| /* Dead store elimination |
| Copyright (C) 2004-2019 Free Software Foundation, Inc. |
| |
| 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/>. */ |
| |
| #include "config.h" |
| #include "system.h" |
| #include "coretypes.h" |
| #include "backend.h" |
| #include "rtl.h" |
| #include "tree.h" |
| #include "gimple.h" |
| #include "tree-pass.h" |
| #include "ssa.h" |
| #include "gimple-pretty-print.h" |
| #include "fold-const.h" |
| #include "gimple-iterator.h" |
| #include "tree-cfg.h" |
| #include "tree-dfa.h" |
| #include "domwalk.h" |
| #include "tree-cfgcleanup.h" |
| #include "params.h" |
| #include "alias.h" |
| #include "tree-ssa-loop.h" |
| #include "gimplify.h" |
| |
| /* 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. */ |
| static bitmap need_eh_cleanup; |
| |
| /* Return value from dse_classify_store */ |
| enum dse_store_status |
| { |
| DSE_STORE_LIVE, |
| DSE_STORE_MAYBE_PARTIAL_DEAD, |
| DSE_STORE_DEAD |
| }; |
| |
| /* STMT is a statement that may write into memory. Analyze it and |
| initialize WRITE to describe how STMT affects memory. |
| |
| Return TRUE if the the statement was analyzed, FALSE otherwise. |
| |
| It is always safe to return FALSE. But typically better optimziation |
| can be achieved by analyzing more statements. */ |
| |
| static bool |
| initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write) |
| { |
| /* It's advantageous to handle certain mem* functions. */ |
| if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL)) |
| { |
| switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt))) |
| { |
| case BUILT_IN_MEMCPY: |
| case BUILT_IN_MEMMOVE: |
| case BUILT_IN_MEMSET: |
| { |
| tree size = NULL_TREE; |
| if (gimple_call_num_args (stmt) == 3) |
| size = gimple_call_arg (stmt, 2); |
| tree ptr = gimple_call_arg (stmt, 0); |
| ao_ref_init_from_ptr_and_size (write, ptr, size); |
| return true; |
| } |
| default: |
| break; |
| } |
| } |
| else if (is_gimple_assign (stmt)) |
| { |
| ao_ref_init (write, gimple_assign_lhs (stmt)); |
| return true; |
| } |
| return false; |
| } |
| |
| /* Given REF from the the alias oracle, return TRUE if it is a valid |
| memory reference for dead store elimination, false otherwise. |
| |
| In particular, the reference must have a known base, known maximum |
| size, start at a byte offset and have a size that is one or more |
| bytes. */ |
| |
| static bool |
| valid_ao_ref_for_dse (ao_ref *ref) |
| { |
| return (ao_ref_base (ref) |
| && known_size_p (ref->max_size) |
| && maybe_ne (ref->size, 0) |
| && known_eq (ref->max_size, ref->size) |
| && known_ge (ref->offset, 0) |
| && multiple_p (ref->offset, BITS_PER_UNIT) |
| && multiple_p (ref->size, BITS_PER_UNIT)); |
| } |
| |
| /* Try to normalize COPY (an ao_ref) relative to REF. Essentially when we are |
| done COPY will only refer bytes found within REF. Return true if COPY |
| is known to intersect at least one byte of REF. */ |
| |
| static bool |
| normalize_ref (ao_ref *copy, ao_ref *ref) |
| { |
| if (!ordered_p (copy->offset, ref->offset)) |
| return false; |
| |
| /* If COPY starts before REF, then reset the beginning of |
| COPY to match REF and decrease the size of COPY by the |
| number of bytes removed from COPY. */ |
| if (maybe_lt (copy->offset, ref->offset)) |
| { |
| poly_int64 diff = ref->offset - copy->offset; |
| if (maybe_le (copy->size, diff)) |
| return false; |
| copy->size -= diff; |
| copy->offset = ref->offset; |
| } |
| |
| poly_int64 diff = copy->offset - ref->offset; |
| if (maybe_le (ref->size, diff)) |
| return false; |
| |
| /* If COPY extends beyond REF, chop off its size appropriately. */ |
| poly_int64 limit = ref->size - diff; |
| if (!ordered_p (limit, copy->size)) |
| return false; |
| |
