From: kelefth <konstantinos.elefther...@vrull.eu>
This pass detects cases of expensive store forwarding and tries to avoid them
by reordering the stores and using suitable bit insertion sequences.
For example it can transform this:
strb w2, [x1, 1]
ldr x0, [x1] # Expensive store forwarding to larger load.
To:
ldr x0, [x1]
strb w2, [x1]
bfi x0, x2, 0, 8
Assembly like this can appear with bitfields or type punning / unions.
On stress-ng when running the cpu-union microbenchmark the following speedups
have been observed.
Neoverse-N1: +29.4%
Intel Coffeelake: +13.1%
AMD 5950X: +17.5%
gcc/ChangeLog:
* Makefile.in: Add avoid-store-forwarding.o
* common.opt: New option -favoid-store-forwarding.
* common.opt.urls: Regenerate.
* doc/invoke.texi: New param store-forwarding-max-distance.
* doc/passes.texi: Document new pass.
* doc/tm.texi: Regenerate.
* doc/tm.texi.in: Document new pass.
* params.opt: New param store-forwarding-max-distance.
* passes.def: Schedule a new pass.
* target.def (HOOK_PREFIX): New target hook avoid_store_forwarding_p.
* target.h (struct store_fwd_info): Declare.
* targhooks.cc (default_avoid_store_forwarding_p):
Add default_avoid_store_forwarding_p.
* targhooks.h (default_avoid_store_forwarding_p):
Add default_avoid_store_forwarding_p.
* tree-pass.h (make_pass_rtl_avoid_store_forwarding): Declare.
* avoid-store-forwarding.cc: New file.
* avoid-store-forwarding.h: New file.
gcc/testsuite/ChangeLog:
* gcc.target/aarch64/avoid-store-forwarding-1.c: New test.
* gcc.target/aarch64/avoid-store-forwarding-2.c: New test.
* gcc.target/aarch64/avoid-store-forwarding-3.c: New test.
* gcc.target/aarch64/avoid-store-forwarding-4.c: New test (XFAIL).
* gcc.target/aarch64/avoid-store-forwarding-5.c: New test (XFAIL).
Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu>
Signed-off-by: Konstantinos Eleftheriou <konstantinos.elefther...@vrull.eu>
Series-version: 6
Series-changes: 6
- Reject the transformation on cases that would cause store_bit_field
to generate subreg expressions on different register classes.
Files avoid-store-forwarding-4.c and avoid-store-forwarding-5.c
contain such cases and have been marked as XFAIL.
- Use optimize_bb_for_speed_p instead of optimize_insn_for_speed_p.
- Inline and remove get_load_mem.
- New implementation for is_store_forwarding.
- Refactor the main loop in avoid_store_forwarding.
- Avoid using the word 'forwardings'.
- Use lowpart_subreg instead of validate_subreg + gen_rtx_subreg.
- Don't use df_insn_rescan where not needed.
- Change order of emitting stores and bit insert instructions.
- Check and reject loads for which the dest register overlaps with src.
- Remove unused variable.
- Change some gen_mov_insn function calls to gen_rtx_SET.
- Subtract the cost of eliminated load, instead of 1, for the total
cost.
- Use delete_insn instead of set_insn_deleted.
- Regenerate common.opt.urls.
- Add some more comments.
Series-changes: 5
- Fix bug with BIG_ENDIAN targets.
- Fix bug with unrecognized instructions.
- Fix / simplify pass init/fini.
Series-changes: 4
- Change pass scheduling to run after sched1.
- Add target hook to decide whether a store forwarding instance
should be avoided or not.
- Fix bugs.
Series-changes: 3
- Only emit SUBREG after calling validate_subreg.
- Fix memory corruption due to vec self-reference.
- Fix bitmap_bit_in_range_p ICE due to BLKMode.
- Reject MEM to MEM sets.
- Add get_load_mem comment.
- Add new testcase.
Series-changes: 2
- Allow modes that are not scalar_int_mode.
- Introduce simple costing to avoid unprofitable transformations.
- Reject bit insert sequences that spill to memory.
- Document new pass.
- Fix and add testcases.
---
gcc/Makefile.in | 1 +
gcc/avoid-store-forwarding.cc | 630 ++++++++++++++++++
gcc/avoid-store-forwarding.h | 56 ++
gcc/common.opt | 4 +
gcc/common.opt.urls | 3 +
gcc/doc/invoke.texi | 9 +
gcc/doc/passes.texi | 8 +
gcc/doc/tm.texi | 8 +
gcc/doc/tm.texi.in | 2 +
gcc/params.opt | 4 +
gcc/passes.def | 1 +
gcc/target.def | 10 +
gcc/target.h | 3 +
gcc/targhooks.cc | 25 +
gcc/targhooks.h | 3 +
.../aarch64/avoid-store-forwarding-1.c | 28 +
.../aarch64/avoid-store-forwarding-2.c | 39 ++
.../aarch64/avoid-store-forwarding-3.c | 31 +
.../aarch64/avoid-store-forwarding-4.c | 26 +
.../aarch64/avoid-store-forwarding-5.c | 41 ++
gcc/tree-pass.h | 1 +
21 files changed, 933 insertions(+)
create mode 100644 gcc/avoid-store-forwarding.cc
create mode 100644 gcc/avoid-store-forwarding.h
create mode 100644 gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-1.c
create mode 100644 gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-2.c
create mode 100644 gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-3.c
create mode 100644 gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-4.c
create mode 100644 gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-5.c
diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 059cf2e8f79..8b3e3e45d2e 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1682,6 +1682,7 @@ OBJS = \
statistics.o \
stmt.o \
stor-layout.o \
+ avoid-store-forwarding.o \
store-motion.o \
streamer-hooks.o \
stringpool.o \
diff --git a/gcc/avoid-store-forwarding.cc b/gcc/avoid-store-forwarding.cc
new file mode 100644
index 00000000000..2e1efdaa06e
--- /dev/null
+++ b/gcc/avoid-store-forwarding.cc
@@ -0,0 +1,630 @@
+/* Avoid store forwarding optimization pass.
