GCC Middle and Back End API Reference
fwprop.c File Reference

Data Structures

class  single_def_use_dom_walker




static df_ref get_def_for_use ()
static void process_defs ()
static void process_uses ()
static void build_single_def_use_links ()
static bool can_simplify_addr ()
static void canonicalize_address ()
static bool should_replace_address (rtx old_rtx, rtx new_rtx, enum machine_mode mode, addr_space_t as, bool speed)
static bool propagate_rtx_1 ()
static int varying_mem_p ()
static rtx propagate_rtx (rtx x, enum machine_mode mode, rtx old_rtx, rtx new_rtx, bool speed)
static bool local_ref_killed_between_p ()
static bool use_killed_between ()
static bool all_uses_available_at ()
static void register_active_defs ()
static void update_df_init ()
static void update_uses ()
static void update_df ()
static bool try_fwprop_subst ()
static bool free_load_extend ()
static bool forward_propagate_subreg ()
static bool forward_propagate_asm ()
static bool forward_propagate_and_simplify ()
static bool forward_propagate_into ()
static void fwprop_init ()
static void fwprop_done ()
static bool gate_fwprop ()
static unsigned int fwprop ()
rtl_opt_passmake_pass_rtl_fwprop ()
static unsigned int fwprop_addr ()
rtl_opt_passmake_pass_rtl_fwprop_addr ()


static int num_changes
static vec< df_refuse_def_ref
static vec< df_refreg_defs
static vec< df_refreg_defs_stack
static bitmap local_md
static bitmap local_lr
static df_refactive_defs
static sparseset active_defs_check

Enumeration Type Documentation

anonymous enum
   Flags for the last parameter of propagate_rtx_1.  
     If PR_CAN_APPEAR is true, propagate_rtx_1 always returns true;
     if it is false, propagate_rtx_1 returns false if, for at least
     one occurrence OLD, it failed to collapse the result to a constant.
     For example, (mult:M (reg:M A) (minus:M (reg:M B) (reg:M A))) may
     collapse to zero if replacing (reg:M B) with (reg:M A).

     PR_CAN_APPEAR is disregarded inside MEMs: in that case,
     propagate_rtx_1 just tries to make cheaper and valid memory
     If PR_HANDLE_MEM is not set, propagate_rtx_1 won't attempt any replacement
     outside memory addresses.  This is needed because propagate_rtx_1 does
     not do any analysis on memory; thus it is very conservative and in general
     it will fail if non-read-only MEMs are found in the source expression.

     PR_HANDLE_MEM is set when the source of the propagation was not
     another MEM.  Then, it is safe not to treat non-read-only MEMs as
     ``opaque'' objects.  
     Set when costs should be optimized for speed.  

Function Documentation

static bool all_uses_available_at ( )
   Check if all uses in DEF_INSN can be used in TARGET_INSN.  This
   would require full computation of available expressions;
   we check only restricted conditions, see use_killed_between.  
     If target_insn comes right after def_insn, which is very common
     for addresses, we can use a quicker test.  Ignore debug insns
     other than target insns for this.  
         If the insn uses the reg that it defines, the substitution is
         Look at all the uses of DEF_INSN, and see if they are not
         killed between DEF_INSN and TARGET_INSN.  
static void build_single_def_use_links ( )
   Build a vector holding the reaching definitions of uses reached by a
   single dominating definition.  
     We use the multiple definitions problem to compute our restricted
     use-def chains.  
     Walk the dominator tree looking for single reaching definitions
     dominating the uses.  This is similar to how SSA form is built.  
static bool can_simplify_addr ( )
   Do not try to replace constant addresses or addresses of local and
   argument slots.  These MEM expressions are made only once and inserted
   in many instructions, as well as being used to control symbol table
   output.  It is not safe to clobber them.

   There are some uncommon cases where the address is already in a register
   for some reason, but we cannot take advantage of that because we have
   no easy way to unshare the MEM.  In addition, looking up all stack
   addresses is costly.  
static void canonicalize_address ( )
   Returns a canonical version of X for the address, from the point of view,
   that all multiplications are represented as MULT instead of the multiply
   by a power of 2 being represented as ASHIFT.

