+++ /dev/null
-/* Dead-code elimination pass for the GNU compiler.
- Copyright (C) 2000, 2001, 2002 Free Software Foundation, Inc.
- Written by Jeffrey D. Oldham <oldham@codesourcery.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 2, 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 COPYING. If not, write to the Free
-Software Foundation, 59 Temple Place - Suite 330, Boston, MA
-02111-1307, USA. */
-
-/* Dead-code elimination is the removal of instructions which have no
- impact on the program's output. "Dead instructions" have no impact
- on the program's output, while "necessary instructions" may have
- impact on the output.
-
- The algorithm consists of three phases:
- 1) marking as necessary all instructions known to be necessary,
- e.g., writing a value to memory,
- 2) propagating necessary instructions, e.g., the instructions
- giving values to operands in necessary instructions, and
- 3) removing dead instructions (except replacing dead conditionals
- with unconditional jumps).
-
- Side Effects:
- The last step can require adding labels, deleting insns, and
- modifying basic block structures. Some conditional jumps may be
- converted to unconditional jumps so the control-flow graph may be
- out-of-date.
-
- Edges from some infinite loops to the exit block can be added to
- the control-flow graph, but will be removed after this pass is
- complete.
-
- It Does Not Perform:
- We decided to not simultaneously perform jump optimization and dead
- loop removal during dead-code elimination. Thus, all jump
- instructions originally present remain after dead-code elimination
- but 1) unnecessary conditional jump instructions are changed to
- unconditional jump instructions and 2) all unconditional jump
- instructions remain.
-
- Assumptions:
- 1) SSA has been performed.
- 2) The basic block and control-flow graph structures are accurate.
- 3) The flow graph permits constructing an edge_list.
- 4) note rtxes should be saved.
-
- Unfinished:
- When replacing unnecessary conditional jumps with unconditional
- jumps, the control-flow graph is not updated. It should be.
-
- References:
- Building an Optimizing Compiler
- Robert Morgan
- Butterworth-Heinemann, 1998
- Section 8.9
-*/
-
-#include "config.h"
-#include "system.h"
-
-#include "rtl.h"
-#include "hard-reg-set.h"
-#include "basic-block.h"
-#include "ssa.h"
-#include "insn-config.h"
-#include "recog.h"
-#include "output.h"
-
-\f
-/* A map from blocks to the edges on which they are control dependent. */
-typedef struct {
- /* An dynamically allocated array. The Nth element corresponds to
- the block with index N + 2. The Ith bit in the bitmap is set if
- that block is dependent on the Ith edge. */
- bitmap *data;
- /* The number of elements in the array. */
- int length;
-} control_dependent_block_to_edge_map_s, *control_dependent_block_to_edge_map;
-
-/* Local function prototypes. */
-static control_dependent_block_to_edge_map control_dependent_block_to_edge_map_create
- PARAMS((size_t num_basic_blocks));
-static void set_control_dependent_block_to_edge_map_bit
- PARAMS ((control_dependent_block_to_edge_map c, basic_block bb,
- int edge_index));
-static void control_dependent_block_to_edge_map_free
- PARAMS ((control_dependent_block_to_edge_map c));
-static void find_all_control_dependences
- PARAMS ((struct edge_list *el, int *pdom,
- control_dependent_block_to_edge_map cdbte));
-static void find_control_dependence
- PARAMS ((struct edge_list *el, int edge_index, int *pdom,
- control_dependent_block_to_edge_map cdbte));
-static basic_block find_pdom
- PARAMS ((int *pdom, basic_block block));
-static int inherently_necessary_register_1
- PARAMS ((rtx *current_rtx, void *data));