| if (maybe_gt (copy->size, limit)) |
| copy->size = limit; |
| return true; |
| } |
| |
| /* Clear any bytes written by STMT from the bitmap LIVE_BYTES. The base |
| address written by STMT must match the one found in REF, which must |
| have its base address previously initialized. |
| |
| This routine must be conservative. If we don't know the offset or |
| actual size written, assume nothing was written. */ |
| |
| static void |
| clear_bytes_written_by (sbitmap live_bytes, gimple *stmt, ao_ref *ref) |
| { |
| ao_ref write; |
| if (!initialize_ao_ref_for_dse (stmt, &write)) |
| return; |
| |
| /* Verify we have the same base memory address, the write |
| has a known size and overlaps with REF. */ |
| HOST_WIDE_INT start, size; |
| if (valid_ao_ref_for_dse (&write) |
| && operand_equal_p (write.base, ref->base, OEP_ADDRESS_OF) |
| && known_eq (write.size, write.max_size) |
| && normalize_ref (&write, ref) |
| && (write.offset - ref->offset).is_constant (&start) |
| && write.size.is_constant (&size)) |
| bitmap_clear_range (live_bytes, start / BITS_PER_UNIT, |
| size / BITS_PER_UNIT); |
| } |
| |
| /* REF is a memory write. Extract relevant information from it and |
| initialize the LIVE_BYTES bitmap. If successful, return TRUE. |
| Otherwise return FALSE. */ |
| |
| static bool |
| setup_live_bytes_from_ref (ao_ref *ref, sbitmap live_bytes) |
| { |
| HOST_WIDE_INT const_size; |
| if (valid_ao_ref_for_dse (ref) |
| && ref->size.is_constant (&const_size) |
| && (const_size / BITS_PER_UNIT |
| <= PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE))) |
| { |
| bitmap_clear (live_bytes); |
| bitmap_set_range (live_bytes, 0, const_size / BITS_PER_UNIT); |
| return true; |
| } |
| return false; |
| } |
| |
| /* Compute the number of elements that we can trim from the head and |
| tail of ORIG resulting in a bitmap that is a superset of LIVE. |
| |
| Store the number of elements trimmed from the head and tail in |
| TRIM_HEAD and TRIM_TAIL. |
| |
| STMT is the statement being trimmed and is used for debugging dump |
| output only. */ |
| |
| static void |
| compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail, |
| gimple *stmt) |
| { |
| /* We use sbitmaps biased such that ref->offset is bit zero and the bitmap |
| extends through ref->size. So we know that in the original bitmap |
| bits 0..ref->size were true. We don't actually need the bitmap, just |
| the REF to compute the trims. */ |
| |
| /* Now identify how much, if any of the tail we can chop off. */ |
| HOST_WIDE_INT const_size; |
| int last_live = bitmap_last_set_bit (live); |
| if (ref->size.is_constant (&const_size)) |
| { |
| int last_orig = (const_size / BITS_PER_UNIT) - 1; |
| /* We can leave inconvenient amounts on the tail as |
| residual handling in mem* and str* functions is usually |
| reasonably efficient. */ |
| *trim_tail = last_orig - last_live; |
| |
| /* But don't trim away out of bounds accesses, as this defeats |
| proper warnings. |
| |
| We could have a type with no TYPE_SIZE_UNIT or we could have a VLA |
| where TYPE_SIZE_UNIT is not a constant. */ |
| if (*trim_tail |
| && TYPE_SIZE_UNIT (TREE_TYPE (ref->base)) |
| && TREE_CODE (TYPE_SIZE_UNIT (TREE_TYPE (ref->base))) == INTEGER_CST |
| && compare_tree_int (TYPE_SIZE_UNIT (TREE_TYPE (ref->base)), |
| last_orig) <= 0) |
| *trim_tail = 0; |
| } |
| else |
| *trim_tail = 0; |
| |
| /* Identify how much, if any of the head we can chop off. */ |
| int first_orig = 0; |
| int first_live = bitmap_first_set_bit (live); |
| *trim_head = first_live - first_orig; |
| |
| /* If more than a word remains, then make sure to keep the |
| starting point at least word aligned. */ |
| if (last_live - first_live > UNITS_PER_WORD) |
| *trim_head &= ~(UNITS_PER_WORD - 1); |
| |