+ Copyright (C) 2024 Free Software Foundation, Inc.
+ Contributed by VRULL GmbH.
+
+ 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 "avoid-store-forwarding.h"
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "target.h"
+#include "rtl.h"
+#include "alias.h"
+#include "rtlanal.h"
+#include "cfgrtl.h"
+#include "tree-pass.h"
+#include "cselib.h"
+#include "predict.h"
+#include "insn-config.h"
+#include "expmed.h"
+#include "recog.h"
+#include "regset.h"
+#include "df.h"
+#include "expr.h"
+#include "memmodel.h"
+#include "emit-rtl.h"
+#include "vec.h"
+
+/* This pass tries to detect and avoid cases of store forwarding.
+ On many processors there is a large penalty when smaller stores are
+ forwarded to larger loads. The idea used to avoid the stall is to move
+ the store after the load and in addition emit a bit insert sequence so
+ the load register has the correct value. For example the following:
+
+ strb w2, [x1, 1]
+ ldr x0, [x1]
+
+ Will be transformed to:
+
+ ldr x0, [x1]
+ strb w2, [x1]
+ bfi x0, x2, 0, 8
+*/
+
+namespace {
+
+const pass_data pass_data_avoid_store_forwarding =
+{
+ RTL_PASS, /* type. */
+ "avoid_store_forwarding", /* name. */
+ OPTGROUP_NONE, /* optinfo_flags. */
+ TV_NONE, /* tv_id. */
+ 0, /* properties_required. */
+ 0, /* properties_provided. */
+ 0, /* properties_destroyed. */
+ 0, /* todo_flags_start. */
+ TODO_df_finish /* todo_flags_finish. */
+};
+
+class pass_rtl_avoid_store_forwarding : public rtl_opt_pass
+{
+public:
+ pass_rtl_avoid_store_forwarding (gcc::context *ctxt)
+ : rtl_opt_pass (pass_data_avoid_store_forwarding, ctxt)
+ {}
+
+ /* opt_pass methods: */
+ virtual bool gate (function *)
+ {
+ return flag_avoid_store_forwarding && optimize >= 1;
+ }
+
+ virtual unsigned int execute (function *) override;
+}; // class pass_rtl_avoid_store_forwarding
+
+/* Handler for finding and avoiding store forwardings. */
+
+class store_forwarding_analyzer
+{
+public:
+ unsigned int stats_sf_detected = 0;
+ unsigned int stats_sf_avoided = 0;
+
+ bool is_store_forwarding (rtx store_mem, rtx load_mem,
+ HOST_WIDE_INT *off_val);
+ bool process_store_forwarding (vec<store_fwd_info> &, rtx_insn *load_insn,
+ rtx load_mem);
+ void avoid_store_forwarding (basic_block);
+ void update_stats (function *);
+};
+
+/* Return a bit insertion sequence that would make DEST have the correct value
+ if the store represented by STORE_INFO were to be moved after DEST. */
+
+static rtx_insn *
+generate_bit_insert_sequence (store_fwd_info *store_info, rtx dest)
+{
+ /* Memory size should be a constant at this stage. */
+ unsigned HOST_WIDE_INT store_size
+ = MEM_SIZE (store_info->store_mem).to_constant ();
+
+ start_sequence ();
+
+ rtx mov_reg = store_info->mov_reg;
+ store_bit_field (dest, store_size * BITS_PER_UNIT,
+ store_info->offset * BITS_PER_UNIT, 0, 0,
+ GET_MODE (mov_reg), mov_reg,
+ false, false);
+
+ rtx_insn *insns = get_insns ();
+ unshare_all_rtl_in_chain (insns);
+ end_sequence ();
+
+ for (rtx_insn *insn = insns; insn; insn = NEXT_INSN (insn))
+ if (contains_mem_rtx_p (PATTERN (insn))
+ || recog_memoized (insn) < 0)
+ return NULL;
+
+ return insns;
+}
+
+/* Return true iff a store to STORE_MEM would write to a sub-region of bytes
+ from what LOAD_MEM would read. If true also store the relative byte offset
+ of the store within the load to OFF_VAL. */
+
+bool store_forwarding_analyzer::
+is_store_forwarding (rtx store_mem, rtx load_mem, HOST_WIDE_INT *off_val)
+{
+ poly_int64 load_offset, store_offset;
+ rtx load_base = strip_offset (XEXP (load_mem, 0), &load_offset);
+ rtx store_base = strip_offset (XEXP (store_mem, 0), &store_offset);
+ return (MEM_SIZE (load_mem).is_constant ()
+ && rtx_equal_p (load_base, store_base)
+ && known_subrange_p (store_offset, MEM_SIZE (store_mem),
+ load_offset, MEM_SIZE (load_mem))
+ && (store_offset - load_offset).is_constant (off_val));
+}
+
+/* Given a list of small stores that are forwarded to LOAD_INSN, try to
+ rearrange them so that a store-forwarding penalty doesn't occur.
