[WTF] Use quadratic-probing instead of double-hashing

https://bugs.webkit.org/show_bug.cgi?id=239813

Reviewed by Geoffrey Garen.

If table size is always power-of-two, we can safely use quadratic-probing[1] even if the load exceeds 50%.
Quadratic-probing and double-hashing have different tradeoff between clustering possibility and CPU cache
friendliness. Double-hashing can avoid clustering more, but it is not CPU cache friendly. Quadratic-probing
can cause clustering more but better for CPU cache. And while linear-probing caused a lot of trouble and not
practical, quadratic-probing with 75% load factor is practical configuration which is used in production-ready
hashtables e.g. Google's sparsetable / densetable[2].

This patch replaces WTF::HashTable's probing algorithm from double-hashing to quadratic-probing to make it more
cache friendly while keeping reasonable clustering characteristics (compared to linear-probing etc.). Due to
this cache friendliness, we get large performance improvements.

1. 0.52% improvement in Speedometer2 with 95% probability.
2. 0.72% improvement in JetStream2 with 98% probability.

[1]: https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/
[2]: http://goog-sparsehash.sourceforge.net/doc/implementation.html

Canonical link: https://commits.webkit.org/250093@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@293579 268f45cc-cd09-0410-ab3c-d52691b4dbfc

LayoutTests/ChangeLog

@@ -1,3 +1,19 @@
+2022-04-28  Yusuke Suzuki  <ysuzuki@apple.com>
+
+        [WTF] Use quadratic-probing instead of double-hashing
+        https://bugs.webkit.org/show_bug.cgi?id=239813
+
+        Reviewed by Geoffrey Garen.
+
+        The following tests are relying on HashTable's ordering which is not guaranteed.
+
+        * media/media-session/actionHandlerInternalMappings-expected.txt:
+        * platform/glib/accessibility/content-editable-as-textarea-expected.txt:
+        * platform/gtk/accessibility/native-text-control-attributed-string-expected.txt:
+        * storage/indexeddb/getdatabases-expected.txt:
+        * storage/indexeddb/getdatabases-private-expected.txt:
+        * storage/indexeddb/resources/getdatabases.js:
+
 2022-04-28  Antti Koivisto  <antti@apple.com>
 
         [ iOS ] tables/mozilla/bugs/bug26178.html is a constant failure

LayoutTests/media/media-session/actionHandlerInternalMappings-expected.txt

@@ -22,12 +22,12 @@ ACTION: seekto
 Command: SeekToPlaybackPositionCommand
 Iterate over all possible actions
 Command: PauseCommand
+Command: PlayCommand
 Command: SkipBackwardCommand
 Command: SkipForwardCommand
-Command: NextTrackCommand
-Command: PreviousTrackCommand
 Command: SeekToPlaybackPositionCommand
-Command: PlayCommand
+Command: PreviousTrackCommand
+Command: NextTrackCommand
 Command: StopCommand
 Iterate over possible actions after video element src is cleared
 RUN(video.src = "")

LayoutTests/platform/glib/accessibility/content-editable-as-textarea-expected.txt

@@ -15,13 +15,13 @@ Attributed string with range:
 		family-name:-webkit-standard
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
 		fg-color:0,0,0
-		size:12pt
 		weight:400
+		size:12pt
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for 'ello<\n>':
 	Range attributes for 'worl':
 		weight:700

LayoutTests/platform/gtk/accessibility/native-text-control-attributed-string-expected.txt

@@ -10,13 +10,13 @@ Attributed string for range (0, 9):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for 'some word':
 Attributed string for range (0, 3):
 	Default text attributes:
@@ -25,13 +25,13 @@ Attributed string for range (0, 3):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for 'som':
 Attributed string for range (5, 4):
 	Default text attributes:
@@ -40,13 +40,13 @@ Attributed string for range (5, 4):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for 'word':
 Attributed string for range (5, 10):
 Attributed string for range (10, 1):
@@ -57,13 +57,13 @@ Attributed string for range (-1, 1):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for 's':
 
