#include "tree-dfa.h"
#include "domwalk.h"
#include "tree-cfgcleanup.h"
+#include "params.h"
/* This file implements dead store elimination.
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)
+ && ref->max_size != -1
+ && (ref->offset % BITS_PER_UNIT) == 0
+ && (ref->size % BITS_PER_UNIT) == 0
+ && (ref->size != -1));
+}
+
+/* Normalize COPY (an ao_ref) relative to REF. Essentially when we are
+ done COPY will only refer bytes found within REF.
+
+ We have already verified that COPY intersects at least one
+ byte with REF. */
+
+static void
+normalize_ref (ao_ref *copy, ao_ref *ref)
+{
+ /* 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 (copy->offset < ref->offset)
+ {
+ copy->size -= (ref->offset - copy->offset);
+ copy->offset = ref->offset;
+ }
+
+ /* If COPY extends beyond REF, chop off its size appropriately. */
+ if (copy->offset + copy->size > ref->offset + ref->size)
+ copy->size -= (copy->offset + copy->size - (ref->offset + ref->size));
+}
+
+/* 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. */
+ if (valid_ao_ref_for_dse (&write)
+ && write.base == ref->base
+ && write.size == write.max_size
+ && ((write.offset < ref->offset
+ && write.offset + write.size > ref->offset)
+ || (write.offset >= ref->offset
+ && write.offset < ref->offset + ref->size)))
+ {
+ normalize_ref (&write, ref);
+ bitmap_clear_range (live_bytes,
+ (write.offset - ref->offset) / BITS_PER_UNIT,
+ write.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)
+{
+ if (valid_ao_ref_for_dse (ref)
+ && (ref->size / BITS_PER_UNIT
+ <= PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE)))
+ {
+ bitmap_clear (live_bytes);
+ bitmap_set_range (live_bytes, 0, ref->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. */
+
+static void
+compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail)
+{
+ /* 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. */
+ *trim_tail = 0;
+ int last_orig = (ref->size / BITS_PER_UNIT) - 1;
+ int last_live = bitmap_last_set_bit (live);
+ *trim_tail = (last_orig - last_live) & ~0x1;
+
+ /* 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) & ~0x1;
+}
+
+/* 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);
+
+ /* 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 (trim_tail * 2 >= ref->size / BITS_PER_UNIT)
+ {
+ /* 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 (trim_head * 2 >= ref->size / BITS_PER_UNIT)
+ {
+ /* 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 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))
+ {
+ switch (gimple_assign_rhs_code (stmt))
+ {
+ case COMPLEX_CST:
+ maybe_trim_complex_store (ref, live, stmt);
+ break;
+ default:
+ break;
+ }
+ }
+}
/* A helper of dse_optimize_stmt.
Given a GIMPLE_ASSIGN in STMT that writes to REF, find a candidate
statement *USE_STMT that may prove STMT to be dead.
Return TRUE if the above conditions are met, otherwise FALSE. */
-static bool
-dse_possible_dead_store_p (ao_ref *ref, gimple *stmt, gimple **use_stmt)
+static dse_store_status
+dse_classify_store (ao_ref *ref, gimple *stmt, gimple **use_stmt,
+ bool byte_tracking_enabled, sbitmap live_bytes)
{
gimple *temp;
unsigned cnt = 0;
/* Limit stmt walking to be linear in the number of possibly
dead stores. */
if (++cnt > 256)
- return false;
+ return DSE_STORE_LIVE;
if (gimple_code (temp) == GIMPLE_PHI)
defvar = PHI_RESULT (temp);
fail = true;
BREAK_FROM_IMM_USE_STMT (ui);
}
- /* Do not consider the PHI as use if it dominates the
+ /* Do not consider the PHI as use if it dominates the
stmt defining the virtual operand we are processing,
we have processed it already in this case. */
if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
}
if (fail)
- return false;
+ {
+ /* 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
if (!temp)
{
if (ref_may_alias_global_p (ref))
- return false;
+ return DSE_STORE_LIVE;
temp = stmt;
break;
}
+
+ if (byte_tracking_enabled && temp)
+ clear_bytes_written_by (live_bytes, temp, ref);
}
- /* Continue walking until we reach a kill. */
- while (!stmt_kills_ref_p (temp, ref));
