add -fmove-loop-stores option to control GIMPLE loop store-motion

This adds the -fmove-loop-stores option, mainly as a way to disable
the store-motion part of GIMPLE invariant motion (-ftree-loop-im)
which is enabled by default.  It might be sensible to turn off
-fmove-loop-stores at -O1 since it can result in compile-time
as well as memory usage issues but this patch tries to preserve
existing behavior besides introducing the new option with the
exception of -Og where I've disabled it.

Controlling store-motion has been made easy by earlier refactoring
for the invariant motion only use after loop interchange.

2021-07-02  Richard Biener  <rguenther@suse.de>

	* doc/invoke.texi (fmove-loop-stores): Document.
	* common.opt (fmove-loop-stores): New option.
	* opts.c (default_options_table): Enable -fmove-loop-stores
	at -O1 but not -Og.
	* tree-ssa-loop-im.c (pass_lim::execute): Pass
	flag_move_loop_stores instead of true to
	loop_invariant_motion_in_fun.

4 files changed, 18 insertions, 3 deletions

diff --git a/gcc/common.opt b/gcc/common.opt
index 5b03bbc..d9da113 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2084,6 +2084,10 @@ fmove-loop-invariants
 Common Var(flag_move_loop_invariants) Optimization
 Move loop invariant computations out of loops.
 
+fmove-loop-stores
+Common Var(flag_move_loop_stores) Optimization
+Move stores out of loops.
+
 fdce
 Common Var(flag_dce) Init(1) Optimization
 Use the RTL dead code elimination pass.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index f470fc6..e67d47a 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -528,7 +528,7 @@ Objective-C and Objective-C++ Dialects}.
 -floop-parallelize-all  -flra-remat  -flto  -flto-compression-level @gol
 -flto-partition=@var{alg}  -fmerge-all-constants @gol
 -fmerge-constants  -fmodulo-sched  -fmodulo-sched-allow-regmoves @gol
--fmove-loop-invariants  -fno-branch-count-reg @gol
+-fmove-loop-invariants  -fmove-loop-stores  -fno-branch-count-reg @gol
 -fno-defer-pop  -fno-fp-int-builtin-inexact  -fno-function-cse @gol
 -fno-guess-branch-probability  -fno-inline  -fno-math-errno  -fno-peephole @gol
 -fno-peephole2  -fno-printf-return-value  -fno-sched-interblock @gol
@@ -10260,6 +10260,7 @@ compilation time.
 -fipa-reference-addressable @gol
 -fmerge-constants @gol
 -fmove-loop-invariants @gol
+-fmove-loop-stores@gol
 -fomit-frame-pointer @gol
 -freorder-blocks @gol
 -fshrink-wrap @gol
@@ -10403,7 +10404,7 @@ optimization flags except for those that may interfere with debugging:
 @gccoptlist{-fbranch-count-reg  -fdelayed-branch @gol
 -fdse  -fif-conversion  -fif-conversion2  @gol
 -finline-functions-called-once @gol
--fmove-loop-invariants  -fssa-phiopt @gol
+-fmove-loop-invariants  -fmove-loop-stores  -fssa-phiopt @gol
 -ftree-bit-ccp  -ftree-dse  -ftree-pta  -ftree-sra}
 
 @end table
@@ -13011,6 +13012,15 @@ Enabled by @option{-O3}, @option{-fprofile-use}, and @option{-fauto-profile}.
 Enables the loop invariant motion pass in the RTL loop optimizer.  Enabled
 at level @option{-O1} and higher, except for @option{-Og}.
 
+@item -fmove-loop-stores
+@opindex fmove-loop-stores
+Enables the loop store motion pass in the GIMPLE loop optimizer.  This
+moves invariant stores to after the end of the loop in exchange for
+carrying the stored value in a register across the iteration.
+Note for this option to have an effect @option{-ftree-loop-im} has to
+be enabled as well.  Enabled at level @option{-O1} and higher, except
+for @option{-Og}.
+
 @item -fsplit-loops
 @opindex fsplit-loops
 Split a loop into two if it contains a condition that's always true
diff --git a/gcc/opts.c b/gcc/opts.c
index f159bb3..25282f7 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -575,6 +575,7 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fif_conversion2, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_finline_functions_called_once, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fmove_loop_invariants, NULL, 1 },
+    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fmove_loop_stores, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fssa_phiopt, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fipa_modref, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_bit_ccp, NULL, 1 },
diff --git a/gcc/tree-ssa-loop-im.c b/gcc/tree-ssa-loop-im.c
index e7a3050..9ac390b 100644
--- a/gcc/tree-ssa-loop-im.c
+++ b/gcc/tree-ssa-loop-im.c
@@ -3258,7 +3258,7 @@ pass_lim::execute (function *fun)
 
   if (number_of_loops (fun) <= 1)
     return 0;
-  unsigned int todo = loop_invariant_motion_in_fun (fun, true);
+  unsigned int todo = loop_invariant_motion_in_fun (fun, flag_move_loop_stores);
 
   if (!in_loop_pipeline)
     loop_optimizer_finalize ();