Don't run DSE at -Og

This patch stops gimple and rtl DSE from running by default at -Og.
The idea is both to improve compile time and to stop us from deleting
stores that we can't track in debug info.

We could rein this back in future for stores to local variables
with is_gimple_reg_type, but at the moment we don't have any
infrastructure for switching between binds to specific values
and binds to evolving memory locations.  Even then, location
tracking only works for direct references to the variables, and doesn't
for example help with printing dereferenced pointers (see the next patch
in the series for an example).

I'm also not sure that DSE is important enough for -Og to justify the
compile time cost --  especially in the case of RTL DSE, which is pretty
expensive.

2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* common.opt (Og): Change the initial value of flag_dse to 0.
	* opts.c (default_options_table): Move OPT_ftree_dse from
	OPT_LEVELS_1_PLUS to OPT_LEVELS_1_PLUS_NOT_DEBUG.  Also add
	OPT_fdse to OPT_LEVELS_1_PLUS_NOT_DEBUG.  Put the OPT_ftree_pta
	entry before the OPT_ftree_sra entry.
	* doc/invoke.texi (Og): Add -fdse and -ftree-dse to the list
	of flags disabled by Og.

gcc/testsuite/
	* c-c++-common/guality/Og-global-dse-1.c: New test.

From-SVN: r273871

6 files changed, 37 insertions, 5 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 3eb52020e68..3e84bafe8bd 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,15 @@
 2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* common.opt (Og): Change the initial value of flag_dse to 0.
+	* opts.c (default_options_table): Move OPT_ftree_dse from
+	OPT_LEVELS_1_PLUS to OPT_LEVELS_1_PLUS_NOT_DEBUG.  Also add
+	OPT_fdse to OPT_LEVELS_1_PLUS_NOT_DEBUG.  Put the OPT_ftree_pta
+	entry before the OPT_ftree_sra entry.
+	* doc/invoke.texi (Og): Add -fdse and -ftree-dse to the list
+	of flags disabled by Og.
+
+2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* tree-cfg.c (execute_fixup_cfg): Don't delete stores to write-only
 	variables for -Og.
 
diff --git a/gcc/common.opt b/gcc/common.opt
index 11637c8cab0..c1605385712 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1955,7 +1955,7 @@ Common Var(flag_dce) Init(1) Optimization
 Use the RTL dead code elimination pass.
 
 fdse
-Common Var(flag_dse) Init(1) Optimization
+Common Var(flag_dse) Init(0) Optimization
 Use the RTL dead store elimination pass.
 
 freschedule-modulo-scheduled-loops
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 77a2d561e38..d1dfa2a5034 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8423,10 +8423,10 @@ no effect.  Otherwise @option{-Og} enables all @option{-O1}
 optimization flags except for those that may interfere with debugging:
 
 @gccoptlist{-fbranch-count-reg  -fdelayed-branch @gol
--fif-conversion  -fif-conversion2  @gol
+-fdse  -fif-conversion  -fif-conversion2  @gol
 -finline-functions-called-once @gol
 -fmove-loop-invariants  -fssa-phiopt @gol
--ftree-bit-ccp  -ftree-pta  -ftree-sra}
+-ftree-bit-ccp  -ftree-dse  -ftree-pta  -ftree-sra}
 
 @end table
 
diff --git a/gcc/opts.c b/gcc/opts.c
index 296f0f61802..28142cef044 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -469,7 +469,6 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_1_PLUS, OPT_ftree_copy_prop, NULL, 1 },
     { OPT_LEVELS_1_PLUS, OPT_ftree_dce, NULL, 1 },
     { OPT_LEVELS_1_PLUS, OPT_ftree_dominator_opts, NULL, 1 },
-    { OPT_LEVELS_1_PLUS, OPT_ftree_dse, NULL, 1 },
     { OPT_LEVELS_1_PLUS, OPT_ftree_fre, NULL, 1 },
     { OPT_LEVELS_1_PLUS, OPT_ftree_sink, NULL, 1 },
     { OPT_LEVELS_1_PLUS, OPT_ftree_slsr, NULL, 1 },
@@ -480,14 +479,16 @@ static const struct default_options default_options_table[] =
 #if DELAY_SLOTS
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fdelayed_branch, NULL, 1 },
 #endif
+    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fdse, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_fif_conversion, NULL, 1 },
     { 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_fssa_phiopt, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_bit_ccp, NULL, 1 },
-    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_sra, NULL, 1 },
+    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_dse, NULL, 1 },
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_pta, NULL, 1 },
+    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_ftree_sra, NULL, 1 },
 
     /* -O2 and -Os optimizations.  */
     { OPT_LEVELS_2_PLUS, OPT_fcaller_saves, NULL, 1 },
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 1dd2819e9e8..6dab4515305 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,9 @@
 2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* c-c++-common/guality/Og-global-dse-1.c: New test.
+
+2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* c-c++-common/guality/Og-static-wo-1.c: New test.
 	* g++.dg/guality/guality.exp: Separate the c-c++-common tests into
 	"Og" and "general" tests.  Run the latter at -O0 and -Og only.
diff --git a/gcc/testsuite/c-c++-common/guality/Og-global-dse-1.c b/gcc/testsuite/c-c++-common/guality/Og-global-dse-1.c
new file mode 100644
index 00000000000..3d4b4e60e15
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/guality/Og-global-dse-1.c
@@ -0,0 +1,17 @@
+/* { dg-do run } */
+/* { dg-options "-g" } */
+
+struct s { int i, j; };
+struct s gs1, gs2 = { 3, 4 };
+
+void __attribute__((noipa)) consume (void) {};
+
+int
+main (void)
+{
+  gs1.i = 1;
+  gs1.j = 2;	/* { dg-final { gdb-test . "gs1.i" "1" } } */
+  gs1 = gs2;	/* { dg-final { gdb-test . "gs1.j" "2" } } */
+  consume ();	/* { dg-final { gdb-test . "gs1.i" "3" } } */
+  return 0;	/* { dg-final { gdb-test . "gs1.j" "4" } } */
+}