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[Bug tree-optimization/53844] [4.6/4.7 Regression] GCC generates suboptimal code for unused members of classes in some cases on multiple targets.
- From: "rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Wed, 04 Jul 2012 09:55:27 +0000
- Subject: [Bug tree-optimization/53844] [4.6/4.7 Regression] GCC generates suboptimal code for unused members of classes in some cases on multiple targets.
- Auto-submitted: auto-generated
- References: <bug-53844-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53844
--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-07-04 09:55:27 UTC ---
For trunk dse_possible_dead_store_p fails to look through the loop, when
being at the VDEF .MEM_165:
<bb 3>:
# i_93 = PHI <i_78(3), 0(2)>
# .MEM_100 = PHI <.MEM_165(3), .MEM_68(2)>
# VUSE <.MEM_100>
D.3879_71 = MEM[(struct VBase *)out_2(D)].my_data;
D.3880_73 = (long unsigned int) i_93;
D.3881_74 = D.3880_73 * 8;
D.3878_75 = D.3879_71 + D.3881_74;
# VUSE <.MEM_100>
D.3975_138 = MEM[(const struct VBase *)in_1(D)].my_data;
D.3974_141 = D.3975_138 + D.3881_74;
# VUSE <.MEM_100>
D.3981_142 = *D.3974_141;
# .MEM_165 = VDEF <.MEM_100>
*D.3878_75 = D.3981_142;
i_78 = i_93 + 1;
if (i_78 != 100)
goto <bb 3>;
else
goto <bb 4>;
<bb 4>:
# .MEM_28 = VDEF <.MEM_165>
D.2811 ={v} {CLOBBER};
we then see two uses that have a VDEF - the PHI (which we processed before),
and the store after the loop.
The following fixes that
Index: gcc/tree-ssa-dse.c
===================================================================
--- gcc/tree-ssa-dse.c (revision 189248)
+++ gcc/tree-ssa-dse.c (working copy)
@@ -94,7 +94,7 @@ dse_possible_dead_store_p (gimple stmt,
temp = stmt;
do
{
- gimple use_stmt;
+ gimple use_stmt, defvar_def;
imm_use_iterator ui;
bool fail = false;
tree defvar;
@@ -108,6 +108,7 @@ dse_possible_dead_store_p (gimple stmt,
defvar = PHI_RESULT (temp);
else
defvar = gimple_vdef (temp);
+ defvar_def = temp;
temp = NULL;
FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
{
@@ -139,7 +140,13 @@ dse_possible_dead_store_p (gimple stmt,
fail = true;
BREAK_FROM_IMM_USE_STMT (ui);
}
- temp = use_stmt;
+ /* Do not consider the PHI as use if it dominates the
+ stmt defining the virtual operand we are processing. */
+ if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
+ && !dominated_by_p (CDI_DOMINATORS,
+ gimple_bb (defvar_def),
+ gimple_bb (use_stmt)))
+ temp = use_stmt;
}
/* If the statement is a use the store is not dead. */
else if (ref_maybe_used_by_stmt_p (use_stmt,
but the existing loop code is weird ...
I'm anyway testing the above.