-static int inherently_necessary_register
- PARAMS ((rtx current_rtx));
-static int find_inherently_necessary
- PARAMS ((rtx current_rtx));
-static int propagate_necessity_through_operand
- PARAMS ((rtx *current_rtx, void *data));
-static void note_inherently_necessary_set
- PARAMS ((rtx, rtx, void *));
-\f
-/* Unnecessary insns are indicated using insns' in_struct bit. */
-
-/* Indicate INSN is dead-code; returns nothing. */
-#define KILL_INSN(INSN) INSN_DEAD_CODE_P(INSN) = 1
-/* Indicate INSN is necessary, i.e., not dead-code; returns nothing. */
-#define RESURRECT_INSN(INSN) INSN_DEAD_CODE_P(INSN) = 0
-/* Return nonzero if INSN is unnecessary. */
-#define UNNECESSARY_P(INSN) INSN_DEAD_CODE_P(INSN)
-static void mark_all_insn_unnecessary
- PARAMS ((void));
-/* Execute CODE with free variable INSN for all unnecessary insns in
- an unspecified order, producing no output. */
-#define EXECUTE_IF_UNNECESSARY(INSN, CODE) \
-{ \
- rtx INSN; \
- \
- for (INSN = get_insns (); INSN != NULL_RTX; INSN = NEXT_INSN (INSN)) \
- if (INSN_DEAD_CODE_P (INSN)) { \
- CODE; \
- } \
-}
-/* Find the label beginning block BB. */
-static rtx find_block_label
- PARAMS ((basic_block bb));
-/* Remove INSN, updating its basic block structure. */
-static void delete_insn_bb
- PARAMS ((rtx insn));
-\f
-/* Recording which blocks are control dependent on which edges. We
- expect each block to be control dependent on very few edges so we
- use a bitmap for each block recording its edges. An array holds
- the bitmap. Its position 0 entry holds the bitmap for block
- INVALID_BLOCK+1 so that all blocks, including the entry and exit
- blocks can participate in the data structure. */
-
-/* Create a control_dependent_block_to_edge_map, given the number
- NUM_BASIC_BLOCKS of non-entry, non-exit basic blocks, e.g.,
- n_basic_blocks. This memory must be released using
- control_dependent_block_to_edge_map_free (). */
-
-static control_dependent_block_to_edge_map
-control_dependent_block_to_edge_map_create (num_basic_blocks)
- size_t num_basic_blocks;
-{
- int i;
- control_dependent_block_to_edge_map c
- = xmalloc (sizeof (control_dependent_block_to_edge_map_s));
- c->length = num_basic_blocks - (INVALID_BLOCK+1);
- c->data = xmalloc ((size_t) c->length*sizeof (bitmap));
- for (i = 0; i < c->length; ++i)
- c->data[i] = BITMAP_XMALLOC ();
-
- return c;
-}
-
-/* Indicate block BB is control dependent on an edge with index
- EDGE_INDEX in the mapping C of blocks to edges on which they are
- control-dependent. */
-
-static void
-set_control_dependent_block_to_edge_map_bit (c, bb, edge_index)
- control_dependent_block_to_edge_map c;
- basic_block bb;
- int edge_index;
-{
- if (bb->index - (INVALID_BLOCK+1) >= c->length)
- abort ();
-
- bitmap_set_bit (c->data[bb->index - (INVALID_BLOCK+1)],
- edge_index);
-}
-
-/* Execute CODE for each edge (given number EDGE_NUMBER within the
- CODE) for which the block containing INSN is control dependent,
- returning no output. CDBTE is the mapping of blocks to edges on
- which they are control-dependent. */
-
-#define EXECUTE_IF_CONTROL_DEPENDENT(CDBTE, INSN, EDGE_NUMBER, CODE) \
- EXECUTE_IF_SET_IN_BITMAP \
- (CDBTE->data[BLOCK_NUM (INSN) - (INVALID_BLOCK+1)], 0, \
- EDGE_NUMBER, CODE)
-
-/* Destroy a control_dependent_block_to_edge_map C. */
-
-static void
-control_dependent_block_to_edge_map_free (c)
- control_dependent_block_to_edge_map c;
-{
- int i;
- for (i = 0; i < c->length; ++i)
- BITMAP_XFREE (c->data[i]);
- free ((PTR) c);
-}
-
-/* Record all blocks' control dependences on all edges in the edge
- list EL, ala Morgan, Section 3.6. The mapping PDOM of blocks to
- their postdominators are used, and results are stored in CDBTE,
- which should be empty. */
-
-static void
