Re: [PATCH net-next v4 2/8] 6lowpan: add uncompress header size function

On Wed, 2014-02-26 at 16:10 +0000, David Laight wrote:

From: Alexander Aring

[]

quoted

diff --git a/net/ieee802154/6lowpan.h b/net/ieee802154/6lowpan.h

[]

quoted

+static inline u8 lowpan_addr_mode_size(const u8 addr_mode)
+{
+	switch (addr_mode) {
+	case LOWPAN_IPHC_ADDR_00:
+		return 16;
+	case LOWPAN_IPHC_ADDR_01:
+		return 8;
+	case LOWPAN_IPHC_ADDR_02:
+		return 2;
+	default:
+		return 0;
+	}
+}

The compiler will generate much better code if you index an array instead
of using a switch statement.

Are you sure of that?

Perhaps the compiler would inline the assignment
anyway if addr_mode is __builtin_constant_p

gcc 4.8 here does the same thing with:

static inline unsigned char f1(unsigned char a)
{
	switch (a) {
	case 0: return 16;
	case 1: return 8;
	case 2: return 2;
	default: return 0;
	}
}

static inline unsigned char f2(unsigned char a)
{
	static const unsigned char rtns[] = { 16, 8, 2, 0 };

	return rtns[a & 3];
}