| if ((*trim_head || *trim_tail) |
| && dump_file && (dump_flags & TDF_DETAILS)) |
| { |
| fprintf (dump_file, " Trimming statement (head = %d, tail = %d): ", |
| *trim_head, *trim_tail); |
| print_gimple_stmt (dump_file, stmt, 0, dump_flags); |
| fprintf (dump_file, "\n"); |
| } |
| } |
| |
| /* STMT initializes an object from COMPLEX_CST where one or more of the |
| bytes written may be dead stores. REF is a representation of the |
| memory written. LIVE is the bitmap of stores that are actually live. |
| |
| Attempt to rewrite STMT so that only the real or imaginary part of |
| the object is actually stored. */ |
| |
| static void |
| maybe_trim_complex_store (ao_ref *ref, sbitmap live, gimple *stmt) |
| { |
| int trim_head, trim_tail; |
| compute_trims (ref, live, &trim_head, &trim_tail, stmt); |
| |
| /* The amount of data trimmed from the head or tail must be at |
| least half the size of the object to ensure we're trimming |
| the entire real or imaginary half. By writing things this |
| way we avoid more O(n) bitmap operations. */ |
| if (known_ge (trim_tail * 2 * BITS_PER_UNIT, ref->size)) |
| { |
| /* TREE_REALPART is live */ |
| tree x = TREE_REALPART (gimple_assign_rhs1 (stmt)); |
| tree y = gimple_assign_lhs (stmt); |
| y = build1 (REALPART_EXPR, TREE_TYPE (x), y); |
| gimple_assign_set_lhs (stmt, y); |
| gimple_assign_set_rhs1 (stmt, x); |
| } |
| else if (known_ge (trim_head * 2 * BITS_PER_UNIT, ref->size)) |
| { |
| /* TREE_IMAGPART is live */ |
| tree x = TREE_IMAGPART (gimple_assign_rhs1 (stmt)); |
| tree y = gimple_assign_lhs (stmt); |
| y = build1 (IMAGPART_EXPR, TREE_TYPE (x), y); |
| gimple_assign_set_lhs (stmt, y); |
| gimple_assign_set_rhs1 (stmt, x); |
| } |
| |
| /* Other cases indicate parts of both the real and imag subobjects |
| are live. We do not try to optimize those cases. */ |
| } |
| |
| /* STMT initializes an object using a CONSTRUCTOR where one or more of the |
| bytes written are dead stores. ORIG is the bitmap of bytes stored by |
| STMT. LIVE is the bitmap of stores that are actually live. |
| |
| Attempt to rewrite STMT so that only the real or imaginary part of |
| the object is actually stored. |
| |
| The most common case for getting here is a CONSTRUCTOR with no elements |
| being used to zero initialize an object. We do not try to handle other |
| cases as those would force us to fully cover the object with the |
| CONSTRUCTOR node except for the components that are dead. */ |
| |
| static void |
| maybe_trim_constructor_store (ao_ref *ref, sbitmap live, gimple *stmt) |
| { |
| tree ctor = gimple_assign_rhs1 (stmt); |
| |
| /* This is the only case we currently handle. It actually seems to |
| catch most cases of actual interest. */ |
| gcc_assert (CONSTRUCTOR_NELTS (ctor) == 0); |
| |
| int head_trim = 0; |
| int tail_trim = 0; |
| compute_trims (ref, live, &head_trim, &tail_trim, stmt); |
| |
| /* Now we want to replace the constructor initializer |
| with memset (object + head_trim, 0, size - head_trim - tail_trim). */ |
| if (head_trim || tail_trim) |
| { |
| /* We want &lhs for the MEM_REF expression. */ |
| tree lhs_addr = build_fold_addr_expr (gimple_assign_lhs (stmt)); |
| |
| if (! is_gimple_min_invariant (lhs_addr)) |
| return; |
| |
| /* The number of bytes for the new constructor. */ |
| poly_int64 ref_bytes = exact_div (ref->size, BITS_PER_UNIT); |
| poly_int64 count = ref_bytes - head_trim - tail_trim; |
| |
| /* And the new type for the CONSTRUCTOR. Essentially it's just |
| a char array large enough to cover the non-trimmed parts of |
| the original CONSTRUCTOR. Note we want explicit bounds here |
| so that we know how many bytes to clear when expanding the |
| CONSTRUCTOR. */ |
| tree type = build_array_type_nelts (char_type_node, count); |
