Collaboration diagram for temp_expr_table_d:

Data Fields
var_map	map
bitmap *	partition_dependencies
bitmap	replaceable_expressions
bitmap *	expr_decl_uids
bitmap *	kill_list
int	virtual_partition
bitmap	partition_in_use
bitmap	new_replaceable_dependencies
int *	num_in_part
int *	call_cnt

Detailed Description

Routines for performing Temporary Expression Replacement (TER) in SSA trees. Copyright (C) 2003-2013 Free Software Foundation, Inc. Contributed by Andrew MacLeod amacl.nosp@m.eod@.nosp@m.redha.nosp@m.t.co.nosp@m.m

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/. Temporary Expression Replacement (TER)

Replace SSA version variables during out-of-ssa with their defining expression if there is only one use of the variable.

This pass is required in order for the RTL expansion pass to see larger chunks of code. This allows it to make better choices on RTL pattern selection. When expand is rewritten and merged with out-of-ssa, and understands SSA, this should be eliminated.

A pass is made through the function, one block at a time. No cross block information is tracked.

Variables which only have one use, and whose defining stmt is considered a replaceable expression (see ssa_is_replaceable_p) are tracked to see whether they can be replaced at their use location.

n_12 = C * 10 a_2 = b_5 + 6 v_9 = a_2 * n_12

if there are the only use of n_12 and a_2, TER will substitute in their expressions in v_9, and end up with:

v_9 = (b_5 + 6) * (C * 10)

which will then have the ssa_name assigned to regular variables, and the resulting code which will be passed to the expander looks something like:

v = (b + 6) * (C * 10)

This requires ensuring that none of the variables used by the expression change between the def point and where it is used. Furthermore, if any of the ssa_names used in this expression are themselves replaceable, we have to ensure none of that expressions' arguments change either. Although SSA_NAMES themselves don't change, this pass is performed after coalescing has coalesced different SSA_NAMES together, so there could be a definition of an SSA_NAME which is coalesced with a use that causes a problem, i.e.,

PHI b_5 = <b_8(2), b_14(1)> <...> a_2 = b_5 + 6 b_8 = c_4 + 4 v_9 = a_2 * n_12 <...>

If b_5, b_8 and b_14 are all coalesced together... The expression b_5 + 6 CANNOT replace the use in the statement defining v_9 because b_8 is in fact killing the value of b_5 since they share a partition and will be assigned the same memory or register location.

TER implements this but stepping through the instructions in a block and tracking potential expressions for replacement, and the partitions they are dependent on. Expressions are represented by the SSA_NAME_VERSION of the DEF on the LHS of a GIMPLE_ASSIGN and the expression is the RHS.

When a stmt is determined to be a possible replacement expression, the following steps are taken:

EXPR_DECL_UID bitmap is allocated and set to the base variable UID of the def and any uses in the expression. non-NULL means the expression is being tracked. The UID's themselves are used to prevent TER substitution into accumulating sequences, i.e.,

x = x + y x = x + z x = x + w etc. this can result in very large expressions which don't accomplish anything see PR tree-optimization/17549.

PARTITION_DEPENDENCIES is another bitmap array, and it has a bit set for any partition which is used in the expression. This is primarily used to remove an expression from the partition kill lists when a decision is made whether to replace it or not. This is indexed by ssa version number as well, and indicates a partition number. virtual operands are not tracked individually, but they are summarized by an artificial partition called VIRTUAL_PARTITION. This means a MAY or MUST def will kill ALL expressions that are dependent on a virtual operand. Note that the EXPR_DECL_UID and this bitmap represent very similar information, but the info in one is not easy to obtain from the other.

KILL_LIST is yet another bitmap array, this time it is indexed by partition number, and represents a list of active expressions which will will no longer be valid if a definition into this partition takes place.

PARTITION_IN_USE is simply a bitmap which is used to track which partitions currently have something in their kill list. This is used at the end of a block to clear out the KILL_LIST bitmaps at the end of each block.

NEW_REPLACEABLE_DEPENDENCIES is used as a temporary place to store dependencies which will be reused by the current definition. All the uses on an expression are processed before anything else is done. If a use is determined to be a replaceable expression AND the current stmt is also going to be replaceable, all the dependencies of this replaceable use will be picked up by the current stmt's expression. Rather than recreate them, they are simply copied here and then copied into the new expression when it is processed.

a_2 = b_5 + 6 v_8 = a_2 + c_4

a_2's expression 'b_5 + 6' is determined to be replaceable at the use location. It is dependent on the partition 'b_5' is in. This is cached into the NEW_REPLACEABLE_DEPENDENCIES bitmap, and when v_8 is examined for replaceability, it is a candidate, and it is dependent on the partition b_5 is in NOT a_2, as well as c_4's partition.

if v_8 is also replaceable:

x_9 = v_8 * 5

x_9 is dependent on partitions b_5, and c_4

if a statement is found which has either of those partitions written to before x_9 is used, then x_9 itself is NOT replaceable. Temporary Expression Replacement (TER) table information.