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[PATCH][ARM] Restrict MAX_CONDITIONAL_EXECUTE when -mrestrict-it is in place


Hi all,

When using the short Thumb2 IT blocks we want to also restrict ifcvt so that it will not end up generating a number of back-to-back cond_execs
that will later end up being back to back single-instruction IT blocks. Branching over them should be a better choice.

This patch implements that by setting max_insns_skipped to 1 when arm_restrict_it.

With this patch, I've seen GCC replace a number of sequences in places like SPEC2006 from:
    it    eq
    moveq    r1, r5
    it    ne
    movne    r1, r10
    it    eq
    moveq    r8, r4

to a branch over them.

Bootstrapped and tested on arm.
Ok for trunk?

Thanks,
Kyrill

2015-05-18  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * config/arm/arm.c (arm_option_params_internal): When optimising
    for speed set max_insns_skipped when arm_restrict_it.

2015-05-18  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * gcc.target/arm/short-it-ifcvt-1.c: New test.
    * gcc.target/arm/short-it-ifcvt-2.c: Likewise.
commit f23178a102c3f35d42d36e14ed69d6b03f5cd300
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Thu May 14 12:08:14 2015 +0100

    [ARM] Restrict MAX_CONDITIONAL_EXECUTE when -mrestrict-it is in place

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 3f1e48a..6b570af 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -2770,7 +2770,10 @@ arm_option_params_internal (struct gcc_options *opts)
         max_insns_skipped = opts->x_arm_restrict_it ? 1 : 4;
     }
   else
-    max_insns_skipped = current_tune->max_insns_skipped;
+    /* When -mrestrict-it is in use tone down the if-conversion.  */
+    max_insns_skipped
+      = (TREE_TARGET_THUMB2 (opts) && opts->x_arm_restrict_it)
+         ? 1 : current_tune->max_insns_skipped;
 }
 
 /* Reset options between modes that the user has specified.  */
diff --git a/gcc/testsuite/gcc.target/arm/short-it-ifcvt-1.c b/gcc/testsuite/gcc.target/arm/short-it-ifcvt-1.c
new file mode 100644
index 0000000..f3d29b7
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/short-it-ifcvt-1.c
@@ -0,0 +1,23 @@
+/* Test that ifcvt is not being too aggressive when -mrestrict-it.  */
+/* { dg-do compile } */
+/* { dg-options "-O2 -mrestrict-it" } */
+/* { dg-require-effective-target arm_thumb2_ok } */
+
+int
+f1(int x, int y, int z)
+{
+  if (x > 100)
+    {
+      x++;
+      z = -z;
+    }
+  else
+    {
+      x = -x;
+      y = -y;
+      z = 1;
+    }
+  return x + y + z;
+}
+
+/* { dg-final { scan-assembler "b(gt|le)" } } */
diff --git a/gcc/testsuite/gcc.target/arm/short-it-ifcvt-2.c b/gcc/testsuite/gcc.target/arm/short-it-ifcvt-2.c
new file mode 100644
index 0000000..9ac8153
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/short-it-ifcvt-2.c
@@ -0,0 +1,21 @@
+/* Test that ifcvt is not being too aggressive when -mrestrict-it.  */
+/* { dg-do compile } */
+/* { dg-options "-O2 -mrestrict-it" } */
+/* { dg-require-effective-target arm_thumb2_ok } */
+
+int
+f1(int x, int y, int z)
+{
+  if (x > 100)
+    {
+      x++;
+      z = -z;
+    }
+  else
+    {
+      x = -x;
+      y = -y;
+    }
+  return x + y + z;
+}
+/* { dg-final { scan-assembler "b(gt|le)" } } */

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