   Every ASHIFT we find has been made by simplify_gen_binary and was not
   there before, so it is not shared.  So we can do this in place.  
static bool forward_propagate_and_simplify ( )
   Try to replace USE with SRC (defined in DEF_INSN) and simplify the
     Do not propagate into PC, CC0, etc.  
     If def and use are subreg, check if they match.  
     Check if the def had a subreg, but the use has the whole reg.  
     Check if the use has a subreg, but the def had the whole reg.  Unlike the
     previous case, the optimization is possible and often useful indeed.  
     Make sure that we can treat REG as having the same mode as the
     source of DEF_SET.  
     Check if the substitution is valid (last, because it's the most
     expensive check!).  
     Check if the def is loading something from the constant pool; in this
     case we would undo optimization such as compress_float_constant.
     Still, we can set a REG_EQUAL note.  
     Else try simplifying.  
         Do not replace an existing REG_EQUAL note if the insn is not
         recognized.  Either we're already replacing in the note, or we'll
         separately try plugging the definition in the note and simplifying.
         And only install a REQ_EQUAL note when the destination is a REG
         that isn't mentioned in USE_SET, as the note would be invalid
         otherwise.  We also don't want to install a note if we are merely
         propagating a pseudo since verifying that this pseudo isn't dead
         is a pain; moreover such a note won't help anything.  
static bool forward_propagate_asm ( )
   Try to replace USE with SRC (defined in DEF_INSN) in __asm.  
     In __asm don't replace if src might need more registers than
     reg, as that could increase register pressure on the __asm.  
static bool forward_propagate_into ( )
   Given a use USE of an insn, if it has a single reaching
   definition, try to forward propagate it into that insn.
   Return true if cfg cleanup will be needed.  
     Only consider uses that have a single definition.  
     Do not propagate loop invariant definitions inside the loop.  
     Check if the use is still present in the insn!  
     Only try one kind of propagation.  If two are possible, we'll
     do it on the following iterations.  
static bool forward_propagate_subreg ( )
   If USE is a subreg, see if it can be replaced by a pseudo.  
     Only consider subregs... 
     If this is a paradoxical SUBREG...  
         If this is a paradoxical SUBREG, we have no idea what value the
         extra bits would have.  However, if the operand is equivalent to
         a SUBREG whose operand is the same as our mode, and all the modes
         are within a word, we can just use the inner operand because
         these SUBREGs just say how to treat the register.  
     If this is a SUBREG of a ZERO_EXTEND or SIGN_EXTEND, and the SUBREG
     is the low part of the reg being extended then just use the inner
     operand.  Don't do this if the ZERO_EXTEND or SIGN_EXTEND insn will
     be removed due to it matching a LOAD_EXTEND_OP load from memory,
     or due to the operation being a no-op when applied to registers.
     For example, if we have:

         A: (set (reg:DI X) (sign_extend:DI (reg:SI Y)))
         B: (... (subreg:SI (reg:DI X)) ...)

     and mode_rep_extended says that Y is already sign-extended,
     the backend will typically allow A to be combined with the
     definition of Y or, failing that, allow A to be deleted after
     reload through register tying.  Introducing more uses of Y
     prevents both optimisations.  
static bool free_load_extend ( )
   For the given single_set INSN, containing SRC known to be a
   ZERO_EXTEND or SIGN_EXTEND of a register, return true if INSN
   is redundant due to the register being set by a LOAD_EXTEND_OP
   load from memory.  
static unsigned int fwprop ( )
     Go through all the uses.  df_uses_create will create new ones at the
     end, and we'll go through them as well.

     Do not forward propagate addresses into loops until after unrolling.
     CSE did so because it was able to fix its own mess, but we are not.  
               The outer most loop is not really a loop.  
static unsigned int fwprop_addr ( )
     Go through all the uses.  df_uses_create will create new ones at the
     end, and we'll go through them as well.  
               The outer most loop is not really a loop.  
static void fwprop_done ( )

References gate_fwprop().

static void fwprop_init ( )
     We do not always want to propagate into loops, so we have to find
     loops and be careful about them.  Avoid CFG modifications so that
     we don't have to update dominance information afterwards for

References RTL_PASS.

static bool gate_fwprop ( )
   Main entry point.  

References DF_REF_REG_USE.

Referenced by fwprop_done().

static df_ref get_def_for_use ( )
   Return the only def in USE's use-def chain, or NULL if there is
   more than one def in the chain.  
static bool local_ref_killed_between_p ( )
   Return true if the register from reference REF is killed
   between FROM to (but not including) TO.  
rtl_opt_pass* make_pass_rtl_fwprop ( )
rtl_opt_pass* make_pass_rtl_fwprop_addr ( )
static void process_defs ( )
             Do not store anything if "transitioning" from NULL to NULL.  But
             otherwise, push a special entry on the stack to tell the
             leave_block callback that the entry in reg_defs was NULL.  
static void process_uses ( )
   Fill the use_def_ref vector with values for the uses in USE_REC,
   taking reaching definitions info from LOCAL_MD and REG_DEFS.
   TOP_FLAG says which artificials uses should be used, when USE_REC
   is an artificial use vector.  
static rtx propagate_rtx ( rtx  x,
enum machine_mode  mode,
rtx  old_rtx,
rtx  new_rtx,
bool  speed 
   Replace all occurrences of OLD in X with NEW and try to simplify the
   resulting expression (in mode MODE).  Return a new expression if it is
   a constant, otherwise X.