+ The stores must be given in reverse program order, starting from the
+ one closer to LOAD_INSN. */
+
+bool store_forwarding_analyzer::
+process_store_forwarding (vec<store_fwd_info> &stores, rtx_insn *load_insn,
+ rtx load_mem)
+{
+ machine_mode load_mem_mode = GET_MODE (load_mem);
+ /* Memory sizes should be constants at this stage. */
+ HOST_WIDE_INT load_size = MEM_SIZE (load_mem).to_constant ();
+
+ /* If the stores cover all the bytes of the load without overlap then we can
+ eliminate the load entirely and use the computed value instead. */
+
+ sbitmap forwarded_bytes = sbitmap_alloc (load_size);
+ bitmap_clear (forwarded_bytes);
+
+ unsigned int i;
+ store_fwd_info* it;
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ HOST_WIDE_INT store_size = MEM_SIZE (it->store_mem).to_constant ();
+ if (bitmap_bit_in_range_p (forwarded_bytes, it->offset,
+ it->offset + store_size - 1))
+ break;
+ bitmap_set_range (forwarded_bytes, it->offset, store_size);
+ }
+
+ bitmap_not (forwarded_bytes, forwarded_bytes);
+ bool load_elim = bitmap_empty_p (forwarded_bytes);
+
+ stats_sf_detected++;
+
+ if (dump_file)
+ {
+ fprintf (dump_file, "Store forwarding detected:\n");
+
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ fprintf (dump_file, "From: ");
+ print_rtl_single (dump_file, it->store_insn);
+ }
+
+ fprintf (dump_file, "To: ");
+ print_rtl_single (dump_file, load_insn);
+
+ if (load_elim)
+ fprintf (dump_file, "(Load elimination candidate)\n");
+ }
+
+ rtx load = single_set (load_insn);
+ rtx dest;
+
+ if (load_elim)
+ dest = gen_reg_rtx (load_mem_mode);
+ else
+ dest = SET_DEST (load);
+
+ int move_to_front = -1;
+ int total_cost = 0;
+
+ /* Check if we can emit bit insert instructions for all forwarded stores. */
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ it->mov_reg = gen_reg_rtx (GET_MODE (it->store_mem));
+ rtx_insn *insns = NULL;
+
+ /* If we're eliminating the load then find the store with zero offset
+ and use it as the base register to avoid a bit insert if possible. */
+ if (load_elim && it->offset == 0)
+ {
+ start_sequence ();
+
+ /* We can use a paradoxical subreg to force this to a wider mode, as
+ the only use will be inserting the bits (i.e., we don't care about
+ the value of the higher bits). */
+ rtx ext0 = lowpart_subreg (GET_MODE (dest), it->mov_reg,
+ GET_MODE (it->mov_reg));
+ if (ext0)
+ {
+ rtx_insn *move0 = emit_move_insn (dest, ext0);
+ if (recog_memoized (move0) >= 0)
+ {
+ insns = get_insns ();
+ move_to_front = (int) i;
+ }
+ }
+
+ end_sequence ();
+ }
+
+ if (!insns)
+ insns = generate_bit_insert_sequence (&(*it), dest);
+
+ if (!insns)
+ {
+ if (dump_file)
+ {
+ fprintf (dump_file, "Failed due to: ");
+ print_rtl_single (dump_file, it->store_insn);
+ }
+ return false;
+ }
+
+ total_cost += seq_cost (insns, true);
+ it->bits_insert_insns = insns;
+
+ rtx store_set = single_set (it->store_insn);
+
+ /* Create a register move at the store's original position to save the
+ stored value. */
+ start_sequence ();
+ rtx_insn *insn1
+ = emit_insn (gen_rtx_SET (it->mov_reg, SET_SRC (store_set)));
+ end_sequence ();
+
+ if (recog_memoized (insn1) < 0)
+ {
+ if (dump_file)
+ {
+ fprintf (dump_file, "Failed due to unrecognizable insn: ");
+ print_rtl_single (dump_file, insn1);
+ }
+ return false;
+ }
+
+ it->save_store_value_insn = insn1;
+
+ /* Create a new store after the load with the saved original value.
