Bug 1850746 - Part 5: Factor out conversion between arena and page indexes r=sfink

This also adds an assertion that the first arena is on a page boundary which is
required for decommit to work correctly.

Differential Revision: https://phabricator.services.mozilla.com/D220414

js/src/gc/Allocator.cpp

@@ -524,7 +524,7 @@ void ArenaChunk::commitOnePage(GCRuntime* gc) {
   decommittedPages[pageIndex] = false;
 
   for (size_t i = 0; i < ArenasPerPage; i++) {
-    size_t arenaIndex = pageIndex * ArenasPerPage + i;
+    size_t arenaIndex = pageToArenaIndex(pageIndex) + i;
     MOZ_ASSERT(!freeCommittedArenas[arenaIndex]);
     freeCommittedArenas[arenaIndex] = true;
     ++info.numArenasFreeCommitted;

js/src/gc/Heap.cpp

@@ -94,7 +94,7 @@ const uint8_t Arena::ThingsPerArena[] = {
 
 bool Arena::allocated() const {
   size_t arenaIndex = ArenaChunk::arenaIndex(this);
-  size_t pageIndex = arenaIndex / ArenasPerPage;
+  size_t pageIndex = ArenaChunk::arenaToPageIndex(arenaIndex);
   bool result = !chunk()->decommittedPages[pageIndex] &&
                 !chunk()->freeCommittedArenas[arenaIndex] &&
                 IsValidAllocKind(allocKind);
@@ -321,7 +321,7 @@ inline bool ArenaChunk::canDecommitPage(size_t pageIndex) const {
     return false;
   }
 
-  size_t arenaIndex = pageIndex * ArenasPerPage;
+  size_t arenaIndex = pageToArenaIndex(pageIndex);
   for (size_t i = 0; i < ArenasPerPage; i++) {
     if (!freeCommittedArenas[arenaIndex + i]) {
       return false;
@@ -374,7 +374,7 @@ bool ArenaChunk::decommitOneFreePage(GCRuntime* gc, size_t pageIndex,
 
   // Temporarily mark the page as allocated while we decommit.
   for (size_t i = 0; i < ArenasPerPage; i++) {
-    size_t arenaIndex = pageIndex * ArenasPerPage + i;
+    size_t arenaIndex = pageToArenaIndex(pageIndex) + i;
     MOZ_ASSERT(freeCommittedArenas[arenaIndex]);
     freeCommittedArenas[arenaIndex] = false;
   }
@@ -397,7 +397,7 @@ bool ArenaChunk::decommitOneFreePage(GCRuntime* gc, size_t pageIndex,
     decommittedPages[pageIndex] = true;
   } else {
     for (size_t i = 0; i < ArenasPerPage; i++) {
-      size_t arenaIndex = pageIndex * ArenasPerPage + i;
+      size_t arenaIndex = pageToArenaIndex(pageIndex) + i;
       MOZ_ASSERT(!freeCommittedArenas[arenaIndex]);
       freeCommittedArenas[arenaIndex] = true;
     }
@@ -430,7 +430,7 @@ void ArenaChunk::decommitFreeArenasWithoutUnlocking(const AutoLockGC& lock) {
 
     decommittedPages[i] = true;
     for (size_t j = 0; j < ArenasPerPage; ++j) {
-      size_t arenaIndex = i * ArenasPerPage + j;
+      size_t arenaIndex = pageToArenaIndex(i) + j;
       MOZ_ASSERT(freeCommittedArenas[arenaIndex]);
       freeCommittedArenas[arenaIndex] = false;
     }
@@ -636,7 +636,8 @@ void ArenaChunk::verify() const {
   MOZ_ASSERT(freeCommittedCount == info.numArenasFreeCommitted);
 
   for (size_t i = 0; i < ArenasPerChunk; ++i) {
-    MOZ_ASSERT(!(decommittedPages[pageIndex(i)] && freeCommittedArenas[i]));
+    MOZ_ASSERT(
+        !(decommittedPages[arenaToPageIndex(i)] && freeCommittedArenas[i]));
   }
 }
 

js/src/gc/Heap.h

@@ -491,6 +491,20 @@ class ArenaChunk : public ArenaChunkBase {
     return (offset - offsetof(ArenaChunk, arenas)) >> ArenaShift;
   }
 
+  static size_t pageIndex(const Arena* arena) {
+    return arenaToPageIndex(arenaIndex(arena));
+  }
+
+  static size_t arenaToPageIndex(size_t arenaIndex) {
+    static_assert((offsetof(ArenaChunk, arenas) % PageSize) == 0,
+                  "First arena should be on a page boundary");
+    return arenaIndex / ArenasPerPage;
+  }
+
+  static size_t pageToArenaIndex(size_t pageIndex) {
+    return pageIndex * ArenasPerPage;
+  }
+
   explicit ArenaChunk(JSRuntime* runtime) : ArenaChunkBase(runtime) {}
 
   uintptr_t address() const {
@@ -549,16 +563,8 @@ class ArenaChunk : public ArenaChunkBase {
   // build.
   bool isPageFree(const Arena* arena) const;
 
-  // Get the page index of the arena.
-  size_t pageIndex(const Arena* arena) const {
-    return pageIndex(arenaIndex(arena));
-  }
-  size_t pageIndex(size_t arenaIndex) const {
-    return arenaIndex / ArenasPerPage;
-  }
-
-  Arena* pageAddress(size_t pageIndex) {
-    return &arenas[pageIndex * ArenasPerPage];
+  void* pageAddress(size_t pageIndex) {
+    return &arenas[pageToArenaIndex(pageIndex)];
   }
 };
 

js/src/gc/Verifier.cpp

@@ -692,7 +692,8 @@ void js::gc::MarkingValidator::validate() {
     ChunkMarkBitmap* incBitmap = &chunk->markBits;
 
     for (size_t i = 0; i < ArenasPerChunk; i++) {
-      if (chunk->decommittedPages[chunk->pageIndex(i)]) {
+      size_t pageIndex = ArenaChunk::arenaToPageIndex(i);
+      if (chunk->decommittedPages[pageIndex]) {
         continue;
       }
       Arena* arena = &chunk->arenas[i];