Bitcode reader: replace DecodeChar6() with a lookup table (NFC)

[mehdi_amini]

Summary

Measured to be more performant when reading bitcode.

[davide]

I wonder if you can use a similar tecnique for encoding. They should be symmetric.

[mehdi_amini]

That's a good point, I've been focused on the Reader only because this is where my current bottle-neck is :)

[• rafael]

What test have you used? I will give this a quick try on "opt -disable-verifier -disable-output" on a LTO of clang.

[• rafael]

LGTM.

The results I got with 10 runs were

master:

11.167095958 seconds time elapsed ( +- 0.34% )

patch:

10.860838399 seconds time elapsed ( +- 0.39% )

[mehdi_amini]

r260455

[mehdi_amini]

r260455

include/llvm/Bitcode/BitCodes.h

static unsigned EncodeChar6(char C) {
     if (C >= '0' && C <= '9') return C-'0'+26+26;
     if (C == '.')             return 62;
     if (C == '_')             return 63;
     llvm_unreachable("Not a value Char6 character!");
   }
 
   static char DecodeChar6(unsigned V) {
     assert((V & ~63) == 0 && "Not a Char6 encoded character!");
-    if (V < 26)       return V+'a';
-    if (V < 26+26)    return V-26+'A';
-    if (V < 26+26+10) return V-26-26+'0';
-    if (V == 62)      return '.';
-    if (V == 63)      return '_';
-    llvm_unreachable("Not a value Char6 character!");
+    return "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789._"
+        [V];
   }
 
 };
 
 template <> struct isPodLike<BitCodeAbbrevOp> { static const bool value=true; };
 
 /// BitCodeAbbrev - This class represents an abbreviation record.  An
 /// abbreviation allows a complex record that has redundancy to be stored in a