| |
| /* Build a suitable alias type rather than using alias set zero |
| to avoid pessimizing. */ |
| tree alias_type = reference_alias_ptr_type (gimple_assign_lhs (stmt)); |
| |
| /* Build a MEM_REF representing the whole accessed area, starting |
| at the first byte not trimmed. */ |
| tree exp = fold_build2 (MEM_REF, type, lhs_addr, |
| build_int_cst (alias_type, head_trim)); |
| |
| /* Now update STMT with a new RHS and LHS. */ |
| gimple_assign_set_lhs (stmt, exp); |
| gimple_assign_set_rhs1 (stmt, build_constructor (type, NULL)); |
| } |
| } |
| |
| /* STMT is a memcpy, memmove or memset. Decrement the number of bytes |
| copied/set by DECREMENT. */ |
| static void |
| decrement_count (gimple *stmt, int decrement) |
| { |
| tree *countp = gimple_call_arg_ptr (stmt, 2); |
| gcc_assert (TREE_CODE (*countp) == INTEGER_CST); |
| *countp = wide_int_to_tree (TREE_TYPE (*countp), (TREE_INT_CST_LOW (*countp) |
| - decrement)); |
| } |
| |
| static void |
| increment_start_addr (gimple *stmt, tree *where, int increment) |
| { |
| if (tree lhs = gimple_call_lhs (stmt)) |
| if (where == gimple_call_arg_ptr (stmt, 0)) |
| { |
| gassign *newop = gimple_build_assign (lhs, unshare_expr (*where)); |
| gimple_stmt_iterator gsi = gsi_for_stmt (stmt); |
| gsi_insert_after (&gsi, newop, GSI_SAME_STMT); |
| gimple_call_set_lhs (stmt, NULL_TREE); |
| update_stmt (stmt); |
| } |
| |
| if (TREE_CODE (*where) == SSA_NAME) |
| { |
| tree tem = make_ssa_name (TREE_TYPE (*where)); |
| gassign *newop |
| = gimple_build_assign (tem, POINTER_PLUS_EXPR, *where, |
| build_int_cst (sizetype, increment)); |
| gimple_stmt_iterator gsi = gsi_for_stmt (stmt); |
| gsi_insert_before (&gsi, newop, GSI_SAME_STMT); |
| *where = tem; |
| update_stmt (stmt); |
| return; |
| } |
| |
| *where = build_fold_addr_expr (fold_build2 (MEM_REF, char_type_node, |
| *where, |
| build_int_cst (ptr_type_node, |
| increment))); |
| } |
| |
| /* STMT is builtin call that writes bytes in bitmap ORIG, some bytes are dead |
| (ORIG & ~NEW) and need not be stored. Try to rewrite STMT to reduce |
| the amount of data it actually writes. |
| |
| Right now we only support trimming from the head or the tail of the |
| memory region. In theory we could split the mem* call, but it's |
| likely of marginal value. */ |
| |
| static void |
| maybe_trim_memstar_call (ao_ref *ref, sbitmap live, gimple *stmt) |
| { |
| switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt))) |
| { |
| case BUILT_IN_MEMCPY: |
| case BUILT_IN_MEMMOVE: |
| { |
| int head_trim, tail_trim; |
| compute_trims (ref, live, &head_trim, &tail_trim, stmt); |
| |
| /* Tail trimming is easy, we can just reduce the count. */ |
| if (tail_trim) |
| decrement_count (stmt, tail_trim); |
| |
| /* Head trimming requires adjusting all the arguments. */ |
| if (head_trim) |
| { |
| tree *dst = gimple_call_arg_ptr (stmt, 0); |
| increment_start_addr (stmt, dst, head_trim); |
| tree *src = gimple_call_arg_ptr (stmt, 1); |
| increment_start_addr (stmt, src, head_trim); |
| decrement_count (stmt, head_trim); |
| } |
| break; |
| } |
| |
| case BUILT_IN_MEMSET: |
| { |
| int head_trim, tail_trim; |
| compute_trims (ref, live, &head_trim, &tail_trim, stmt); |
| |
| /* Tail trimming is easy, we can just reduce the count. */ |
| if (tail_trim) |
| decrement_count (stmt, tail_trim); |
| |
| /* Head trimming requires adjusting all the arguments. */ |
| if (head_trim) |
| { |
| tree *dst = gimple_call_arg_ptr (stmt, 0); |
| increment_start_addr (stmt, dst, head_trim); |
| decrement_count (stmt, head_trim); |
| } |
| break; |
| } |
| |
| default: |
| break; |
| } |
| } |
| |
| /* STMT is a memory write where one or more bytes written are dead |
| stores. ORIG is the bitmap of bytes stored by STMT. LIVE is the |
