Speed up identifying keywords in scanner, by using extra table.

Speed up identifying keywords in scanner, by using extra table.

The table maps the first character of a identifier, to the first keyword
intry in the sorted token-list. If no keyword exists that starts with a
given character, we don't enter the loop at all.

BUG=
R=hausner@google.com

Committed: https://code.google.com/p/dart/source/detail?r=36934

[Anders Johnsen, 2014-06-03 17:23:40]

This gives a slight speedup when scanning.

Ivan, can you forward to the persons?

[hausner, 2014-06-03 17:54:07]

DBC: have you measured a speedup using an actual program, e.g. compiling all of
dart2js?

[hausner, 2014-06-03 17:59:26]

Another DBC.

https://codereview.chromium.org/316633002/diff/1/runtime/vm/scanner.cc
File runtime/vm/scanner.cc (right):

https://codereview.chromium.org/316633002/diff/1/runtime/vm/scanner.cc#newcod...
runtime/vm/scanner.cc:294: if (ch >= 0 && ch < kNumLowercaseChars) {
why not if (('a' <= ident_char0) && (ident_char0 <= 'z')) {....

Less cryptic IMO and saves you computing ch for anything not starting with a
lowercase letter.

[Anders Johnsen, 2014-06-03 18:52:59]

I'll work on getting some numbers as well.

https://codereview.chromium.org/316633002/diff/1/runtime/vm/scanner.cc
File runtime/vm/scanner.cc (right):

https://codereview.chromium.org/316633002/diff/1/runtime/vm/scanner.cc#newcod...
runtime/vm/scanner.cc:294: if (ch >= 0 && ch < kNumLowercaseChars) {
On 2014/06/03 17:59:26, hausner wrote:
> why not if (('a' <= ident_char0) && (ident_char0 <= 'z')) {....
> 
> Less cryptic IMO and saves you computing ch for anything not starting with a
> lowercase letter.

Good idea. I'll work on a updated version with this guard in, instead.

[Anders Johnsen, 2014-06-03 19:26:13]

Updated.

Numbers (10 runs, sorted):

Before:
Dart2JSCompileAll(RunTime): 1344110
Dart2JSCompileAll(RunTime): 1349788
Dart2JSCompileAll(RunTime): 1355867
Dart2JSCompileAll(RunTime): 1366156
Dart2JSCompileAll(RunTime): 1367951
Dart2JSCompileAll(RunTime): 1376330
Dart2JSCompileAll(RunTime): 1397344
Dart2JSCompileAll(RunTime): 1419839
Dart2JSCompileAll(RunTime): 1422471
Dart2JSCompileAll(RunTime): 1448912

After:
Dart2JSCompileAll(RunTime): 1331067
Dart2JSCompileAll(RunTime): 1335008
Dart2JSCompileAll(RunTime): 1339462
Dart2JSCompileAll(RunTime): 1339801
Dart2JSCompileAll(RunTime): 1341255
Dart2JSCompileAll(RunTime): 1342645
Dart2JSCompileAll(RunTime): 1343230
Dart2JSCompileAll(RunTime): 1345596
Dart2JSCompileAll(RunTime): 1346633
Dart2JSCompileAll(RunTime): 1351431

[hausner, 2014-06-03 19:42:39]

Thank you. If I take the average before and after, it seems the speedup is about
3%.

Before:	
1344110	
1349788	
1355867	
1366156	
1367951	
1376330	
1397344	
1419839	
1422471	
1448912	
1384876.8	avg
	
After:	
1331067	
1335008	
1339462	
1339801	
1341255	
1342645	
1343230	
1345596	
1346633	
1351431	
1341612.8	avg
	
0.968759676	avg after / avg before

[hausner, 2014-06-03 20:08:26]

LGTM, ship it!

[Anders Johnsen, 2014-06-03 20:15:52]

Committed patchset #2 manually as r36934 (presubmit successful).