Data Structures
class	dse_dom_walker

Functions
static bool	gate_dse (void)
static unsigned int	tree_ssa_dse (void)
static bool	dse_possible_dead_store_p ()
static void	dse_optimize_stmt ()
gimple_opt_pass *	make_pass_dse ()

Variables
static bitmap	need_eh_cleanup

Function Documentation

static void dse_optimize_stmt ( )

static

   Attempt to eliminate dead stores in the statement referenced by BSI.

   A dead store is a store into a memory location which will later be
   overwritten by another store without any intervening loads.  In this
   case the earlier store can be deleted.

   In our SSA + virtual operand world we use immediate uses of virtual
   operands to detect dead stores.  If a store's virtual definition
   is used precisely once by a later store to the same location which
   post dominates the first store, then the first store is dead.

     If this statement has no virtual defs, then there is nothing
     to do.

     We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
     that's not also a function call, then record it into our table.

     Don't return early on *this_2(D) ={v} {CLOBBER}.

         But only remove *this_2(D) ={v} {CLOBBER} if killed by
         another clobber stmt.

         If we have precisely one immediate use at this point and the
         stores are to the same memory location or there is a chain of
         virtual uses from stmt and the stmt which stores to that same
         memory location, then we may have found redundant store.

             If use_stmt is or might be a nop assignment, e.g. for
             struct { ... } S a, b, *p; ...
             b = a; b = b;
             or
             b = a; b = *p; where p might be &b,
             or
             *p = a; *p = b; where p might be &b,
             or
             *p = *u; *p = *v; where p might be v, then USE_STMT
             acts as a use as well as definition, so store in STMT
             is not dead.

             Then we need to fix the operand of the consuming stmt.

             Remove the dead store.

             And release any SSA_NAMEs set in this statement back to the
             SSA_NAME manager.

References bitmap_set_bit(), dse_possible_dead_store_p(), dump_file, dump_flags, gimple_assign_lhs(), gimple_bb(), gimple_clobber_p(), gimple_get_lhs(), gimple_has_lhs(), gsi_remove(), gsi_stmt(), basic_block_def::index, operand_equal_p(), print_gimple_stmt(), ref_maybe_used_by_stmt_p(), release_defs(), stmt_kills_ref_p(), and unlink_stmt_vdef().

static bool dse_possible_dead_store_p ( )

static

   A helper of dse_optimize_stmt.
   Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
   may prove STMT to be dead.
   Return TRUE if the above conditions are met, otherwise FALSE.

     Self-assignments are zombies.

     Find the first dominated statement that clobbers (part of) the
     memory stmt stores to with no intermediate statement that may use
     part of the memory stmt stores.  That is, find a store that may
     prove stmt to be a dead store.

         Limit stmt walking to be linear in the number of possibly
         dead stores.

             If we ever reach our DSE candidate stmt again fail.  We
             cannot handle dead stores in loops.

             In simple cases we can look through PHI nodes, but we
             have to be careful with loops and with memory references
             containing operands that are also operands of PHI nodes.
             See gcc.c-torture/execute/20051110-*.c.

                     Make sure we are not in a loop latch block.

                     We can look through PHIs to regions post-dominating
                     the DSE candidate stmt.

                 Do not consider the PHI as use if it dominates the 
                 stmt defining the virtual operand we are processing,
                 we have processed it already in this case.

             If the statement is a use the store is not dead.

             If this is a store, remember it or bail out if we have
             multiple ones (the will be in different CFG parts then).

         If we didn't find any definition this means the store is dead
         if it isn't a store to global reachable memory.  In this case
         just pretend the stmt makes itself dead.  Otherwise fail.

     We deliberately stop on clobbering statements and not only on
     killing ones to make walking cheaper.  Otherwise we can just
     continue walking until both stores have equal reference trees.

Referenced by dse_optimize_stmt().

static bool gate_dse ( void )

static

gimple_opt_pass* make_pass_dse ( )

static unsigned int tree_ssa_dse ( )

static

   Main entry point.

     We might consider making this a property of each pass so that it
     can be [re]computed on an as-needed basis.  Particularly since
     this pass could be seen as an extension of DCE which needs post
     dominators.

     Dead store elimination is fundamentally a walk of the post-dominator
     tree and a backwards walk of statements within each block.

     Removal of stores may make some EH edges dead.  Purge such edges from
     the CFG as needed.

     For now, just wipe the post-dominator information.

Variable Documentation

bitmap need_eh_cleanup

static

@verbatim

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/.

   This file implements dead store elimination.

   A dead store is a store into a memory location which will later be
   overwritten by another store without any intervening loads.  In this
   case the earlier store can be deleted.

   In our SSA + virtual operand world we use immediate uses of virtual
   operands to detect dead stores.  If a store's virtual definition
   is used precisely once by a later store to the same location which
   post dominates the first store, then the first store is dead.

   The single use of the store's virtual definition ensures that
   there are no intervening aliased loads and the requirement that
   the second load post dominate the first ensures that if the earlier
   store executes, then the later stores will execute before the function
   exits.

   It may help to think of this as first moving the earlier store to
   the point immediately before the later store.  Again, the single
   use of the virtual definition and the post-dominance relationship
   ensure that such movement would be safe.  Clearly if there are
   back to back stores, then the second is redundant.

   Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
   may also help in understanding this code since it discusses the
   relationship between dead store and redundant load elimination.  In
   fact, they are the same transformation applied to different views of
   the CFG.

   Bitmap of blocks that have had EH statements cleaned.  We should
   remove their dead edges eventually.

Data Structures

Functions

Variables

Function Documentation

Variable Documentation