+ This avoids the forwarding stall. */
+ start_sequence ();
+ rtx_insn *insn2
+ = emit_insn (gen_rtx_SET (SET_DEST (store_set), it->mov_reg));
+ end_sequence ();
+
+ if (recog_memoized (insn2) < 0)
+ {
+ if (dump_file)
+ {
+ fprintf (dump_file, "Failed due to unrecognizable insn: ");
+ print_rtl_single (dump_file, insn2);
+ }
+ return false;
+ }
+
+ it->store_saved_value_insn = insn2;
+ }
+
+ if (load_elim)
+ total_cost -= insn_cost (load_insn, true);
+
+ /* Let the target decide if transforming this store forwarding instance is
+ profitable. */
+ if (!targetm.avoid_store_forwarding_p (stores, load_mem, total_cost,
+ load_elim))
+ {
+ if (dump_file)
+ fprintf (dump_file, "Not transformed due to target decision.\n");
+
+ return false;
+ }
+
+ /* If we have a move instead of bit insert, it needs to be emitted first in
+ the resulting sequence. */
+ if (move_to_front != -1)
+ {
+ store_fwd_info copy = stores[move_to_front];
+ stores.safe_push (copy);
+ stores.ordered_remove (move_to_front);
+ }
+
+ if (load_elim)
+ {
+ machine_mode outer_mode = GET_MODE (SET_DEST (load));
+ if (outer_mode != load_mem_mode)
+ {
+ start_sequence ();
+ rtx load_value = simplify_gen_unary (GET_CODE (SET_SRC (load)),
+ outer_mode, dest, load_mem_mode);
+ rtx load_move = gen_rtx_SET (SET_DEST (load), load_value);
+ rtx_insn *insn = emit_insn (load_move);
+ rtx_insn *seq = get_insns ();
+ end_sequence ();
+
+ if (recog_memoized (insn) < 0)
+ return false;
+
+ emit_insn_after (seq, load_insn);
+ }
+ }
+
+ if (dump_file)
+ {
+ fprintf (dump_file, "Store forwarding avoided with bit inserts:\n");
+
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ if (stores.length () > 1)
+ {
+ fprintf (dump_file, "For: ");
+ print_rtl_single (dump_file, it->store_insn);
+ }
+
+ fprintf (dump_file, "With sequence:\n");
+
+ for (rtx_insn *insn = it->bits_insert_insns; insn;
+ insn = NEXT_INSN (insn))
+ {
+ fprintf (dump_file, " ");
+ print_rtl_single (dump_file, insn);
+ }
+ }
+ }
+
+ stats_sf_avoided++;
+
+ /* Done, emit all the generated instructions and delete the stores.
+ Note that STORES are in reverse program order. */
+
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ emit_insn_after (it->bits_insert_insns, load_insn);
+ emit_insn_after (it->store_saved_value_insn, load_insn);
+ }
+
+ FOR_EACH_VEC_ELT (stores, i, it)
+ {
+ emit_insn_before (it->save_store_value_insn, it->store_insn);
+ delete_insn (it->store_insn);
+ }
+
+ df_insn_rescan (load_insn);
+
+ if (load_elim)
+ delete_insn (load_insn);
+
+ return true;
+}
+
+/* Try to modify BB so that expensive store forwarding cases are avoided. */
+
+void store_forwarding_analyzer::avoid_store_forwarding (basic_block bb)
+{
+ if (!optimize_bb_for_speed_p (bb))
+ return;
+
+ auto_vec<store_fwd_info, 8> store_exprs;
+ rtx_insn *insn;
+ unsigned int insn_cnt = 0;
+
+ FOR_BB_INSNS (bb, insn)
+ {
+ if (!NONDEBUG_INSN_P (insn))
+ continue;
+
+ vec_rtx_properties properties;
+ properties.add_insn (insn, false);
+
+ rtx set = single_set (insn);
+
+ if (!set)
+ {
+ store_exprs.truncate (0);
+ continue;
+ }
+
+ /* The inner mem RTX if INSN is a load, NULL_RTX otherwise. */
+ rtx load_mem = SET_SRC (set);
+
+ if (GET_CODE (load_mem) == ZERO_EXTEND
+ || GET_CODE (load_mem) == SIGN_EXTEND)
+ load_mem = XEXP (load_mem, 0);
+
+ if (!MEM_P (load_mem))
+ load_mem = NULL_RTX;
+
+ /* The mem RTX if INSN is a store, NULL_RTX otherwise. */
+ rtx store_mem = MEM_P (SET_DEST (set)) ? SET_DEST (set) : NULL_RTX;
+
+ /* We cannot analyze memory RTXs that have unknown size. */
+ if ((store_mem && (!MEM_SIZE_KNOWN_P (store_mem)
+ || !MEM_SIZE (store_mem).is_constant ()))
+ || (load_mem && (!MEM_SIZE_KNOWN_P (load_mem)
+ || !MEM_SIZE (load_mem).is_constant ())))
+ {
+ store_exprs.truncate (0);
+ continue;
+ }
+
+ bool is_simple = !properties.has_asm
+ && !properties.has_side_effects ();
+ bool is_simple_store = is_simple
+ && store_mem
+ && !contains_mem_rtx_p (SET_SRC (set));
+ bool is_simple_load = is_simple
+ && load_mem
+ && !contains_mem_rtx_p (SET_DEST (set));
+
+ int removed_count = 0;
+
+ if (is_simple_store)
+ {
+ /* Record store forwarding candidate. */
+ store_fwd_info info;
+ info.store_insn = insn;
+ info.store_mem = store_mem;
+ info.insn_cnt = insn_cnt;
+ info.remove = false;
+ info.forwarded = false;
+ store_exprs.safe_push (info);
+ }
+
+ bool reads_mem = false;
+ bool writes_mem = false;
+ for (auto ref : properties.refs ())
+ if (ref.is_mem ())
+ {
+ reads_mem |= ref.is_read ();
+ writes_mem |= ref.is_write ();
+ }
+ else if (ref.is_write ())
+ {
+ /* Drop store forwarding candidates when the address register is
+ overwritten. */
+ bool remove_rest = false;
+ unsigned int i;
+ store_fwd_info *it;
+ FOR_EACH_VEC_ELT_REVERSE (store_exprs, i, it)
+ {
+ if (remove_rest
+ || reg_overlap_mentioned_p (regno_reg_rtx[ref.regno],
+ it->store_mem))
+ {
+ it->remove = true;
+ removed_count++;
+ remove_rest = true;
+ }
+ }
+ }
+
+ if (is_simple_load)
+ {
+ /* Process load for possible store forwarding cases.