-find_all_control_dependences (el, pdom, cdbte)
- struct edge_list *el;
- int *pdom;
- control_dependent_block_to_edge_map cdbte;
-{
- int i;
-
- for (i = 0; i < NUM_EDGES (el); ++i)
- find_control_dependence (el, i, pdom, cdbte);
-}
-
-/* Determine all blocks' control dependences on the given edge with
- edge_list EL index EDGE_INDEX, ala Morgan, Section 3.6. The
- mapping PDOM of blocks to their postdominators are used, and
- results are stored in CDBTE, which is assumed to be initialized
- with zeros in each (block b', edge) position. */
-
-static void
-find_control_dependence (el, edge_index, pdom, cdbte)
- struct edge_list *el;
- int edge_index;
- int *pdom;
- control_dependent_block_to_edge_map cdbte;
-{
- basic_block current_block;
- basic_block ending_block;
-
- if (INDEX_EDGE_PRED_BB (el, edge_index) == EXIT_BLOCK_PTR)
- abort ();
- ending_block =
- (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
- ? BASIC_BLOCK (0)
- : find_pdom (pdom, INDEX_EDGE_PRED_BB (el, edge_index));
-
- for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
- current_block != ending_block && current_block != EXIT_BLOCK_PTR;
- current_block = find_pdom (pdom, current_block))
- {
- set_control_dependent_block_to_edge_map_bit (cdbte,
- current_block,
- edge_index);
- }
-}
-\f
-/* Find the immediate postdominator PDOM of the specified basic block
- BLOCK. This function is necessary because some blocks have
- negative numbers. */
-
-static basic_block
-find_pdom (pdom, block)
- int *pdom;
- basic_block block;
-{
- if (!block)
- abort ();
- if (block->index == INVALID_BLOCK)
- abort ();
-
- if (block == ENTRY_BLOCK_PTR)
- return BASIC_BLOCK (0);
- else if (block == EXIT_BLOCK_PTR || pdom[block->index] == EXIT_BLOCK)
- return EXIT_BLOCK_PTR;
- else
- return BASIC_BLOCK (pdom[block->index]);
-}
-
-/* Determine if the given CURRENT_RTX uses a hard register not
- converted to SSA. Returns nonzero only if it uses such a hard
- register. DATA is not used.
-
- The program counter (PC) is not considered inherently necessary
- since code should be position-independent and thus not depend on
- particular PC values. */
-
-static int
-inherently_necessary_register_1 (current_rtx, data)
- rtx *current_rtx;
- void *data ATTRIBUTE_UNUSED;
-{
- rtx x = *current_rtx;
-
- if (x == NULL_RTX)
- return 0;
- switch (GET_CODE (x))
- {
- case CLOBBER:
- /* Do not traverse the rest of the clobber. */
- return -1;
- break;
- case PC:
- return 0;
- break;
- case REG:
- if (CONVERT_REGISTER_TO_SSA_P (REGNO (x)) || x == pc_rtx)
- return 0;
- else
- return !0;
- break;
- default:
- return 0;
- break;
- }
-}
-
-/* Return nonzero if the insn CURRENT_RTX is inherently necessary. */
-
-static int
-inherently_necessary_register (current_rtx)
- rtx current_rtx;
-{
- return for_each_rtx (¤t_rtx,
- &inherently_necessary_register_1, NULL);
-}
-
-
-/* Called via note_stores for each store in an insn. Note whether
- or not a particular store is inherently necessary. Store a
- nonzero value in inherently_necessary_p if such a store is found. */
-
-static void
-note_inherently_necessary_set (dest, set, data)
- rtx set ATTRIBUTE_UNUSED;
- rtx dest;
- void *data;
-{
- int *inherently_necessary_set_p = (int *) data;
-
- while (GET_CODE (dest) == SUBREG
- || GET_CODE (dest) == STRICT_LOW_PART
- || GET_CODE (dest) == ZERO_EXTRACT
- || GET_CODE (dest) == SIGN_EXTRACT)
- dest = XEXP (dest, 0);
-
- if (GET_CODE (dest) == MEM
- || GET_CODE (dest) == UNSPEC
- || GET_CODE (dest) == UNSPEC_VOLATILE)
- *inherently_necessary_set_p = 1;
-}
-
-/* Mark X as inherently necessary if appropriate. For example,
- function calls and storing values into memory are inherently
- necessary. This function is to be used with for_each_rtx ().