| bitmap of stores that are actually live. |
| |
| Attempt to rewrite STMT so that it writes fewer memory locations. Right |
| now we only support trimming at the start or end of the memory region. |
| It's not clear how much there is to be gained by trimming from the middle |
| of the region. */ |
| |
| static void |
| maybe_trim_partially_dead_store (ao_ref *ref, sbitmap live, gimple *stmt) |
| { |
| if (is_gimple_assign (stmt) |
| && TREE_CODE (gimple_assign_lhs (stmt)) != TARGET_MEM_REF) |
| { |
| switch (gimple_assign_rhs_code (stmt)) |
| { |
| case CONSTRUCTOR: |
| maybe_trim_constructor_store (ref, live, stmt); |
| break; |
| case COMPLEX_CST: |
| maybe_trim_complex_store (ref, live, stmt); |
| break; |
| default: |
| break; |
| } |
| } |
| } |
| |
| /* Return TRUE if USE_REF reads bytes from LIVE where live is |
| derived from REF, a write reference. |
| |
| While this routine may modify USE_REF, it's passed by value, not |
| location. So callers do not see those modifications. */ |
| |
| static bool |
| live_bytes_read (ao_ref use_ref, ao_ref *ref, sbitmap live) |
| { |
| /* We have already verified that USE_REF and REF hit the same object. |
| Now verify that there's actually an overlap between USE_REF and REF. */ |
| HOST_WIDE_INT start, size; |
| if (normalize_ref (&use_ref, ref) |
| && (use_ref.offset - ref->offset).is_constant (&start) |
| && use_ref.size.is_constant (&size)) |
| { |
| /* If USE_REF covers all of REF, then it will hit one or more |
| live bytes. This avoids useless iteration over the bitmap |
| below. */ |
| if (start == 0 && known_eq (size, ref->size)) |
| return true; |
| |
| /* Now check if any of the remaining bits in use_ref are set in LIVE. */ |
| return bitmap_bit_in_range_p (live, start / BITS_PER_UNIT, |
| (start + size - 1) / BITS_PER_UNIT); |
| } |
| return true; |
| } |
| |
| /* Callback for dse_classify_store calling for_each_index. Verify that |
| indices are invariant in the loop with backedge PHI in basic-block DATA. */ |
| |
| static bool |
| check_name (tree, tree *idx, void *data) |
| { |
| basic_block phi_bb = (basic_block) data; |
| if (TREE_CODE (*idx) == SSA_NAME |
| && !SSA_NAME_IS_DEFAULT_DEF (*idx) |
| && dominated_by_p (CDI_DOMINATORS, gimple_bb (SSA_NAME_DEF_STMT (*idx)), |
| phi_bb)) |
| return false; |
| return true; |
| } |
| |
| /* A helper of dse_optimize_stmt. |
| Given a GIMPLE_ASSIGN in STMT that writes to REF, classify it |
| according to downstream uses and defs. Sets *BY_CLOBBER_P to true |
| if only clobber statements influenced the classification result. |
| Returns the classification. */ |
| |
| static dse_store_status |
| dse_classify_store (ao_ref *ref, gimple *stmt, |
| bool byte_tracking_enabled, sbitmap live_bytes, |
| bool *by_clobber_p = NULL) |
| { |
| gimple *temp; |
| int cnt = 0; |
| auto_bitmap visited; |
| |
| if (by_clobber_p) |
| *by_clobber_p = true; |
| |
| /* 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. */ |
| temp = stmt; |
| do |
| { |
| gimple *use_stmt; |
| imm_use_iterator ui; |
| bool fail = false; |
| tree defvar; |
| |
| if (gimple_code (temp) == GIMPLE_PHI) |
| { |
| /* If we visit this PHI by following a backedge then we have to |
| make sure ref->ref only refers to SSA names that are invariant |
| with respect to the loop represented by this PHI node. */ |
| if (dominated_by_p (CDI_DOMINATORS, gimple_bb (stmt), |
| gimple_bb (temp)) |
| && !for_each_index (ref->ref ? &ref->ref : &ref->base, |
| check_name, gimple_bb (temp))) |
| return DSE_STORE_LIVE; |
| defvar = PHI_RESULT (temp); |
| bitmap_set_bit (visited, SSA_NAME_VERSION (defvar)); |
| } |
| else |
| defvar = gimple_vdef (temp); |
| auto_vec<gimple *, 10> defs; |
| gimple *phi_def = NULL; |
| FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar) |
| { |
| /* Limit stmt walking. */ |
| if (++cnt > PARAM_VALUE (PARAM_DSE_MAX_ALIAS_QUERIES_PER_STORE)) |
| { |
| fail = true; |
| BREAK_FROM_IMM_USE_STMT (ui); |
| } |
| |
| /* We have visited ourselves already so ignore STMT for the |
| purpose of chaining. */ |
| if (use_stmt == stmt) |
| ; |
| /* 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. */ |
| else if (gimple_code (use_stmt) == GIMPLE_PHI) |
| { |
| /* If we already visited this PHI ignore it for further |
| processing. */ |
| if (!bitmap_bit_p (visited, |
| SSA_NAME_VERSION (PHI_RESULT (use_stmt)))) |
| { |
| defs.safe_push (use_stmt); |
| phi_def = use_stmt; |
| } |
| } |
| /* If the statement is a use the store is not dead. */ |
| else if (ref_maybe_used_by_stmt_p (use_stmt, ref)) |
| { |
| /* Handle common cases where we can easily build an ao_ref |
| structure for USE_STMT and in doing so we find that the |
| references hit non-live bytes and thus can be ignored. */ |
| if (byte_tracking_enabled |
| && is_gimple_assign (use_stmt)) |
| { |
| ao_ref use_ref; |
| ao_ref_init (&use_ref, gimple_assign_rhs1 (use_stmt)); |
| if (valid_ao_ref_for_dse (&use_ref) |
| && use_ref.base == ref->base |
| && known_eq (use_ref.size, use_ref.max_size) |
| && !live_bytes_read (use_ref, ref, live_bytes)) |
| { |
| /* If this is a store, remember it as we possibly |
| need to walk the defs uses. */ |
| if (gimple_vdef (use_stmt)) |
| defs.safe_push (use_stmt); |
| continue; |
| } |
| } |
| |
| fail = true; |
| BREAK_FROM_IMM_USE_STMT (ui); |
| } |
| /* If this is a store, remember it as we possibly need to walk the |
| defs uses. */ |
| else if (gimple_vdef (use_stmt)) |
| defs.safe_push (use_stmt); |
| } |
| |
| if (fail) |
| { |
| /* STMT might be partially dead and we may be able to reduce |
| how many memory locations it stores into. */ |
| if (byte_tracking_enabled && !gimple_clobber_p (stmt)) |
| return DSE_STORE_MAYBE_PARTIAL_DEAD; |
| return DSE_STORE_LIVE; |
| } |
| |
| /* 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. */ |
| if (defs.is_empty ()) |
| { |
| if (ref_may_alias_global_p (ref)) |
| return DSE_STORE_LIVE; |
| |
| if (by_clobber_p) |
| *by_clobber_p = false; |
| return DSE_STORE_DEAD; |
| } |
| |
| /* Process defs and remove those we need not process further. */ |
| for (unsigned i = 0; i < defs.length ();) |
| { |
| gimple *def = defs[i]; |
| gimple *use_stmt; |
| use_operand_p use_p; |
| /* If the path to check starts with a kill we do not need to |
| process it further. |
| ??? With byte tracking we need only kill the bytes currently |
| live. */ |
| if (stmt_kills_ref_p (def, ref)) |
| { |
| if (by_clobber_p && !gimple_clobber_p (def)) |
| *by_clobber_p = false; |
| defs.unordered_remove (i); |
| } |
| /* In addition to kills we can remove defs whose only use |
| is another def in defs. That can only ever be PHIs of which |
| we track a single for simplicity reasons (we fail for multiple |
| PHIs anyways). We can also ignore defs that feed only into |
| already visited PHIs. */ |
| else if (gimple_code (def) != GIMPLE_PHI |
| && single_imm_use (gimple_vdef (def), &use_p, &use_stmt) |
| && (use_stmt == phi_def |
| || (gimple_code (use_stmt) == GIMPLE_PHI |
| && bitmap_bit_p (visited, |
| SSA_NAME_VERSION |
| (PHI_RESULT (use_stmt)))))) |
| defs.unordered_remove (i); |
| else |
| ++i; |
| } |
| |
| /* If all defs kill the ref we are done. */ |
| if (defs.is_empty ()) |
| return DSE_STORE_DEAD; |
| /* If more than one def survives fail. */ |
| if (defs.length () > 1) |
| { |
| /* STMT might be partially dead and we may be able to reduce |
| how many memory locations it stores into. */ |
| if (byte_tracking_enabled && !gimple_clobber_p (stmt)) |
