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

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

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.

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   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 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

int* temp_expr_table_d::call_cnt
bitmap* temp_expr_table_d::expr_decl_uids
var_map temp_expr_table_d::map
bitmap temp_expr_table_d::new_replaceable_dependencies
int* temp_expr_table_d::num_in_part
bitmap* temp_expr_table_d::partition_dependencies
bitmap temp_expr_table_d::partition_in_use
bitmap temp_expr_table_d::replaceable_expressions
int temp_expr_table_d::virtual_partition

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