Bug 941804 - Reduce chunk size to 256K for B2G r=terrence

b2g/confvars.sh

@@ -68,3 +68,5 @@ MOZ_BUNDLED_FONTS=1
 
 # Enable exact rooting on b2g.
 JSGC_USE_EXACT_ROOTING=1
+
+JS_GC_SMALL_CHUNK_SIZE=1

configure.in

@@ -7229,6 +7229,17 @@ if test -n "$JSGC_GENERATIONAL"; then
     AC_DEFINE(JSGC_GENERATIONAL)
 fi
 
+dnl ========================================================
+dnl = Use a smaller chunk size for GC chunks
+dnl ========================================================
+MOZ_ARG_ENABLE_BOOL(small-chunk-size,
+[  --enable-small-chunk-size  Allocate memory for JS GC things in smaller chunks],
+    JS_GC_SMALL_CHUNK_SIZE=1,
+    JS_GC_SMALL_CHUNK_SIZE= )
+if test -n "$JS_GC_SMALL_CHUNK_SIZE"; then
+    AC_DEFINE(JS_GC_SMALL_CHUNK_SIZE)
+fi
+
 dnl ========================================================
 dnl Zealous JavaScript GC
 dnl ========================================================
@@ -9252,6 +9263,9 @@ fi
 if test -z "$JSGC_GENERATIONAL" ; then
     ac_configure_args="$ac_configure_args --disable-gcgenerational"
 fi
+if test -n "$JS_GC_SMALL_CHUNK_SIZE" ; then
+    ac_configure_args="$ac_configure_args --enable-small-chunk-size"
+fi
 if test -z "$MOZ_NATIVE_NSPR"; then
     ac_configure_args="$ac_configure_args --with-nspr-cflags='$NSPR_CFLAGS'"
     ac_configure_args="$ac_configure_args --with-nspr-libs='$NSPR_LIBS'"

js/public/HeapAPI.h

@@ -32,7 +32,11 @@ const size_t ArenaShift = 12;
 const size_t ArenaSize = size_t(1) << ArenaShift;
 const size_t ArenaMask = ArenaSize - 1;
 
+#ifdef JS_GC_SMALL_CHUNK_SIZE
+const size_t ChunkShift = 18;
+#else
 const size_t ChunkShift = 20;
+#endif
 const size_t ChunkSize = size_t(1) << ChunkShift;
 const size_t ChunkMask = ChunkSize - 1;
 
@@ -41,8 +45,13 @@ const size_t CellSize = size_t(1) << CellShift;
 const size_t CellMask = CellSize - 1;
 
 /* These are magic constants derived from actual offsets in gc/Heap.h. */
+#ifdef JS_GC_SMALL_CHUNK_SIZE
+const size_t ChunkMarkBitmapOffset = 258104;
+const size_t ChunkMarkBitmapBits = 31744;
+#else
 const size_t ChunkMarkBitmapOffset = 1032352;
 const size_t ChunkMarkBitmapBits = 129024;
+#endif
 const size_t ChunkRuntimeOffset = ChunkSize - sizeof(void*);
 const size_t ChunkLocationOffset = ChunkSize - 2 * sizeof(void*) - sizeof(uint64_t);
 

js/src/configure.in

@@ -3214,6 +3214,19 @@ if test -n "$JSGC_USE_EXACT_ROOTING"; then
     AC_DEFINE(JSGC_USE_EXACT_ROOTING)
 fi
 
+dnl ========================================================
+dnl = Use a smaller chunk size for GC chunks
+dnl ========================================================
+dnl Use large (1MB) chunks by default.  For B2G this option is used to give
+dnl smaller (currently 256K) chunks.
+MOZ_ARG_ENABLE_BOOL(small-chunk-size,
+[  --enable-small-chunk-size  Allocate memory for JS GC things in smaller chunks],
+    JS_GC_SMALL_CHUNK_SIZE=1,
+    JS_GC_SMALL_CHUNK_SIZE= )
+if test -n "$JS_GC_SMALL_CHUNK_SIZE"; then
+    AC_DEFINE(JS_GC_SMALL_CHUNK_SIZE)
+fi
+
 dnl ========================================================
 dnl = Use GC tracing
 dnl ========================================================

js/src/gc/Heap.h

@@ -707,7 +707,12 @@ const size_t BytesPerArenaWithHeader = ArenaSize + ArenaBitmapBytes;
 const size_t ChunkDecommitBitmapBytes = ChunkSize / ArenaSize / JS_BITS_PER_BYTE;
 const size_t ChunkBytesAvailable = ChunkSize - sizeof(ChunkInfo) - ChunkDecommitBitmapBytes;
 const size_t ArenasPerChunk = ChunkBytesAvailable / BytesPerArenaWithHeader;
+
+#ifdef JS_GC_SMALL_CHUNK_SIZE
+static_assert(ArenasPerChunk == 62, "Do not accidentally change our heap's density.");
+#else
 static_assert(ArenasPerChunk == 252, "Do not accidentally change our heap's density.");
+#endif
 
 /* A chunk bitmap contains enough mark bits for all the cells in a chunk. */
 struct ChunkBitmap