- Return nonzero iff inherently necessary. */
-
-static int
-find_inherently_necessary (x)
- rtx x;
-{
- if (x == NULL_RTX)
- return 0;
- else if (inherently_necessary_register (x))
- return !0;
- else
- switch (GET_CODE (x))
- {
- case CALL_INSN:
- case BARRIER:
- case PREFETCH:
- return !0;
- case CODE_LABEL:
- case NOTE:
- return 0;
- case JUMP_INSN:
- return JUMP_TABLE_DATA_P (x) || computed_jump_p (x) != 0;
- case INSN:
- {
- int inherently_necessary_set = 0;
- note_stores (PATTERN (x),
- note_inherently_necessary_set,
- &inherently_necessary_set);
-
- /* If we found an inherently necessary set or an asm
- instruction, then we consider this insn inherently
- necessary. */
- return (inherently_necessary_set
- || GET_CODE (PATTERN (x)) == ASM_INPUT
- || asm_noperands (PATTERN (x)) >= 0);
- }
- default:
- /* Found an impossible insn type. */
- abort ();
- break;
- }
-}
-
-/* Propagate necessity through REG and SUBREG operands of CURRENT_RTX.
- This function is called with for_each_rtx () on necessary
- instructions. The DATA must be a varray of unprocessed
- instructions. */
-
-static int
-propagate_necessity_through_operand (current_rtx, data)
- rtx *current_rtx;
- void *data;
-{
- rtx x = *current_rtx;
- varray_type *unprocessed_instructions = (varray_type *) data;
-
- if (x == NULL_RTX)
- return 0;
- switch ( GET_CODE (x))
- {
- case REG:
- if (CONVERT_REGISTER_TO_SSA_P (REGNO (x)))
- {
- rtx insn = VARRAY_RTX (ssa_definition, REGNO (x));
- if (insn != NULL_RTX && UNNECESSARY_P (insn))
- {
- RESURRECT_INSN (insn);
- VARRAY_PUSH_RTX (*unprocessed_instructions, insn);
- }
- }
- return 0;
-
- default:
- return 0;
- }
-}
-
-/* Indicate all insns initially assumed to be unnecessary. */
-
-static void
-mark_all_insn_unnecessary ()
-{
- rtx insn;
- for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
- KILL_INSN (insn);
-}
-
-/* Find the label beginning block BB, adding one if necessary. */
-
-static rtx
-find_block_label (bb)
- basic_block bb;
-{
- rtx insn = bb->head;
- if (LABEL_P (insn))
- return insn;
- else
- {
- rtx new_label = emit_label_before (gen_label_rtx (), insn);
- if (insn == bb->head)
- bb->head = new_label;
- return new_label;
- }
-}
-
-/* Remove INSN, updating its basic block structure. */
-
-static void
-delete_insn_bb (insn)
- rtx insn;
-{
- if (!insn)
- abort ();
-
- /* Do not actually delete anything that is not an INSN.
