Don't run DSE at -Og

[Richard Sandiford <richard.sandiford@arm.com>]

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.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Richard


2019-07-07  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.

Index: gcc/common.opt
===================================================================
--- gcc/common.opt	2019-07-03 20:50:45.902320943 +0100
+++ gcc/common.opt	2019-07-07 10:29:23.683336097 +0100
@@ -1955,7 +1955,7 @@ Common Var(flag_dce) Init(1) Optimizatio
 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
Index: gcc/opts.c
===================================================================
--- gcc/opts.c	2019-06-29 17:20:49.000000000 +0100
+++ gcc/opts.c	2019-07-07 10:29:23.691336032 +0100
@@ -466,7 +466,6 @@ static const struct default_options defa
     { 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 },
@@ -477,14 +476,16 @@ static const struct default_options defa
 #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 },
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	2019-07-03 20:50:42.346350375 +0100
+++ gcc/doc/invoke.texi	2019-07-07 10:29:23.687336062 +0100
@@ -8421,10 +8421,10 @@ no effect.  Otherwise @option{-Og} enabl
 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
 
Index: gcc/testsuite/c-c++-common/guality/Og-global-dse-1.c
===================================================================
--- /dev/null	2019-06-14 15:59:19.298479944 +0100
+++ gcc/testsuite/c-c++-common/guality/Og-global-dse-1.c	2019-07-07 10:29:23.691336032 +0100
@@ -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" } } */
+}