+ Possible newly created/moved stores, resulted from a successful
+ forwarding, will be processed in subsequent iterations. */
+ auto_vec<store_fwd_info> forwardings;
+ bool partial_forwarding = false;
+ bool remove_rest = false;
+
+ bool vector_load = VECTOR_MODE_P (GET_MODE (load_mem));
+
+ unsigned int i;
+ store_fwd_info *it;
+ FOR_EACH_VEC_ELT_REVERSE (store_exprs, i, it)
+ {
+ rtx store_mem = it->store_mem;
+ HOST_WIDE_INT off_val;
+
+ bool vector_store = VECTOR_MODE_P (GET_MODE (store_mem));
+
+ if (remove_rest)
+ {
+ it->remove = true;
+ removed_count++;
+ }
+ else if (vector_load ^ vector_store)
+ {
+ /* Vector stores followed by a non-vector load or the
+ opposite, cause store_bit_field to generate non-canonical
+ expressions, like (subreg:V4SI (reg:DI ...) 0)).
+ Cases like that should be handled using vec_duplicate,
+ so we reject the transformation in those cases. */
+ it->remove = true;
+ removed_count++;
+ remove_rest = true;
+ }
+ else if (is_store_forwarding (store_mem, load_mem, &off_val))
+ {
+ /* Check if moving this store after the load is legal. */
+ bool write_dep = false;
+ for (unsigned int j = store_exprs.length () - 1; j != i; j--)
+ if (!store_exprs[j].forwarded
+ && output_dependence (store_mem,
+ store_exprs[j].store_mem))
+ {
+ write_dep = true;
+ break;
+ }
+
+ if (!write_dep)
+ {
+ it->forwarded = true;
+ it->offset = off_val;
+ forwardings.safe_push (*it);
+ }
+ else
+ partial_forwarding = true;
+
+ it->remove = true;
+ removed_count++;
+ }
+ else if (true_dependence (store_mem, GET_MODE (store_mem),
+ load_mem))
+ {
+ /* We cannot keep a store forwarding candidate if it possibly
+ interferes with this load. */
+ it->remove = true;
+ removed_count++;
+ remove_rest = true;
+ }
+ }
+
+ if (!forwardings.is_empty () && !partial_forwarding)
+ process_store_forwarding (forwardings, insn, load_mem);
+ }
+
+ if ((writes_mem && !is_simple_store)
+ || (reads_mem && !is_simple_load))
+ store_exprs.truncate (0);
+
+ if (removed_count)
+ {
+ unsigned int i, j;
+ store_fwd_info *it;
+ VEC_ORDERED_REMOVE_IF (store_exprs, i, j, it, it->remove);
+ }
+
+ /* Don't consider store forwarding if the RTL instruction distance is
+ more than PARAM_STORE_FORWARDING_MAX_DISTANCE. */
+ if (!store_exprs.is_empty ()
+ && (store_exprs[0].insn_cnt
+ + param_store_forwarding_max_distance <= insn_cnt))
+ store_exprs.ordered_remove (0);
+
+ insn_cnt++;
+ }
+}
+
+/* Update pass statistics. */
+
+void store_forwarding_analyzer::update_stats (function *fn)
+{
+ statistics_counter_event (fn, "Cases of store forwarding detected: ",
+ stats_sf_detected);
+ statistics_counter_event (fn, "Cases of store forwarding avoided: ",
+ stats_sf_detected);
+}
+
+unsigned int
+pass_rtl_avoid_store_forwarding::execute (function *fn)
+{
+ df_set_flags (DF_DEFER_INSN_RESCAN);
+
+ init_alias_analysis ();
+
+ store_forwarding_analyzer analyzer;
+
+ basic_block bb;
+ FOR_EACH_BB_FN (bb, fn)
+ analyzer.avoid_store_forwarding (bb);
+
+ end_alias_analysis ();
+
+ analyzer.update_stats (fn);
+
+ return 0;
+}
+
+} // anon namespace.
+
+rtl_opt_pass *
+make_pass_rtl_avoid_store_forwarding (gcc::context *ctxt)
+{
+ return new pass_rtl_avoid_store_forwarding (ctxt);
+}
diff --git a/gcc/avoid-store-forwarding.h b/gcc/avoid-store-forwarding.h
new file mode 100644
index 00000000000..55a0c97f008
--- /dev/null
+++ b/gcc/avoid-store-forwarding.h
@@ -0,0 +1,56 @@
+/* Avoid store forwarding optimization pass.
+ Copyright (C) 2024 Free Software Foundation, Inc.
+ Contributed by VRULL GmbH.
+
+ 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/>. */
+
+#ifndef GCC_AVOID_STORE_FORWARDING_H
+#define GCC_AVOID_STORE_FORWARDING_H
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "rtl.h"
+
+struct store_fwd_info
+{
+ /* The store instruction that is a store forwarding candidate. */
+ rtx_insn *store_insn;
+ /* SET_DEST (single_set (store_insn)). */
+ rtx store_mem;
+ /* The temporary that will hold the stored value at the original store
+ position. */
+ rtx mov_reg;
+ /* The instruction sequence that inserts the stored value's bits at the
+ appropriate position in the loaded value. */
+ rtx_insn *bits_insert_insns;
+ /* An instruction that saves the store's value in a register temporarily,
+ (set (reg X) (SET_SRC (store_insn))). */
+ rtx_insn *save_store_value_insn;
+ /* An instruction that stores the saved value back to memory,
+ (set (SET_DEST (store_insn)) (reg X)). */
+ rtx_insn *store_saved_value_insn;
+ /* The byte offset for the store's position within the load. */
+ HOST_WIDE_INT offset;
+
+ unsigned int insn_cnt;
+ bool remove;
+ bool forwarded;
+};
+
+#endif /* GCC_AVOID_STORE_FORWARDING_H */
diff --git a/gcc/common.opt b/gcc/common.opt
index 12b25ff486d..346a996ef07 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1755,6 +1755,10 @@ fgcse-sm
Common Var(flag_gcse_sm) Init(0) Optimization
Perform store motion after global common subexpression elimination.