-
- We can get here because we only consider INSNs as
- potentially necessary. We leave it to later passes
- to remove unnecessary notes, unused labels, etc. */
- if (! INSN_P (insn))
- return;
-
- delete_insn (insn);
-}
-\f
-/* Perform the dead-code elimination. */
-
-void
-ssa_eliminate_dead_code ()
-{
- int i;
- rtx insn;
- /* Necessary instructions with operands to explore. */
- varray_type unprocessed_instructions;
- /* Map element (b,e) is nonzero if the block is control dependent on
- edge. "cdbte" abbreviates control dependent block to edge. */
- control_dependent_block_to_edge_map cdbte;
- /* Element I is the immediate postdominator of block I. */
- int *pdom;
- struct edge_list *el;
-
- int max_insn_uid = get_max_uid ();
-
- /* Initialize the data structures. */
- mark_all_insn_unnecessary ();
- VARRAY_RTX_INIT (unprocessed_instructions, 64,
- "unprocessed instructions");
- cdbte = control_dependent_block_to_edge_map_create (n_basic_blocks);
-
- /* Prepare for use of BLOCK_NUM (). */
- connect_infinite_loops_to_exit ();
- /* Be careful not to clear the added edges. */
- compute_bb_for_insn (max_insn_uid);
-
- /* Compute control dependence. */
- pdom = (int *) xmalloc (n_basic_blocks * sizeof (int));
- for (i = 0; i < n_basic_blocks; ++i)
- pdom[i] = INVALID_BLOCK;
- calculate_dominance_info (pdom, NULL, CDI_POST_DOMINATORS);
- /* Assume there is a path from each node to the exit block. */
- for (i = 0; i < n_basic_blocks; ++i)
- if (pdom[i] == INVALID_BLOCK)
- pdom[i] = EXIT_BLOCK;
- el = create_edge_list ();
- find_all_control_dependences (el, pdom, cdbte);
-
- /* Find inherently necessary instructions. */
- for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
- if (find_inherently_necessary (insn))
- {
- RESURRECT_INSN (insn);
- VARRAY_PUSH_RTX (unprocessed_instructions, insn);
- }
-
- /* Propagate necessity using the operands of necessary instructions. */
- while (VARRAY_ACTIVE_SIZE (unprocessed_instructions) > 0)
- {
- rtx current_instruction;
- int edge_number;
-
- current_instruction = VARRAY_TOP_RTX (unprocessed_instructions);
- VARRAY_POP (unprocessed_instructions);
-
- /* Make corresponding control dependent edges necessary. */
- /* Assume the only JUMP_INSN is the block's last insn. It appears
- that the last instruction of the program need not be a
- JUMP_INSN. */
-
- if (INSN_P (current_instruction)
- && !JUMP_TABLE_DATA_P (current_instruction))
- {
- /* Notes and labels contain no interesting operands. */
- EXECUTE_IF_CONTROL_DEPENDENT
- (cdbte, current_instruction, edge_number,
- {
- rtx jump_insn = (INDEX_EDGE_PRED_BB (el, edge_number))->end;
- if (GET_CODE (jump_insn) == JUMP_INSN
- && UNNECESSARY_P (jump_insn))
- {
- RESURRECT_INSN (jump_insn);
- VARRAY_PUSH_RTX (unprocessed_instructions, jump_insn);
- }
- });
-
- /* Propagate through the operands. */
- for_each_rtx (¤t_instruction,
- &propagate_necessity_through_operand,
- (PTR) &unprocessed_instructions);
-
- /* PHI nodes are somewhat special in that each PHI alternative
- has data and control dependencies. The data dependencies
- are handled via propagate_necessity_through_operand. We
- handle the control dependency here.
-
- We consider the control dependent edges leading to the
- predecessor block associated with each PHI alternative
- as necessary. */
- if (PHI_NODE_P (current_instruction))
- {
- rtvec phi_vec = XVEC (SET_SRC (PATTERN (current_instruction)), 0);
- int num_elem = GET_NUM_ELEM (phi_vec);
- int v;
-
- for (v = num_elem - 2; v >= 0; v -= 2)
- {
- basic_block bb;
-
- bb = BASIC_BLOCK (INTVAL (RTVEC_ELT (phi_vec, v + 1)));
- EXECUTE_IF_CONTROL_DEPENDENT
- (cdbte, bb->end, edge_number,
- {
- rtx jump_insn;
-
- jump_insn = (INDEX_EDGE_PRED_BB (el, edge_number))->end;
- if (((GET_CODE (jump_insn) == JUMP_INSN))
- && UNNECESSARY_P (jump_insn))
- {
- RESURRECT_INSN (jump_insn);
- VARRAY_PUSH_RTX (unprocessed_instructions, jump_insn);
- }
- });
-
- }
- }
- }
- }
-
- /* Remove the unnecessary instructions. */
- EXECUTE_IF_UNNECESSARY (insn,
- {
- if (any_condjump_p (insn))
- {
- basic_block bb = BLOCK_FOR_INSN (insn);
- basic_block pdom_bb = find_pdom (pdom, bb);
- rtx lbl;
- edge e;
-
- /* Egad. The immediate post dominator is the exit block. We
- would like to optimize this conditional jump to jump directly
- to the exit block. That can be difficult as we may not have
- a suitable CODE_LABEL that allows us to fall unmolested into
- the exit block.
