/* Dead and redundant store elimination
Copyright (C) 2004-2020 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
. */
#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 "alias.h"
#include "tree-ssa-loop.h"
#include "tree-ssa-dse.h"
#include "builtins.h"
#include "gimple-fold.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 or trimmed if the store
was partially dead.
A redundant store is a store into a memory location which stores
the exact same value as a prior store to the same memory location.
While this can often be handled by dead store elimination, removing
the redundant store is often better than removing or trimming the
dead store.
In our SSA + virtual operand world we use immediate uses of virtual
operands to detect these cases. 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
the data stored is the same, then the second store is redundant.
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 makes the first dead. If
the second store stores the same value, then the second store 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. */
static void delete_dead_or_redundant_call (gimple_stmt_iterator *, const char *);
/* Bitmap of blocks that have had EH statements cleaned. We should
remove their dead edges eventually. */
static bitmap need_eh_cleanup;
/* STMT is a statement that may write into memory. Analyze it and
initialize WRITE to describe how STMT affects memory.
Return TRUE if 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:
case BUILT_IN_MEMCPY_CHK:
case BUILT_IN_MEMMOVE_CHK:
case BUILT_IN_MEMSET_CHK:
case BUILT_IN_STRNCPY:
case BUILT_IN_STRNCPY_CHK:
{
tree 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;
}
/* A calloc call can never be dead, but it can make
subsequent stores redundant if they store 0 into
the same memory locations. */
case BUILT_IN_CALLOC:
{
tree nelem = gimple_call_arg (stmt, 0);
tree selem = gimple_call_arg (stmt, 1);
tree lhs;
if (TREE_CODE (nelem) == INTEGER_CST
&& TREE_CODE (selem) == INTEGER_CST
&& (lhs = gimple_call_lhs (stmt)) != NULL_TREE)
{
tree size = fold_build2 (MULT_EXPR, TREE_TYPE (nelem),
nelem, selem);
ao_ref_init_from_ptr_and_size (write, lhs, 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 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_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)
{
int head_trim, tail_trim;
switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt)))
{
case BUILT_IN_STRNCPY:
case BUILT_IN_STRNCPY_CHK:
compute_trims (ref, live, &head_trim, &tail_trim, stmt);
if (head_trim)
{
/* Head trimming of strncpy is only possible if we can
prove all bytes we would trim are non-zero (or we could
turn the strncpy into memset if there must be zero
among the head trimmed bytes). If we don't know anything
about those bytes, the presence or absence of '\0' bytes
in there will affect whether it acts for the non-trimmed
bytes as memset or memcpy/strncpy. */
c_strlen_data lendata = { };
int orig_head_trim = head_trim;
tree srcstr = gimple_call_arg (stmt, 1);
if (!get_range_strlen (srcstr, &lendata, /*eltsize=*/1)
|| !tree_fits_uhwi_p (lendata.minlen))
head_trim = 0;
else if (tree_to_uhwi (lendata.minlen) < (unsigned) head_trim)
{
head_trim = tree_to_uhwi (lendata.minlen);
if ((orig_head_trim & (UNITS_PER_WORD - 1)) == 0)
head_trim &= ~(UNITS_PER_WORD - 1);
}
if (orig_head_trim != head_trim
&& dump_file
&& (dump_flags & TDF_DETAILS))
fprintf (dump_file,
" Adjusting strncpy trimming to (head = %d,"
" tail = %d)\n", head_trim, tail_trim);
}
goto do_memcpy;
case BUILT_IN_MEMCPY:
case BUILT_IN_MEMMOVE:
case BUILT_IN_MEMCPY_CHK:
case BUILT_IN_MEMMOVE_CHK:
compute_trims (ref, live, &head_trim, &tail_trim, stmt);
do_memcpy:
/* 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)
{
/* For __*_chk need to adjust also the last argument. */
if (gimple_call_num_args (stmt) == 4)
{
tree size = gimple_call_arg (stmt, 3);
if (!tree_fits_uhwi_p (size))
break;
if (!integer_all_onesp (size))
{
unsigned HOST_WIDE_INT sz = tree_to_uhwi (size);
if (sz < (unsigned) head_trim)
break;
tree arg = wide_int_to_tree (TREE_TYPE (size),
sz - head_trim);
gimple_call_set_arg (stmt, 3, arg);
}
}
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:
case BUILT_IN_MEMSET_CHK:
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)
{
/* For __*_chk need to adjust also the last argument. */
if (gimple_call_num_args (stmt) == 4)
{
tree size = gimple_call_arg (stmt, 3);
if (!tree_fits_uhwi_p (size))
break;
if (!integer_all_onesp (size))
{
unsigned HOST_WIDE_INT sz = tree_to_uhwi (size);
if (sz < (unsigned) head_trim)
break;
tree arg = wide_int_to_tree (TREE_TYPE (size),
sz - head_trim);
gimple_call_set_arg (stmt, 3, arg);
}
}
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;
}
/* STMT stores the value 0 into one or more memory locations
(via memset, empty constructor, calloc call, etc).
See if there is a subsequent store of the value 0 to one
or more of the same memory location(s). If so, the subsequent
store is redundant and can be removed.