+favoid-store-forwarding
+Common Var(flag_avoid_store_forwarding) Init(0) Optimization
+Try to avoid store forwarding.
+
fgcse-las
Common Var(flag_gcse_las) Init(0) Optimization
Perform redundant load after store elimination in global common subexpression
diff --git a/gcc/common.opt.urls b/gcc/common.opt.urls
index e31736cd994..8325b2c28c3 100644
--- a/gcc/common.opt.urls
+++ b/gcc/common.opt.urls
@@ -700,6 +700,9 @@ UrlSuffix(gcc/Optimize-Options.html#index-fgcse-lm)
fgcse-sm
UrlSuffix(gcc/Optimize-Options.html#index-fgcse-sm)
+favoid-store-forwarding
+UrlSuffix(gcc/Optimize-Options.html#index-favoid-store-forwarding)
+
fgcse-las
UrlSuffix(gcc/Optimize-Options.html#index-fgcse-las)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 4f4ca637549..e09a8e86e85 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -12613,6 +12613,15 @@ loop unrolling.
This option is enabled by default at optimization levels @option{-O1},
@option{-O2}, @option{-O3}, @option{-Os}.
+@opindex favoid-store-forwarding
+@item -favoid-store-forwarding
+@itemx -fno-avoid-store-forwarding
+Many CPUs will stall for many cycles when a load partially depends on previous
+smaller stores. This pass tries to detect such cases and avoid the penalty by
+changing the order of the load and store and then fixing up the loaded value.
+
+Disabled by default.
+
@opindex ffp-contract
@item -ffp-contract=@var{style}
@option{-ffp-contract=off} disables floating-point expression contraction.
diff --git a/gcc/doc/passes.texi b/gcc/doc/passes.texi
index 4ac7a2306a1..639f6b325c8 100644
--- a/gcc/doc/passes.texi
+++ b/gcc/doc/passes.texi
@@ -925,6 +925,14 @@ and addressing mode selection. The pass is run twice,
with values
being propagated into loops only on the second run. The code is
located in @file{fwprop.cc}.
+@item Store forwarding avoidance
+
+This pass attempts to reduce the overhead of store to load forwarding.
+It detects when a load reads from one or more previous smaller stores and
+then rearranges them so that the stores are done after the load. The loaded
+value is adjusted with a series of bit insert instructions so that it stays
+the same. The code is located in @file{avoid-store-forwarding.cc}.
+
@item Common subexpression elimination
This pass removes redundant computation within basic blocks, and
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 4deb3d2c283..eed30a20103 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -7355,6 +7355,14 @@ the @code{POLY_VALUE_MIN}, @code{POLY_VALUE_MAX} and
implementation returns the lowest possible value of @var{val}.
@end deftypefn
+@deftypefn {Target Hook} bool TARGET_AVOID_STORE_FORWARDING_P
(vec<store_fwd_info>, @var{rtx}, @var{int}, @var{bool})
+Given a list of stores and a load instruction that reads from the location
+of the stores, this hook decides if it's profitable to emit additional code
+to avoid a potential store forwarding stall. The additional instructions
+needed, the sequence cost and additional relevant information is given in
+the arguments so that the target can make an informed decision.
+@end deftypefn
+
@node Scheduling
@section Adjusting the Instruction Scheduler
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 9f147ccb95c..59a478971ab 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -4752,6 +4752,8 @@ Define this macro if a non-short-circuit operation
produced by
@hook TARGET_ESTIMATED_POLY_VALUE
+@hook TARGET_AVOID_STORE_FORWARDING_P
+
@node Scheduling
@section Adjusting the Instruction Scheduler
diff --git a/gcc/params.opt b/gcc/params.opt
index a08e4c1042d..9995198e9e4 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -1036,6 +1036,10 @@ Allow the store merging pass to introduce unaligned
stores if it is legal to do
Common Joined UInteger Var(param_store_merging_max_size) Init(65536)
IntegerRange(1, 65536) Param Optimization
Maximum size of a single store merging region in bytes.
+-param=store-forwarding-max-distance=
+Common Joined UInteger Var(param_store_forwarding_max_distance) Init(10)
IntegerRange(1, 1000) Param Optimization
+Maximum number of instruction distance that a small store forwarded to a
larger load may stall.
