Do not store impossible to match filters in HNTrie

Consider the two following filters:

    example.com
    www.example.com

This commit make it so that if the first filter is
already present in a given HNTrie, the second filter
will not be stored, since HNTrie will _always_
return the first filter as a match whenever the
hostname to match is example.com or any subdomain
of example.com.

The detection of such pointless filters is
virtually free when adding a hostname to an HNTrie
instance (given how data is stored in the trie), so
in practice no overhead is incurred to detect such
pointless filters.

The ability to ignore impossible to match filters
in HNTrie instances will _especially_ benefit those
using large hosts files.

Examples of how this helps using real configurations:

- Default lists:
  444 filters out of 100,382 were ignored as a result
  of this commit.

- Default lists + "Energized Ultimate Protection":
  283,669 filters out of 903,235 were ignored as a
  result of this commit.

Side note: There was no measurable difference between
the two configurations above in the performance of
the matching algorithm as reported by the built-in
benchmark tool.

src/js/background.js

@@ -138,7 +138,7 @@ const µBlock = (function() { // jshint ignore:line
         // Read-only
         systemSettings: {
             compiledMagic: 13,  // Increase when compiled format changes
-            selfieMagic: 13     // Increase when selfie format changes
+            selfieMagic: 14     // Increase when selfie format changes
         },
 
         restoreBackupSettings: {

src/js/hntrie.js

@@ -175,7 +175,8 @@ const HNTrieContainer = class {
     matchesJS(iroot) {
         const char0 = this.buf32[HNTRIE_CHAR0_SLOT];
         let ineedle = this.buf[255];
-        let icell = iroot;
+        let icell = this.buf32[iroot+0];
+        if ( icell === 0 ) { return -1; }
         for (;;) {
             if ( ineedle === 0 ) { return -1; }
             ineedle -= 1;
@@ -238,12 +239,6 @@ const HNTrieContainer = class {
     addJS(iroot) {
         let lhnchar = this.buf[255];
         if ( lhnchar === 0 ) { return 0; }
-        let icell = iroot;
-        // special case: first node in trie
-        if ( this.buf32[icell+2] === 0 ) {
-            this.buf32[icell+2] = this.addSegment(lhnchar);
-            return 1;
-        }
         // grow buffer if needed
         if (
             (this.buf32[HNTRIE_CHAR0_SLOT] - this.buf32[HNTRIE_TRIE1_SLOT]) < 24 ||
@@ -251,6 +246,12 @@ const HNTrieContainer = class {
         ) {
             this.growBuf(24, 256);
         }
+        let icell = this.buf32[iroot+0];
+        // special case: first node in trie
+        if ( icell === 0 ) {
+            this.buf32[iroot+0] = this.addCell(0, 0, this.addSegment(lhnchar));
+            return 1;
+        }
         //
         const char0 = this.buf32[HNTRIE_CHAR0_SLOT];
         let inext;
@@ -259,6 +260,9 @@ const HNTrieContainer = class {
             const vseg = this.buf32[icell+2];
             // skip boundary cells
             if ( vseg === 0 ) {
+                // remainder is at label boundary? if yes, no need to add
+                // the rest since the shortest match is always reported
+                if ( this.buf[lhnchar-1] === 0x2E /* '.' */ ) { return -1; }
                 icell = this.buf32[icell+1];
                 continue;
             }
@@ -303,6 +307,9 @@ const HNTrieContainer = class {
                         icell = inext;
                         continue;
                     }
+                    // remainder is at label boundary? if yes, no need to add
+                    // the rest since the shortest match is always reported
+                    if ( this.buf[lhnchar-1] === 0x2E /* '.' */ ) { return -1; }
                     // boundary cell + needle remainder
                     inext = this.addCell(0, 0, 0);
                     this.buf32[icell+1] = inext;
@@ -550,7 +557,7 @@ HNTrieContainer.prototype.HNTrieRef = class {
     }
 
     add(hn) {
-        if ( this.container.setNeedle(hn).add(this.iroot) === 1 ) {
+        if ( this.container.setNeedle(hn).add(this.iroot) > 0 ) {
             this.last = -1;
             this.needle = '';
             this.size += 1;
@@ -560,7 +567,7 @@ HNTrieContainer.prototype.HNTrieRef = class {
     }
 
     addJS(hn) {
-        if ( this.container.setNeedle(hn).addJS(this.iroot) === 1 ) {
+        if ( this.container.setNeedle(hn).addJS(this.iroot) > 0 ) {
             this.last = -1;
             this.needle = '';
             this.size += 1;
@@ -570,7 +577,7 @@ HNTrieContainer.prototype.HNTrieRef = class {
     }
 
     addWASM(hn) {
-        if ( this.container.setNeedle(hn).addWASM(this.iroot) === 1 ) {
+        if ( this.container.setNeedle(hn).addWASM(this.iroot) > 0 ) {
             this.last = -1;
             this.needle = '';
             this.size += 1;

