP_GEN (block), op0);
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