+
-param=switch-conversion-max-branch-ratio=
Common Joined UInteger Var(param_switch_conversion_branch_ratio) Init(8)
IntegerRange(1, 65536) Param Optimization
The maximum ratio between array size and switch branches for a switch
conversion to take place.
diff --git a/gcc/passes.def b/gcc/passes.def
index 7d01227eed1..b736879e4bd 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -503,6 +503,7 @@ along with GCC; see the file COPYING3. If not see
NEXT_PASS (pass_sms);
NEXT_PASS (pass_live_range_shrinkage);
NEXT_PASS (pass_sched);
+ NEXT_PASS (pass_rtl_avoid_store_forwarding);
NEXT_PASS (pass_early_remat);
NEXT_PASS (pass_ira);
NEXT_PASS (pass_reload);
diff --git a/gcc/target.def b/gcc/target.def
index b3155010888..37e07cb373a 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -6979,6 +6979,16 @@ HOOK_VECTOR_END (shrink_wrap)
#undef HOOK_PREFIX
#define HOOK_PREFIX "TARGET_"
+DEFHOOK
+(avoid_store_forwarding_p,
+ "Given a list of stores and a load instruction that reads from the location\n\
+of the stores, this hook decides if it's profitable to emit additional code\n\
+to avoid a potential store forwarding stall. The additional instructions\n\
+needed, the sequence cost and additional relevant information is given in\n\
+the arguments so that the target can make an informed decision.",
+ bool, (vec<store_fwd_info>, rtx, int, bool),
+ default_avoid_store_forwarding_p)
+
/* Determine the type of unwind info to emit for debugging. */
DEFHOOK
(debug_unwind_info,
diff --git a/gcc/target.h b/gcc/target.h
index 837651d273a..bc6845b8023 100644
--- a/gcc/target.h
+++ b/gcc/target.h
@@ -165,6 +165,9 @@ class function_arg_info;
/* This is defined in function-abi.h. */
class predefined_function_abi;
+/* This is defined in avoid-store-forwarding.h. */
+struct store_fwd_info;
+
/* These are defined in tree-vect-stmts.cc. */
extern tree stmt_vectype (class _stmt_vec_info *);
extern bool stmt_in_inner_loop_p (class vec_info *, class _stmt_vec_info *);
diff --git a/gcc/targhooks.cc b/gcc/targhooks.cc
index dc040df9fcd..19927815312 100644
--- a/gcc/targhooks.cc
+++ b/gcc/targhooks.cc
@@ -96,6 +96,7 @@ along with GCC; see the file COPYING3. If not see
#include "tree-vectorizer.h"
#include "options.h"
#include "case-cfn-macros.h"
+#include "avoid-store-forwarding.h"
bool
default_legitimate_address_p (machine_mode mode ATTRIBUTE_UNUSED,
@@ -2281,6 +2282,30 @@ default_class_max_nregs (reg_class_t rclass
ATTRIBUTE_UNUSED,
#endif
}
+/* The default implementation of TARGET_AVOID_STORE_FORWARDING_P. */
+
+bool
+default_avoid_store_forwarding_p (vec<store_fwd_info>, rtx, int total_cost,
+ bool)
+{
+ /* Use a simple cost heurstic base on param_store_forwarding_max_distance.
+ In general the distance should be somewhat correlated to the store
+ forwarding penalty; if the penalty is large then it is justified to
+ increase the window size. Use this to reject sequences that are clearly
+ unprofitable. */
+ int max_cost = COSTS_N_INSNS (param_store_forwarding_max_distance / 2);
+ if (total_cost > max_cost)
+ {
+ if (dump_file)
+ fprintf (dump_file, "Not transformed due to cost: %d > %d.\n",
+ total_cost, max_cost);
+
+ return false;
+ }
+
+ return true;
+}
+
/* Determine the debugging unwind mechanism for the target. */
enum unwind_info_type
diff --git a/gcc/targhooks.h b/gcc/targhooks.h
index 2704d6008f1..7cf22038100 100644
--- a/gcc/targhooks.h
+++ b/gcc/targhooks.h
@@ -254,6 +254,9 @@ extern reg_class_t default_preferred_rename_class
(reg_class_t rclass);
extern bool default_class_likely_spilled_p (reg_class_t);
extern unsigned char default_class_max_nregs (reg_class_t, machine_mode);
+extern bool default_avoid_store_forwarding_p (vec<store_fwd_info>, rtx, int,
+ bool);
+
extern enum unwind_info_type default_debug_unwind_info (void);
extern void default_canonicalize_comparison (int *, rtx *, rtx *, bool);
diff --git a/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-1.c
b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-1.c
new file mode 100644
index 00000000000..4712f101d5e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-1.c
@@ -0,0 +1,28 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -favoid-store-forwarding
-fdump-rtl-avoid_store_forwarding" } */
+
+typedef union {
+ char arr_8[8];
+ long long_value;
+} DataUnion;
+
+long ssll_1 (DataUnion *data, char x)
+{
+ data->arr_8[0] = x;
+ return data->long_value;
+}
+
+long ssll_2 (DataUnion *data, char x)
+{
+ data->arr_8[1] = x;
+ return data->long_value;
+}
+
+long ssll_3 (DataUnion *data, char x)
+{
+ data->arr_8[7] = x;
+ return data->long_value;
+}
+
+/* { dg-final { scan-rtl-dump-times "Store forwarding detected" 3
"avoid_store_forwarding" } } */
+/* { dg-final { scan-rtl-dump-times "Store forwarding avoided" 3
"avoid_store_forwarding" } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-2.c
b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-2.c
new file mode 100644
index 00000000000..b958612173b
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-2.c