   Simplifications where occurrences of NEW collapse to a constant are always
   accepted.  All simplifications are accepted if NEW is a pseudo too.
   Otherwise, we accept simplifications that have a lower or equal cost.  
     gen_lowpart_common will not be able to process VOIDmode entities other
     than CONST_INTs.  
static bool propagate_rtx_1 ( )
   Replace all occurrences of OLD in *PX with NEW and try to simplify the
   resulting expression.  Replace *PX with a new RTL expression if an
   occurrence of OLD was found.

   This is only a wrapper around simplify-rtx.c: do not add any pattern
   matching code here.  (The sole exception is the handling of LO_SUM, but
   that is because there is no simplify_gen_* function for LO_SUM).  
         If unsafe, change MEMs to CLOBBERs or SCRATCHes (to preserve whether
         they have side effects or not).  
     If X is OLD_RTX, return NEW_RTX.  But not if replacing only within an
     address, and we are *not* inside one.  
     If this is an expression, try recursive substitution.  
         The only case we try to handle is a SUBREG.  
             There are some addresses that we cannot work on.  
             Dismiss transformation that we do not want to carry on.  
             Copy propagations are always ok.  Otherwise check the costs.  
             The only simplification we do attempts to remove references to op0
             or make it constant -- in both cases, op0's invalidity will not
             make the result invalid.  
             (lo_sum (high x) x) -> x  
             OP1 is likely not a legitimate address, otherwise there would have
             been no LO_SUM.  We want it to disappear if it is invalid, return
             false in that case.  
     No change, no trouble.  
     The replacement we made so far is valid, if all of the recursive
     replacements were valid, or we could simplify everything to
     a constant.  
static void register_active_defs ( )
   Fill the ACTIVE_DEFS array with the use->def link for the registers
   mentioned in USE_REC.  Register the valid entries in ACTIVE_DEFS_CHECK
   too, for checking purposes.  
static bool should_replace_address ( rtx  old_rtx,
rtx  new_rtx,
enum machine_mode  mode,
addr_space_t  as,
bool  speed 
   OLD is a memory address.  Return whether it is good to use NEW instead,
   for a memory access in the given MODE.  
     Copy propagation is always ok.  
     Prefer the new address if it is less expensive.  
     If the addresses have equivalent cost, prefer the new address
     if it has the highest `set_src_cost'.  That has the potential of
     eliminating the most insns without additional costs, and it
     is the same that cse.c used to do.  
static bool try_fwprop_subst ( )
   Try substituting NEW into LOC, which originated from forward propagation
   of USE's value from DEF_INSN.  SET_REG_EQUAL says whether we are
   substituting the whole SET_SRC, so we can set a REG_EQUAL note if the
   new insn is not recognized.  Return whether the substitution was
     forward_propagate_subreg may be operating on an instruction with
     multiple sets.  If so, assume the cost of the new instruction is
     not greater than the old one.  
         Can also record a simplified value in a REG_EQUAL note,
         making a new one if one does not already exist.  

References copy_rtx(), dump_file, and set_unique_reg_note().

static void update_df ( )
   Update the USE_DEF_REF array for the uses in INSN.  Only update note
   uses if NOTES_ONLY is true.  
static void update_df_init ( )
   Build the use->def links that we use to update the dataflow info
   for new uses.  Note that building the links is very cheap and if
   it were done earlier, they could be used to rule out invalid
   propagations (in addition to what is done in all_uses_available_at).
   I'm not doing this yet, though.  
static void update_uses ( )
   Update the USE_DEF_REF array for the given use, using the active definitions
   in the ACTIVE_DEFS array to match pseudos to their def. 
         Set up the use-def chain.  
static bool use_killed_between ( )
   Check if the given DEF is available in INSN.  This would require full
   computation of available expressions; we check only restricted conditions:
   - if DEF is the sole definition of its register, go ahead;
   - in the same basic block, we check for no definitions killing the
     definition of DEF_INSN;
   - if USE's basic block has DEF's basic block as the sole predecessor,
     we check if the definition is killed after DEF_INSN or before
     TARGET_INSN insn, in their respective basic blocks.  
     We used to have a def reaching a use that is _before_ the def,
     with the def not dominating the use even though the use and def
     are in the same basic block, when a register may be used
     uninitialized in a loop.  This should not happen anymore since
     we do not use reaching definitions, but still we test for such
     cases and assume that DEF is not available.  
     Check if the reg in USE has only one definition.  We already
     know that this definition reaches use, or we wouldn't be here.
     However, this is invalid for hard registers because if they are
     live at the beginning of the function it does not mean that we
     have an uninitialized access.  
     Check locally if we are in the same basic block.  
     Finally, if DEF_BB is the sole predecessor of TARGET_BB.  
         See if USE is killed between DEF_INSN and the last insn in the
         basic block containing DEF_INSN.  
         See if USE is killed between TARGET_INSN and the first insn in the
         basic block containing TARGET_INSN.  
     Otherwise assume the worst case.  
static int varying_mem_p ( )
   for_each_rtx traversal function that returns 1 if BODY points to
   a non-constant mem.  