+ /* Continue walking until we reach a full kill as a single statement
+ or there are no more live bytes. */
+ while (!stmt_kills_ref_p (temp, ref)
+ && !(byte_tracking_enabled && bitmap_empty_p (live_bytes)));
*use_stmt = temp;
+ return DSE_STORE_DEAD;
+}
+
+
+class dse_dom_walker : public dom_walker
+{
+public:
+ dse_dom_walker (cdi_direction direction)
+ : dom_walker (direction), m_byte_tracking_enabled (false)
+
+ { m_live_bytes = sbitmap_alloc (PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE)); }
- return true;
+ ~dse_dom_walker () { sbitmap_free (m_live_bytes); }
+
+ virtual edge before_dom_children (basic_block);
+
+private:
+ sbitmap m_live_bytes;
+ bool m_byte_tracking_enabled;
+ void dse_optimize_stmt (gimple_stmt_iterator *);
+};
+
+/* Delete a dead call STMT, which is mem* call of some kind. */
+static void
+delete_dead_call (gimple *stmt)
+{
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, " Deleted dead call: ");
+ print_gimple_stmt (dump_file, stmt, dump_flags, 0);
+ fprintf (dump_file, "\n");
+ }
+
+ tree lhs = gimple_call_lhs (stmt);
+ gimple_stmt_iterator gsi = gsi_for_stmt (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 STMT, which is a gimple assignment. */
+
+static void
+delete_dead_assignment (gimple *stmt)
+{
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, " Deleted dead store: ");
+ print_gimple_stmt (dump_file, stmt, dump_flags, 0);
+ fprintf (dump_file, "\n");
+ }
+
+ /* Then we need to fix the operand of the consuming stmt. */
+ unlink_stmt_vdef (stmt);
+
+ /* Remove the dead store. */
+ gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
+ 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.
is used precisely once by a later store to the same location which
post dominates the first store, then the first store is dead. */
-static void
-dse_optimize_stmt (gimple_stmt_iterator *gsi)
+void
+dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
{
gimple *stmt = gsi_stmt (*gsi);
|| 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))
case BUILT_IN_MEMSET:
{
gimple *use_stmt;
- ao_ref ref;
- 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 (&ref, ptr, size);
- if (!dse_possible_dead_store_p (&ref, stmt, &use_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, &use_stmt,
+ m_byte_tracking_enabled,
+ m_live_bytes);
+ if (store_status == DSE_STORE_LIVE)
return;
- if (dump_file && (dump_flags & TDF_DETAILS))
- {
- fprintf (dump_file, " Deleted dead call: ");
- print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
- fprintf (dump_file, "\n");
- }
-
- tree lhs = gimple_call_lhs (stmt);
- if (lhs)
+ if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
{
- 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);
+ maybe_trim_partially_dead_store (&ref, m_live_bytes, stmt);
+ return;
}
- 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);
- }
- break;
+ if (store_status == DSE_STORE_DEAD)
+ delete_dead_call (stmt);
+ return;
}
+
default:
return;
}
use_stmt = stmt;
else
{
- ao_ref ref;
- ao_ref_init (&ref, gimple_assign_lhs (stmt));
- if (!dse_possible_dead_store_p (&ref, stmt, &use_stmt))
+ 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, &use_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_partially_dead_store (&ref, m_live_bytes, stmt);
+ return;
+ }
}
/* Now we know that use_stmt kills the LHS of stmt. */
&& !gimple_clobber_p (use_stmt))
return;
- if (dump_file && (dump_flags & TDF_DETAILS))
- {
- fprintf (dump_file, " Deleted dead store: ");
- print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
- 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);
+ delete_dead_assignment (stmt);
}
}
-class dse_dom_walker : public dom_walker
-{
-public:
- dse_dom_walker (cdi_direction direction) : dom_walker (direction) {}
-
- virtual edge before_dom_children (basic_block);
-};
-
edge
dse_dom_walker::before_dom_children (basic_block bb)
{
}
BITMAP_FREE (need_eh_cleanup);
-
+
/* For now, just wipe the post-dominator information. */
free_dominance_info (CDI_POST_DOMINATORS);
return 0;
projects / gcc.git / commitdiff