Cache the subtype of each DecoderInfo to make the Is* checks quicker.

webrtc/modules/audio_coding/neteq/decoder_database.h

Index: webrtc/modules/audio_coding/neteq/decoder_database.h
diff --git a/webrtc/modules/audio_coding/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 36248562bc483332917fc9f1ad4b0603f56ec6be..1289bddadfde344479743d9f59c47c9cea99243f 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -97,6 +97,17 @@ class DecoderDatabase {
       int sample_rate_hz;
     };
     const rtc::Optional<CngDecoder> cng_decoder_;
+
+    enum class Subtype {
+      kNormal,
+      kComfortNoise,
+      kDtmf,
+      kRed
+    };

[kwiberg-webrtc, 2016/09/30 11:28:24]

Do you have to tell the compiler to store this in an 8-bit number, or do you get
that by default? (Is the answer to that question different for enum and enum
class?)

[ossu, 2016/09/30 11:36:47]

According to a quick look-on-the-internet, enum class defaults to int. I can set
it to be 8 bits, but I'm not sure what would be gained from it. The size of
DecoderInfo (with padding) should remain the same and I'm not sure we'll be able
to access an int8_t any faster in general.

[kwiberg-webrtc, 2016/09/30 11:45:37]

Look at it this way: reading and writing 1 byte is at least as cheap as reading
or writing 4 bytes on hardware that we care about, and unless you start to
really track these things you never know when it'll be an actual space saving.

Your call.

[ossu, 2016/09/30 11:50:34]

Just the other day I was asked to change two uint8_t in Priority to int and now
we want it the other way around? The world is strange and confusing! :(

[kwiberg-webrtc, 2016/09/30 13:05:27]

The reason we prefer int for integers rather than something smaller (or
unsigned) is that less confusing things happen when the range in practice turns
out to be larger than it's supposed to be (e.g. because there's a bug).

For enums, especially enum class, you never do any arithmetic or things like
that, so the problems that we're protecting against by picking a larger type
can't happen.

Or at least that's my interpretation.

[ossu, 2016/09/30 13:45:21]

Alright. I've turned it into an int8_t and a quick comparison run shows a small
(likely statistically insignificant) improvement on my workstation.

[kwiberg-webrtc, 2016/09/30 16:55:00]

Wohoo! That's the best kind of improvement, right?

+
+    static Subtype SubtypeFromFormat(const SdpAudioFormat& format);
+
+    const Subtype subtype_;
   };
 
   // Maximum value for 8 bits, and an invalid RTP payload type (since it is