 Attributed string for range (0, 3):
@@ -73,13 +73,13 @@ Attributed string for range (0, 3):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for '•••':
 Attributed string for range (0, 2):
 	Default text attributes:
@@ -88,13 +88,13 @@ Attributed string for range (0, 2):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for '••':
 Attributed string for range (1, 2):
 	Default text attributes:
@@ -103,13 +103,13 @@ Attributed string for range (1, 2):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for '••':
 Attributed string for range (1, 10):
 Attributed string for range (10, 1):
@@ -120,13 +120,13 @@ Attributed string for range (-1, 1):
 		bg-color:255,255,255
 		editable:true
 		strikethrough:false
+		direction:ltr
 		style:normal
+		weight:400
 		fg-color:0,0,0
 		size:12pt
-		weight:400
 		invisible:false
 		underline:none
-		direction:ltr
 	Range attributes for '•':
 
 PASS successfullyParsed is true

LayoutTests/storage/indexeddb/getdatabases-expected.txt

@@ -10,8 +10,8 @@ PASS databases.length is 1
 PASS databases[0].version is 1
 PASS databases[0].name is "getdatabase-first"
 PASS databases.length is 2
-PASS databases[1].version is 2
-PASS databases[1].name is "getdatabase-second"
+PASS databases[0].version is 2
+PASS databases[0].name is "getdatabase-second"
 PASS successfullyParsed is true
 
 TEST COMPLETE

LayoutTests/storage/indexeddb/getdatabases-private-expected.txt

@@ -10,8 +10,8 @@ PASS databases.length is 1
 PASS databases[0].version is 1
 PASS databases[0].name is "getdatabase-first"
 PASS databases.length is 2
-PASS databases[1].version is 2
-PASS databases[1].name is "getdatabase-second"
+PASS databases[0].version is 2
+PASS databases[0].name is "getdatabase-second"
 PASS successfullyParsed is true
 
 TEST COMPLETE

LayoutTests/storage/indexeddb/resources/getdatabases.js

@@ -35,8 +35,8 @@ function continueTest() {
         indexedDB.databases().then((result) => {
             databases = result;
             shouldBeEqualToNumber("databases.length", 2);
-            shouldBeEqualToNumber("databases[1].version", 2);
-            shouldBeEqualToString("databases[1].name", "getdatabase-second");
+            shouldBeEqualToNumber("databases[0].version", 2);
+            shouldBeEqualToString("databases[0].name", "getdatabase-second");
             finishJSTest();
         });
     }

Source/WTF/ChangeLog

@@ -1,3 +1,35 @@
+2022-04-28  Yusuke Suzuki  <ysuzuki@apple.com>
+
+        [WTF] Use quadratic-probing instead of double-hashing
+        https://bugs.webkit.org/show_bug.cgi?id=239813
+
+        Reviewed by Geoffrey Garen.
+
+        If table size is always power-of-two, we can safely use quadratic-probing[1] even if the load exceeds 50%.
+        Quadratic-probing and double-hashing have different tradeoff between clustering possibility and CPU cache
+        friendliness. Double-hashing can avoid clustering more, but it is not CPU cache friendly. Quadratic-probing
+        can cause clustering more but better for CPU cache. And while linear-probing caused a lot of trouble and not
+        practical, quadratic-probing with 75% load factor is practical configuration which is used in production-ready
+        hashtables e.g. Google's sparsetable / densetable[2].
+
+        This patch replaces WTF::HashTable's probing algorithm from double-hashing to quadratic-probing to make it more
+        cache friendly while keeping reasonable clustering characteristics (compared to linear-probing etc.). Due to
+        this cache friendliness, we get large performance improvements.
+
+        1. 0.52% improvement in Speedometer2 with 95% probability.
+        2. 0.72% improvement in JetStream2 with 98% probability.
+
+        [1]: https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/
+        [2]: http://goog-sparsehash.sourceforge.net/doc/implementation.html
+
+        * wtf/HashTable.h:
+        (WTF::KeyTraits>::inlineLookup):
+        (WTF::KeyTraits>::lookupForWriting):
+        (WTF::KeyTraits>::fullLookupForWriting):
+        (WTF::KeyTraits>::addUniqueForInitialization):
+        (WTF::KeyTraits>::add):
+        (WTF::doubleHash): Deleted.
+
 2022-04-28  Per Arne Vollan  <pvollan@apple.com>
 
         [iOS] Enable IOKit blocking in GPUP by default