GCC Middle and Back End API Reference
temp_expr_table Struct Reference
Collaboration diagram for temp_expr_table:

Data Fields

var_map map
bitmap replaceable_expressions
int virtual_partition
bitmap partition_in_use
bitmap new_replaceable_dependencies
int * num_in_part
int * call_cnt
int * reg_vars_cnt

Detailed Description

Routines for performing Temporary Expression Replacement (TER) in SSA trees.
   Copyright (C) 2003-2024 Free Software Foundation, Inc.
   Contributed by Andrew MacLeod  <amacleod@redhat.com>

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
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
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
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 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

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.   

Field Documentation

◆ call_cnt

int* temp_expr_table::call_cnt

◆ expr_decl_uids

bitmap* temp_expr_table::expr_decl_uids

◆ kill_list

bitmap* temp_expr_table::kill_list

◆ map

var_map temp_expr_table::map

◆ new_replaceable_dependencies

bitmap temp_expr_table::new_replaceable_dependencies

◆ num_in_part

int* temp_expr_table::num_in_part

◆ partition_dependencies

bitmap* temp_expr_table::partition_dependencies

◆ partition_in_use

bitmap temp_expr_table::partition_in_use

◆ reg_vars_cnt

int* temp_expr_table::reg_vars_cnt

◆ replaceable_expressions

bitmap temp_expr_table::replaceable_expressions

◆ virtual_partition

int temp_expr_table::virtual_partition

Referenced by new_temp_expr_table().

The documentation for this struct was generated from the following file: