[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

["steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #8 from steven at gcc dot gnu dot org  2010-01-07 21:29 -------
The diff between -O2 and -Os starts in combine:

diff -ur O2/t.c.175r.combine Os/t.c.175r.combine
--- O2/t.c.175r.combine 2010-01-07 22:24:04.000000000 +0100
+++ Os/t.c.175r.combine 2010-01-07 22:23:14.000000000 +0100
@@ -4,18 +4,20 @@
 starting the processing of deferred insns
 ending the processing of deferred insns
 df_analyze called
-insn_cost 7: 12
+insn_cost 7: 8
 insn_cost 8: 4
 insn_cost 9: 4
-insn_cost 10: 12
-insn_cost 11: 12
+insn_cost 10: 8
+insn_cost 11: 4
 insn_cost 12: 4
-insn_cost 46: 4
-insn_cost 17: 12
+insn_cost 46: 16
+insn_cost 17: 8
 insn_cost 19: 4
-rejecting combination of insns 12 and 46
-original costs 4 + 4 = 8
-replacement cost 12
+deferring deletion of insn with uid = 12.
+modifying insn i3    46 {r133:SI={(r138:SI!=0x0)?0x2:0x0};clobber cc:CC;}
+      REG_UNUSED: cc:CC
+      REG_DEAD: r138:SI
+deferring rescan insn with uid = 46.
 (note# 0 # 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

 (note# # # 2 NOTE_INSN_FUNCTION_BEG)
@@ -40,16 +42,17 @@
         (expr_list:REG_EQUAL (mem/c/i:SI (symbol_ref:SI ("bool_var") [flags
0xc0]  <var_decl # bool_var>) [2 bool_var+0 S4 A32])
             (nil))))

-(insn# # # 2 t.c:13 (set (reg:CC 24 cc)
-        (compare:CC (reg:SI 138 [ bool_var ])
-            (const_int 0 [0x0])))# {*arm_cmpsi_insn} (nil))
-
-(insn# # # 2 t.c:8 (set (reg/v:SI 133 [ flags ])
-        (if_then_else:SI (eq (reg:CC 24 cc)
-                (const_int 0 [0x0]))
-            (reg:SI 138 [ bool_var ])
-            (const_int 2 [0x2])))# {*movsicc_insn} (expr_list:REG_DEAD (reg:SI
138 [ bool_var ])
-        (expr_list:REG_DEAD (reg:CC 24 cc)
+(note# # # 2 NOTE_INSN_DELETED)
+
+(insn# # # 2 t.c:8 (parallel [
+            (set (reg/v:SI 133 [ flags ])
+                (if_then_else:SI (ne (reg:SI 138 [ bool_var ])
+                        (const_int 0 [0x0]))
+                    (const_int 2 [0x2])
+                    (const_int 0 [0x0])))
+            (clobber (reg:CC 24 cc))
+        ])# {movcond} (expr_list:REG_UNUSED (reg:CC 24 cc)
+        (expr_list:REG_DEAD (reg:SI 138 [ bool_var ])
             (nil))))

 (insn# # # 2 t.c:14 (set (reg/f:SI 139)
@@ -60,7 +63,10 @@
         (expr_list:REG_DEAD (reg/v:SI 133 [ flags ])
             (nil))))
 starting the processing of deferred insns
+deleting insn with uid = 12.
+rescanning insn with uid = 46.
+deleting insn with uid = 46.
 ending the processing of deferred insns

-;; Combiner totals: 12 attempts, 12 substitutions (2 requiring new space),
-;; 0 successes.
+;; Combiner totals: 13 attempts, 13 substitutions (2 requiring new space),
+;; 1 successes.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494