[-ftree-cselim desoptimizes SH code

[Christian BRUEL <christian dot bruel at st dot com>]

Hello,

On SH, -ftree-cselim transforms a simple conditional store such as

foo(int *a)
{
    *a = 0;
  if (bar())
    *a = 1;
}

into

foo(int *a)
{
    *a = 0;
  if (! bar())
    r = *a;
  else
    r = 1;

   *a = r;
}

which is equivalent to add either a load or a store on the critical path 
(depending if the branch is likely to be taken or not)

more important, those new memory operations are not cleaned up, for 
example the SH code is looks like

(-O2 -fno-schedule-insns -fno-schedule-insns2 -fno-delayed-branch 
-fomit-frame-pointer) to make the code more readable

	tst     r0,r0
        mov     #1,r1
        bf      .L2
        mov.l   @r8,r1
.L2:
        mov.l   r1,@r8
  	...

with -fno-tree-cselim I see the expected

	tst     r0,r0
        bt      .L3
        mov     #1,r1
        mov.l   r1,@r8
.L3:
	...

which is one memory load better and possibly one store better.

Am I right in saying that this option is only useful if conditional 
execution (predication or select) is available (because the conditional 
is made explicit in the phi) ? so a guard on the gate such as p1.patch 
should be ok.

or is it just an SH problem, so this option should only be disabled in 
the sh_optimization_option such as with p2.patch ?

any feedback for the others targets ?

many thanks

Christian




Index: tree-ssa-phiopt.c
===================================================================
--- tree-ssa-phiopt.c	(revision 151937)
+++ tree-ssa-phiopt.c	(working copy)
@@ -1302,7 +1302,11 @@
 static bool
 gate_cselim (void)
 {
-  return flag_tree_cselim;
+#ifdef HAVE_conditional_execution
+  return flag_tree_cselim ;
+#else
+  return 0;
+#endif
 }
 
 struct gimple_opt_pass pass_cselim =

Index: config/sh/sh.c
===================================================================
--- config/sh/sh.c	(revision 151937)
+++ config/sh/sh.c	(working copy)
@@ -674,6 +674,7 @@
   if (flag_schedule_insns > 0)
     flag_schedule_insns = 2;
 
+  flag_tree_cselim = 0;
   set_param_value ("simultaneous-prefetches", 2);
 }