Variable Documentation

df_ref* active_defs
sparseset active_defs_check
bitmap local_lr
bitmap local_md
   The MD bitmaps are trimmed to include only live registers to cut
   memory usage on testcases like insn-recog.c.  Track live registers
   in the basic block and do not perform forward propagation if the
   destination is a dead pseudo occurring in a note.  
int num_changes

RTL-based forward propagation pass for GNU compiler. Copyright (C) 2005-2013 Free Software Foundation, Inc. Contributed by Paolo Bonzini and Steven Bosscher.

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 pass does simple forward propagation and simplification when an
   operand of an insn can only come from a single def.  This pass uses
   df.c, so it is global.  However, we only do limited analysis of
   available expressions.

   1) The pass tries to propagate the source of the def into the use,
   and checks if the result is independent of the substituted value.
   For example, the high word of a (zero_extend:DI (reg:SI M)) is always
   zero, independent of the source register.

   In particular, we propagate constants into the use site.  Sometimes
   RTL expansion did not put the constant in the same insn on purpose,
   to satisfy a predicate, and the result will fail to be recognized;
   but this happens rarely and in this case we can still create a
   REG_EQUAL note.  For multi-word operations, this

      (set (subreg:SI (reg:DI 120) 0) (const_int 0))
      (set (subreg:SI (reg:DI 120) 4) (const_int -1))
      (set (subreg:SI (reg:DI 122) 0)
         (ior:SI (subreg:SI (reg:DI 119) 0) (subreg:SI (reg:DI 120) 0)))
      (set (subreg:SI (reg:DI 122) 4)
         (ior:SI (subreg:SI (reg:DI 119) 4) (subreg:SI (reg:DI 120) 4)))

   can be simplified to the much simpler

      (set (subreg:SI (reg:DI 122) 0) (subreg:SI (reg:DI 119)))
      (set (subreg:SI (reg:DI 122) 4) (const_int -1))

   This particular propagation is also effective at putting together
   complex addressing modes.  We are more aggressive inside MEMs, in
   that all definitions are propagated if the use is in a MEM; if the
   result is a valid memory address we check address_cost to decide
   whether the substitution is worthwhile.

   2) The pass propagates register copies.  This is not as effective as
   the copy propagation done by CSE's canon_reg, which works by walking
   the instruction chain, it can help the other transformations.

   We should consider removing this optimization, and instead reorder the
   RTL passes, because GCSE does this transformation too.  With some luck,
   the CSE pass at the end of rest_of_handle_gcse could also go away.

   3) The pass looks for paradoxical subregs that are actually unnecessary.
   Things like this:

     (set (reg:QI 120) (subreg:QI (reg:SI 118) 0))
     (set (reg:QI 121) (subreg:QI (reg:SI 119) 0))
     (set (reg:SI 122) (plus:SI (subreg:SI (reg:QI 120) 0)
                                (subreg:SI (reg:QI 121) 0)))

   are very common on machines that can only do word-sized operations.
   For each use of a paradoxical subreg (subreg:WIDER (reg:NARROW N) 0),
   if it has a single def and it is (subreg:NARROW (reg:WIDE M) 0),
   we can replace the paradoxical subreg with simply (reg:WIDE M).  The
   above will simplify this to

     (set (reg:QI 120) (subreg:QI (reg:SI 118) 0))
     (set (reg:QI 121) (subreg:QI (reg:SI 119) 0))
     (set (reg:SI 122) (plus:SI (reg:SI 118) (reg:SI 119)))

   where the first two insns are now dead.

   We used to use reaching definitions to find which uses have a
   single reaching definition (sounds obvious...), but this is too
   complex a problem in nasty testcases like PR33928.  Now we use the
   multiple definitions problem in df-problems.c.  The similarity
   between that problem and SSA form creation is taken further, in
   that fwprop does a dominator walk to create its chains; however,
   instead of creating a PHI function where multiple definitions meet
   I just punt and record only singleton use-def chains, which is
   all that is needed by fwprop.  
vec<df_ref> reg_defs
vec<df_ref> reg_defs_stack
vec<df_ref> use_def_ref