src/js/wasm/hntrie.wat

@@ -51,8 +51,9 @@
 ;; offset.
 ;;
 (func (export "matches")
-    (param $icell i32)          ;; offset to root cell of the trie
+    (param $iroot i32)          ;; offset to root cell of the trie
     (result i32)                ;; result = match index, -1 = miss
+    (local $icell i32)          ;; offset to the current cell
     (local $char0 i32)          ;; offset to first character data
     (local $ineedle i32)        ;; current needle offset
     (local $c i32)
@@ -64,15 +65,24 @@
     i32.const 264               ;; start of char section is stored at addr 264
     i32.load
     set_local $char0
-    ;; $icell is an index into an array of 32-bit values
-    get_local $icell
-    i32.const 2
-    i32.shl
-    set_local $icell
     ;; let ineedle = this.buf[255];
     i32.const 255               ;; addr of needle is stored at addr 255
     i32.load8_u
     set_local $ineedle
+    ;; let icell = this.buf32[iroot+0];
+    get_local $iroot
+    i32.const 2
+    i32.shl
+    i32.load
+    i32.const 2
+    i32.shl
+    tee_local $icell
+    ;; if ( icell === 0 ) { return -1; }
+    i32.eqz
+    if
+        i32.const -1
+        return
+    end
     ;; for (;;) {
     block $noSegment loop $nextSegment
         ;; if ( ineedle === 0 ) { return -1; }
@@ -244,8 +254,9 @@
 ;; Add a new hostname to a trie which root cell is passed as argument.
 ;;
 (func (export "add")
-    (param $icell i32)          ;; index of root cell of the trie
+    (param $iroot i32)          ;; index of root cell of the trie
     (result i32)                ;; result: 0 not added, 1 = added
+    (local $icell i32)          ;; index of current cell in the trie
     (local $lhnchar i32)        ;; number of characters left to process in hostname
     (local $char0 i32)          ;; offset to start of character data section
     (local $vseg i32)           ;; integer value describing a segment
@@ -264,24 +275,6 @@
         i32.const 0
         return
     end
-    ;; let icell = iroot;
-    get_local $icell
-    i32.const 2
-    i32.shl
-    tee_local $icell
-    ;; if ( this.buf32[icell+2] === 0 ) {
-    i32.load offset=8
-    i32.eqz
-    if
-        ;;this.buf32[icell+2] = this.addSegment(lhnchar);
-        ;; return 1;
-        get_local $icell
-        get_local $lhnchar
-        call $addSegment
-        i32.store offset=8
-        i32.const 1
-        return
-    end
     ;; if (
     ;;     (this.buf32[HNBIGTRIE_CHAR0_SLOT] - this.buf32[HNBIGTRIE_TRIE1_SLOT]) < 24 ||
     ;;     (this.buf.length - this.buf32[HNBIGTRIE_CHAR1_SLOT]) < 256
@@ -310,6 +303,30 @@
             call $growBuf
         end
     end
+    ;; let icell = this.buf32[iroot+0];
+    get_local $iroot
+    i32.const 2
+    i32.shl
+    tee_local $iroot
+    i32.load
+    i32.const 2
+    i32.shl
+    tee_local $icell
+    ;; if ( this.buf32[icell+2] === 0 ) {
+    i32.eqz
+    if
+        ;; this.buf32[iroot+0] = this.addCell(0, 0, this.addSegment(lhnchar));
+        ;; return 1;
+        get_local $iroot
+        i32.const 0
+        i32.const 0
+        get_local $lhnchar
+        call $addSegment
+        call $addCell
+        i32.store
+        i32.const 1
+        return
+    end
     ;; const char0 = this.buf32[HNBIGTRIE_CHAR0_SLOT];
     i32.const 264
     i32.load
@@ -323,6 +340,19 @@
         ;; if ( vseg === 0 ) {
         i32.eqz
         if
+            ;; if ( this.buf[lhnchar-1] === 0x2E /* '.' */ ) { return -1; }
+            get_local $lhnchar
+            i32.const -1
+            i32.add
+            i32.load8_u
+            i32.const 0x2E
+            i32.eq
+            if
+                i32.const -1
+                return
+            end
+            ;; icell = this.buf32[icell+1];
+            ;; continue;
             get_local $icell
             i32.load offset=4
             i32.const 2
@@ -463,13 +493,23 @@
             else
                 ;; if ( inext !== 0 ) {
                 get_local $inext
-                i32.eqz
+                if
-                if else
                     ;; icell = inext;
                     get_local $inext
                     set_local $icell
                     br $nextSegment
                 end
+                ;; if ( this.buf[lhnchar-1] === 0x2E /* '.' */ ) { return -1; }
+                get_local $lhnchar
+                i32.const -1
+                i32.add
+                i32.load8_u
+                i32.const 0x2E
+                i32.eq
+                if
+                    i32.const -1
+                    return
+                end
                 ;; inext = this.addCell(0, 0, 0);
                 ;; this.buf32[icell+1] = inext;
                 get_local $icell