@@ -0,0 +1,39 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -favoid-store-forwarding
-fdump-rtl-avoid_store_forwarding" } */
+
+typedef union {
+ char arr_8[8];
+ int long_value;
+} DataUnion1;
+
+long no_ssll_1 (DataUnion1 *data, char x)
+{
+ data->arr_8[4] = x;
+ return data->long_value;
+}
+
+long no_ssll_2 (DataUnion1 *data, char x)
+{
+ data->arr_8[5] = x;
+ return data->long_value;
+}
+
+typedef union {
+ char arr_8[8];
+ short long_value[4];
+} DataUnion2;
+
+long no_ssll_3 (DataUnion2 *data, char x)
+{
+ data->arr_8[4] = x;
+ return data->long_value[1];
+}
+
+long no_ssll_4 (DataUnion2 *data, char x)
+{
+ data->arr_8[0] = x;
+ return data->long_value[1];
+}
+
+/* { dg-final { scan-rtl-dump-times "Store forwarding detected" 0
"avoid_store_forwarding" } } */
+/* { dg-final { scan-rtl-dump-times "Store forwarding avoided" 0
"avoid_store_forwarding" } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-3.c
b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-3.c
new file mode 100644
index 00000000000..28191f5aa39
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-3.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -favoid-store-forwarding
-fdump-rtl-avoid_store_forwarding" } */
+
+typedef union {
+ char arr_8[8];
+ long long_value;
+} DataUnion;
+
+long ssll_multi_1 (DataUnion *data, char x)
+{
+ data->arr_8[0] = x;
+ data->arr_8[2] = x;
+ return data->long_value;
+}
+
+long ssll_multi_2 (DataUnion *data, char x)
+{
+ data->arr_8[0] = x;
+ data->arr_8[1] = 11;
+ return data->long_value;
+}
+
+long ssll_multi_3 (DataUnion *data, char x, short y)
+{
+ data->arr_8[1] = x;
+ __builtin_memcpy(data->arr_8 + 4, &y, sizeof(short));
+ return data->long_value;
+}
+
+/* { dg-final { scan-rtl-dump-times "Store forwarding detected" 3
"avoid_store_forwarding" } } */
+/* { dg-final { scan-rtl-dump-times "Store forwarding avoided" 3
"avoid_store_forwarding" } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-4.c
b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-4.c
new file mode 100644
index 00000000000..3572dd5cd39
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-4.c
@@ -0,0 +1,26 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -favoid-store-forwarding
-fdump-rtl-avoid_store_forwarding" } */
+
+typedef int v4si __attribute__ ((vector_size (16)));
+
+typedef union {
+ char arr_16[16];
+ v4si vec_value;
+} DataUnion;
+
+v4si ssll_vect_1 (DataUnion *data, char x)
+{
+ data->arr_16[0] = x;
+ return data->vec_value;
+}
+
+v4si ssll_vect_2 (DataUnion *data, int x)
+{
+ __builtin_memcpy(data->arr_16 + 4, &x, sizeof(int));
+ return data->vec_value;
+}
+
+/* Scalar stores leading to vector loads cause store_bit_field to generate
+ subreg expressions on different register classes. This should be handled
+ using vec_duplicate, so it is marked as an XFAIL for now. */
+/* { dg-final { scan-rtl-dump-times "Store forwarding detected" 2
"avoid_store_forwarding" { xfail *-*-* } } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-5.c
b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-5.c
new file mode 100644
index 00000000000..0b2764256cd
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/avoid-store-forwarding-5.c
@@ -0,0 +1,41 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O2 -favoid-store-forwarding
-fdump-rtl-avoid_store_forwarding" } */
+
+typedef float v4f __attribute__ ((vector_size (16)));
+
+typedef union {
+ float arr_2[4];
+ long long_value;
+ __int128 longlong_value;
+ v4f vec_value;
+} DataUnion;
+
+long ssll_load_elim_1 (DataUnion *data, float x)
+{
+ data->arr_2[0] = x;
+ data->arr_2[1] = 0.0f;
+ return data->long_value;
+}
+
+__int128 ssll_load_elim_2 (DataUnion *data, float x)
+{
+ data->arr_2[0] = x;
+ data->arr_2[1] = 0.0f;
+ data->arr_2[2] = x;
+ data->arr_2[3] = 0.0f;
+ return data->longlong_value;
+}
+
+v4f ssll_load_elim_3 (DataUnion *data, float x)
+{
+ data->arr_2[3] = x;
+ data->arr_2[2] = x;
+ data->arr_2[1] = x;
+ data->arr_2[0] = x;
+ return data->vec_value;
+}
+
+/* Scalar stores leading to vector loads cause store_bit_field to generate
+ subreg expressions on different register classes. This should be handled
+ using vec_duplicate, so it is marked as an XFAIL for now. */
+/* { dg-final { scan-rtl-dump-times "Store forwarding detected" 3
"avoid_store_forwarding" { xfail *-*-* } } } */
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index a928cbe4557..f8bb875cf9f 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -572,6 +572,7 @@ extern rtl_opt_pass *make_pass_rtl_dse3 (gcc::context
*ctxt);
extern rtl_opt_pass *make_pass_rtl_cprop (gcc::context *ctxt);
extern rtl_opt_pass *make_pass_rtl_pre (gcc::context *ctxt);
extern rtl_opt_pass *make_pass_rtl_hoist (gcc::context *ctxt);
+extern rtl_opt_pass *make_pass_rtl_avoid_store_forwarding (gcc::context *ctxt);
extern rtl_opt_pass *make_pass_rtl_store_motion (gcc::context *ctxt);
extern rtl_opt_pass *make_pass_cse_after_global_opts (gcc::context *ctxt);
extern rtl_opt_pass *make_pass_rtl_ifcvt (gcc::context *ctxt);
--
2.46.0