| return DSE_STORE_MAYBE_PARTIAL_DEAD; |
| return DSE_STORE_LIVE; |
| } |
| temp = defs[0]; |
| |
| /* Track partial kills. */ |
| if (byte_tracking_enabled) |
| { |
| clear_bytes_written_by (live_bytes, temp, ref); |
| if (bitmap_empty_p (live_bytes)) |
| { |
| if (by_clobber_p && !gimple_clobber_p (temp)) |
| *by_clobber_p = false; |
| return DSE_STORE_DEAD; |
| } |
| } |
| } |
| /* Continue walking until there are no more live bytes. */ |
| while (1); |
| } |
| |
| |
| class dse_dom_walker : public dom_walker |
| { |
| public: |
| dse_dom_walker (cdi_direction direction) |
| : dom_walker (direction), |
| m_live_bytes (PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE)), |
| m_byte_tracking_enabled (false) {} |
| |
| virtual edge before_dom_children (basic_block); |
| |
| private: |
| auto_sbitmap m_live_bytes; |
| bool m_byte_tracking_enabled; |
| void dse_optimize_stmt (gimple_stmt_iterator *); |
| }; |
| |
| /* Delete a dead call at GSI, which is mem* call of some kind. */ |
| static void |
| delete_dead_call (gimple_stmt_iterator *gsi) |
| { |
| gimple *stmt = gsi_stmt (*gsi); |
| if (dump_file && (dump_flags & TDF_DETAILS)) |
| { |
| fprintf (dump_file, " Deleted dead call: "); |
| print_gimple_stmt (dump_file, stmt, 0, dump_flags); |
| fprintf (dump_file, "\n"); |
| } |
| |
| tree lhs = gimple_call_lhs (stmt); |
| if (lhs) |
| { |
| tree ptr = gimple_call_arg (stmt, 0); |
| gimple *new_stmt = gimple_build_assign (lhs, ptr); |
| unlink_stmt_vdef (stmt); |
| if (gsi_replace (gsi, new_stmt, true)) |
| bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index); |
| } |
| else |
| { |
| /* Then we need to fix the operand of the consuming stmt. */ |
| unlink_stmt_vdef (stmt); |
| |
| /* Remove the dead store. */ |
| if (gsi_remove (gsi, true)) |
| bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index); |
| release_defs (stmt); |
| } |
| } |
| |
| /* Delete a dead store at GSI, which is a gimple assignment. */ |
| |
| static void |
| delete_dead_assignment (gimple_stmt_iterator *gsi) |
| { |
| gimple *stmt = gsi_stmt (*gsi); |
| if (dump_file && (dump_flags & TDF_DETAILS)) |
| { |
| fprintf (dump_file, " Deleted dead store: "); |
| print_gimple_stmt (dump_file, stmt, 0, dump_flags); |
| fprintf (dump_file, "\n"); |
| } |
| |
| /* Then we need to fix the operand of the consuming stmt. */ |
| unlink_stmt_vdef (stmt); |
| |
| /* Remove the dead store. */ |
| basic_block bb = gimple_bb (stmt); |
| if (gsi_remove (gsi, true)) |
| bitmap_set_bit (need_eh_cleanup, bb->index); |
| |
| /* And release any SSA_NAMEs set in this statement back to the |
| SSA_NAME manager. */ |
| release_defs (stmt); |
| } |
| |
| /* 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. */ |
| |
| void |
| dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi) |
| { |
| gimple *stmt = gsi_stmt (*gsi); |
| |
| /* If this statement has no virtual defs, then there is nothing |
| to do. */ |
| if (!gimple_vdef (stmt)) |
| return; |
| |
| /* Don't return early on *this_2(D) ={v} {CLOBBER}. */ |
| if (gimple_has_volatile_ops (stmt) |
| && (!gimple_clobber_p (stmt) |
| || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF)) |
| return; |
| |
| ao_ref ref; |
| if (!initialize_ao_ref_for_dse (stmt, &ref)) |
| return; |
| |
| /* We know we have virtual definitions. We can handle assignments and |
| some builtin calls. */ |
| if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL)) |
| { |
| switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt))) |
| { |
| case BUILT_IN_MEMCPY: |
| case BUILT_IN_MEMMOVE: |
| case BUILT_IN_MEMSET: |
| { |
| /* Occasionally calls with an explicit length of zero |
| show up in the IL. It's pointless to do analysis |
| on them, they're trivially dead. */ |
| tree size = gimple_call_arg (stmt, 2); |