-
- So, we just delete the conditional branch by turning it into
- a deleted note. That is safe, but just not as optimal as
- it could be. */
- if (pdom_bb == EXIT_BLOCK_PTR)
- {
- /* Since we're going to just delete the branch, we need
- look at all the edges and remove all those which are not
- a fallthru edge. */
- e = bb->succ;
- while (e)
- {
- edge temp = e;
-
- e = e->succ_next;
- if ((temp->flags & EDGE_FALLTHRU) == 0)
- {
- /* We've found a non-fallthru edge, find any PHI nodes
- at the target and clean them up. */
- if (temp->dest != EXIT_BLOCK_PTR)
- {
- rtx insn
- = first_insn_after_basic_block_note (temp->dest);
-
- while (PHI_NODE_P (insn))
- {
- remove_phi_alternative (PATTERN (insn), temp->src);
- insn = NEXT_INSN (insn);
- }
- }
-
- remove_edge (temp);
- }
- }
-
- /* Now "delete" the conditional jump. */
- PUT_CODE (insn, NOTE);
- NOTE_LINE_NUMBER (insn) = NOTE_INSN_DELETED;
- continue;
- }
-
- /* We've found a conditional branch that is unnecessary.
-
- First, remove all outgoing edges from this block, updating
- PHI nodes as appropriate. */
- e = bb->succ;
- while (e)
- {
- edge temp = e;
-
- e = e->succ_next;
-
- if (temp->flags & EDGE_ABNORMAL)
- continue;
-
- /* We found an edge that is not executable. First simplify
- the PHI nodes in the target block. */
- if (temp->dest != EXIT_BLOCK_PTR)
- {
- rtx insn = first_insn_after_basic_block_note (temp->dest);
-
- while (PHI_NODE_P (insn))
- {
- remove_phi_alternative (PATTERN (insn), temp->src);
- insn = NEXT_INSN (insn);
- }
- }
-
- remove_edge (temp);
- }
-
- /* Create an edge from this block to the post dominator.
- What about the PHI nodes at the target? */
- make_edge (bb, pdom_bb, 0);
-
- /* Third, transform this insn into an unconditional
- jump to the label for the immediate postdominator. */
- lbl = find_block_label (pdom_bb);
- SET_SRC (PATTERN (insn)) = gen_rtx_LABEL_REF (VOIDmode, lbl);
- INSN_CODE (insn) = -1;
- JUMP_LABEL (insn) = lbl;
- LABEL_NUSES (lbl)++;
-
- /* A barrier must follow any unconditional jump. Barriers
- are not in basic blocks so this must occur after
- deleting the conditional jump. */
- emit_barrier_after (insn);
- }
- else if (!JUMP_P (insn))
- delete_insn_bb (insn);
- });
-
- /* Remove fake edges from the CFG. */
- remove_fake_edges ();
-
- /* Find any blocks with no successors and ensure they are followed
- by a BARRIER. delete_insn has the nasty habit of deleting barriers
- when deleting insns. */
- for (i = 0; i < n_basic_blocks; i++)
- {
- basic_block bb = BASIC_BLOCK (i);
-
- if (bb->succ == NULL)
- {
- rtx next = NEXT_INSN (bb->end);
-
- if (!next || GET_CODE (next) != BARRIER)
- emit_barrier_after (bb->end);
- }
- }
- /* Release allocated memory. */
- for (insn = get_insns (); insn != NULL_RTX; insn = NEXT_INSN (insn))
- RESURRECT_INSN (insn);
- if (VARRAY_ACTIVE_SIZE (unprocessed_instructions) != 0)
- abort ();
- VARRAY_FREE (unprocessed_instructions);
- control_dependent_block_to_edge_map_free (cdbte);
- free ((PTR) pdom);
- free_edge_list (el);
-}
projects / msp430-gcc.git / blobdiff