The subsequent stores could be via memset, empty constructors,
simple MEM stores, etc. */
static void
dse_optimize_redundant_stores (gimple *stmt)
{
int cnt = 0;
/* TBAA state of STMT, if it is a call it is effectively alias-set zero. */
alias_set_type earlier_set = 0;
alias_set_type earlier_base_set = 0;
if (is_gimple_assign (stmt))
{
ao_ref lhs_ref;
ao_ref_init (&lhs_ref, gimple_assign_lhs (stmt));
earlier_set = ao_ref_alias_set (&lhs_ref);
earlier_base_set = ao_ref_base_alias_set (&lhs_ref);
}
/* We could do something fairly complex and look through PHIs
like DSE_CLASSIFY_STORE, but it doesn't seem to be worth
the effort.
Look at all the immediate uses of the VDEF (which are obviously
dominated by STMT). See if one or more stores 0 into the same
memory locations a STMT, if so remove the immediate use statements. */
tree defvar = gimple_vdef (stmt);
imm_use_iterator ui;
gimple *use_stmt;
FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
{
/* Limit stmt walking. */
if (++cnt > param_dse_max_alias_queries_per_store)
BREAK_FROM_IMM_USE_STMT (ui);
/* If USE_STMT stores 0 into one or more of the same locations
as STMT and STMT would kill USE_STMT, then we can just remove
USE_STMT. */
tree fndecl;
if ((is_gimple_assign (use_stmt)
&& gimple_vdef (use_stmt)
&& (gimple_assign_single_p (use_stmt)
&& initializer_zerop (gimple_assign_rhs1 (use_stmt))))
|| (gimple_call_builtin_p (use_stmt, BUILT_IN_NORMAL)
&& (fndecl = gimple_call_fndecl (use_stmt)) != NULL
&& (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
|| DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET_CHK)
&& integer_zerop (gimple_call_arg (use_stmt, 1))))
{
ao_ref write;
if (!initialize_ao_ref_for_dse (use_stmt, &write))
BREAK_FROM_IMM_USE_STMT (ui)
if (valid_ao_ref_for_dse (&write)
&& stmt_kills_ref_p (stmt, &write))
{
gimple_stmt_iterator gsi = gsi_for_stmt (use_stmt);
if (is_gimple_assign (use_stmt))
{
ao_ref lhs_ref;
ao_ref_init (&lhs_ref, gimple_assign_lhs (use_stmt));
if ((earlier_set == ao_ref_alias_set (&lhs_ref)
|| alias_set_subset_of (ao_ref_alias_set (&lhs_ref),
earlier_set))
&& (earlier_base_set == ao_ref_base_alias_set (&lhs_ref)
|| alias_set_subset_of
(ao_ref_base_alias_set (&lhs_ref),
earlier_base_set)))
delete_dead_or_redundant_assignment (&gsi, "redundant",
need_eh_cleanup);
}
else if (is_gimple_call (use_stmt))
{
if ((earlier_set == 0
|| alias_set_subset_of (0, earlier_set))
&& (earlier_base_set == 0
|| alias_set_subset_of (0, earlier_base_set)))
delete_dead_or_redundant_call (&gsi, "redundant");
}
else
gcc_unreachable ();
}
}
}
}
/* 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. */
dse_store_status
dse_classify_store (ao_ref *ref, gimple *stmt,
bool byte_tracking_enabled, sbitmap live_bytes,
bool *by_clobber_p, tree stop_at_vuse)
{
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);
/* If we're instructed to stop walking at region boundary, do so. */
if (defvar == stop_at_vuse)
return DSE_STORE_LIVE;
auto_vec defs;
gimple *phi_def = NULL;
FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
{
/* Limit stmt walking. */
if (++cnt > 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_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_or_redundant_call (gimple_stmt_iterator *gsi, const char *type)
{
gimple *stmt = gsi_stmt (*gsi);
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, " Deleted %s call: ", type);
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. */
void
delete_dead_or_redundant_assignment (gimple_stmt_iterator *gsi, const char *type,
bitmap need_eh_cleanup)
{
gimple *stmt = gsi_stmt (*gsi);
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, " Deleted %s store: ", type);
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) && need_eh_cleanup)
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))
{
tree fndecl = gimple_call_fndecl (stmt);
switch (DECL_FUNCTION_CODE (fndecl))
{
case BUILT_IN_MEMCPY:
case BUILT_IN_MEMMOVE:
case BUILT_IN_STRNCPY:
case BUILT_IN_MEMSET:
case BUILT_IN_MEMCPY_CHK:
case BUILT_IN_MEMMOVE_CHK:
case BUILT_IN_STRNCPY_CHK:
case BUILT_IN_MEMSET_CHK:
{
/* 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_or_redundant_call (gsi, "dead");
return;
}
/* If this is a memset call that initializes an object
to zero, it may be redundant with an earlier memset
or empty CONSTRUCTOR of a larger object. */
if ((DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
|| DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET_CHK)
&& integer_zerop (gimple_call_arg (stmt, 1)))
dse_optimize_redundant_stores (stmt);
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_or_redundant_call (gsi, "dead");
return;
}
case BUILT_IN_CALLOC:
/* We already know the arguments are integer constants. */
dse_optimize_redundant_stores (stmt);
return;
default:
return;
}
}
if (is_gimple_assign (stmt))
{
bool by_clobber_p = false;
/* Check if this statement stores zero to a memory location,
and if there is a subsequent store of zero to the same
memory location. If so, remove the subsequent store. */
if (gimple_assign_single_p (stmt)
&& initializer_zerop (gimple_assign_rhs1 (stmt)))
dse_optimize_redundant_stores (stmt);
/* 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_or_redundant_assignment (gsi, "dead", need_eh_cleanup);
}
}
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);
}