[PATCH, ARM] Set max_insns_skipped to MAX_INSN_PER_IT_BLOCK when optimize_size for THUMB2

[Zhenqiang Chen]

Hi,

Current value for max_insns_skipped is 6. For THUMB2, it needs 2 (IF-THEN)
or 3 (IF-THEN-ELSE) IT blocks to hold all the instructions. The overhead of
IT is 4 or 6 BYTES.

If we do not generate IT blocks, for IF-THEN, the overhead of conditional
jump is 2 or 4; for IF-THEN-ELSE, the overhead is 4, 6, or 8.

Most THUMB2 jump instructions are 2 BYTES. Tests on CSiBE show no one file
has code size regression. So The patch sets max_insns_skipped to
MAX_INSN_PER_IT_BLOCK.

No make check regression on cortex-m3.
For CSiBE, no any file has code size regression. And overall there is >0.01%
code size improvement for cortex-a9 and cortex-m4.

Is it OK?

Thanks!
-Zhenqiang

2014-02-25  Zhenqiang Chen  <zhenqiang.c...@arm.com>

        * config/arm/arm.c (arm_option_override): Set max_insns_skipped
        to MAX_INSN_PER_IT_BLOCK when optimize_size for THUMB2.

testsuite/ChangeLog:
2014-02-25  Zhenqiang Chen  <zhenqiang.c...@arm.com>

        * gcc.target/arm/max-insns-skipped.c: New test.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index b49f43e..99cdbc4 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -2743,6 +2743,15 @@ arm_option_override (void)
       /* If optimizing for size, bump the number of instructions that we
          are prepared to conditionally execute (even on a StrongARM).  */
       max_insns_skipped = 6;
+
+      /* For THUMB2, it needs 2 (IF-THEN) or 3 (IF-THEN-ELSE) IT blocks to
+        hold all the instructions. The overhead of IT is 4 or 6 BYTES.
+        If we do not generate IT blocks, for IF-THEN, the overhead of
+        conditional jump is 2 or 4; for IF-THEN-ELSE, the overhead is 4, 6
+        or 8.  Most THUMB2 jump instructions are 2 BYTES.
+        So set max_insns_skipped to MAX_INSN_PER_IT_BLOCK.  */
+      if (TARGET_THUMB2)
+       max_insns_skipped = MAX_INSN_PER_IT_BLOCK;
     }
   else
     max_insns_skipped = current_tune->max_insns_skipped;
diff --git a/gcc/testsuite/gcc.target/arm/max-insns-skipped.c
b/gcc/testsuite/gcc.target/arm/max-insns-skipped.c
new file mode 100644
index 0000000..0a11554
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/max-insns-skipped.c
@@ -0,0 +1,21 @@
+/* { dg-do assemble { target arm_thumb2 } } */
+/* { dg-options " -Os " } */
+
+int t (int a, int b, int c, int d)
+{
+  int r;
+  if (a > 0) {
+    r = a + b;
+    r += 0x456;
+    r *= 0x1234567;
+    }
+  else {
+    r = b - a;
+    r -= 0x123;
+    r *= 0x12387;
+    r += d;
+   }
+  return r;
+}
+
+/* { dg-final { object-size text <= 40 } } */