| if (integer_zerop (size)) |
| { |
| delete_dead_call (gsi); |
| return; |
| } |
| |
| enum dse_store_status store_status; |
| m_byte_tracking_enabled |
| = setup_live_bytes_from_ref (&ref, m_live_bytes); |
| store_status = dse_classify_store (&ref, stmt, |
| m_byte_tracking_enabled, |
| m_live_bytes); |
| if (store_status == DSE_STORE_LIVE) |
| return; |
| |
| if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD) |
| { |
| maybe_trim_memstar_call (&ref, m_live_bytes, stmt); |
| return; |
| } |
| |
| if (store_status == DSE_STORE_DEAD) |
| delete_dead_call (gsi); |
| return; |
| } |
| |
| default: |
| return; |
| } |
| } |
| |
| if (is_gimple_assign (stmt)) |
| { |
| bool by_clobber_p = false; |
| |
| /* Self-assignments are zombies. */ |
| if (operand_equal_p (gimple_assign_rhs1 (stmt), |
| gimple_assign_lhs (stmt), 0)) |
| ; |
| else |
| { |
| m_byte_tracking_enabled |
| = setup_live_bytes_from_ref (&ref, m_live_bytes); |
| enum dse_store_status store_status; |
| store_status = dse_classify_store (&ref, stmt, |
| m_byte_tracking_enabled, |
| m_live_bytes, &by_clobber_p); |
| if (store_status == DSE_STORE_LIVE) |
| return; |
| |
| if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD) |
| { |
| maybe_trim_partially_dead_store (&ref, m_live_bytes, stmt); |
| return; |
| } |
| } |
| |
| /* Now we know that use_stmt kills the LHS of stmt. */ |
| |
| /* But only remove *this_2(D) ={v} {CLOBBER} if killed by |
| another clobber stmt. */ |
| if (gimple_clobber_p (stmt) |
| && !by_clobber_p) |
| return; |
| |
| delete_dead_assignment (gsi); |
| } |
| } |
| |
| edge |
| dse_dom_walker::before_dom_children (basic_block bb) |
| { |
| gimple_stmt_iterator gsi; |
| |
| for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);) |
| { |
| dse_optimize_stmt (&gsi); |
| if (gsi_end_p (gsi)) |
| gsi = gsi_last_bb (bb); |
| else |
| gsi_prev (&gsi); |
| } |
| return NULL; |
| } |
| |
| namespace { |
| |
| const pass_data pass_data_dse = |
| { |
| GIMPLE_PASS, /* type */ |
| "dse", /* name */ |
| OPTGROUP_NONE, /* optinfo_flags */ |
| TV_TREE_DSE, /* tv_id */ |
| ( PROP_cfg | PROP_ssa ), /* properties_required */ |
| 0, /* properties_provided */ |
| 0, /* properties_destroyed */ |
| 0, /* todo_flags_start */ |
| 0, /* todo_flags_finish */ |
| }; |
| |
| class pass_dse : public gimple_opt_pass |
| { |
| public: |
| pass_dse (gcc::context *ctxt) |
| : gimple_opt_pass (pass_data_dse, ctxt) |
| {} |
| |
| /* opt_pass methods: */ |
| opt_pass * clone () { return new pass_dse (m_ctxt); } |
| virtual bool gate (function *) { return flag_tree_dse != 0; } |
| virtual unsigned int execute (function *); |
| |
| }; // class pass_dse |
| |
| unsigned int |
| pass_dse::execute (function *fun) |
| { |
| need_eh_cleanup = BITMAP_ALLOC (NULL); |
| |
| renumber_gimple_stmt_uids (cfun); |
| |
| /* 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. */ |
| calculate_dominance_info (CDI_POST_DOMINATORS); |
| calculate_dominance_info (CDI_DOMINATORS); |
| |
| /* Dead store elimination is fundamentally a walk of the post-dominator |
| tree and a backwards walk of statements within each block. */ |
| dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr); |
| |
| /* Removal of stores may make some EH edges dead. Purge such edges from |
| the CFG as needed. */ |
| if (!bitmap_empty_p (need_eh_cleanup)) |
| { |
| gimple_purge_all_dead_eh_edges (need_eh_cleanup); |
| cleanup_tree_cfg (); |
| } |
| |
| BITMAP_FREE (need_eh_cleanup); |
| |
| /* For now, just wipe the post-dominator information. */ |
| free_dominance_info (CDI_POST_DOMINATORS); |
| return 0; |
| } |
| |
| } // anon namespace |
| |
| gimple_opt_pass * |
| make_pass_dse (gcc::context *ctxt) |
| { |
| return new pass_dse (ctxt); |
| } |