Fix unaligned accesses in back_edge tables.

This patch fixes the step size of masm->pc_ in back_edge tables to words (4 bytes) to ensure 4 bytes alignment for read/write operations. Read and write of words (4 bytes) data from aligned space (address % 4 == 0) is more efficient on all platforms and especially on MIPS where without this alignment fix a kernel exception handler is used for every unaligned access.

This patch increases the size of back_edge tables by 3 bytes in every row. By the test it seem the back_edge table quite small in every/most cases (maximal length is 18 so in that case there are only 54 additional bytes with this patch).

BUG=

Patch from Douglas Leung <Douglas.Leung@imgtec.com>

Review URL: https://codereview.chromium.org/19248002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15782 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/deoptimizer.cc

@@ -2371,8 +2371,8 @@ void Deoptimizer::PatchInterruptCode(Code* unoptimized_code,
   uint32_t table_length = Memory::uint32_at(back_edge_cursor);
   back_edge_cursor += kIntSize;
   for (uint32_t i = 0; i < table_length; ++i) {
-    uint8_t loop_depth = Memory::uint8_at(back_edge_cursor + 2 * kIntSize);
-    if (loop_depth == loop_nesting_level) {
+    uint32_t loop_depth = Memory::uint32_at(back_edge_cursor + 2 * kIntSize);
+    if (static_cast<int>(loop_depth) == loop_nesting_level) {
       // Loop back edge has the loop depth that we want to patch.
       uint32_t pc_offset = Memory::uint32_at(back_edge_cursor + kIntSize);
       Address pc_after = unoptimized_code->instruction_start() + pc_offset;
@@ -2403,8 +2403,8 @@ void Deoptimizer::RevertInterruptCode(Code* unoptimized_code,
   uint32_t table_length = Memory::uint32_at(back_edge_cursor);
   back_edge_cursor += kIntSize;
   for (uint32_t i = 0; i < table_length; ++i) {
-    uint8_t loop_depth = Memory::uint8_at(back_edge_cursor + 2 * kIntSize);
-    if (loop_depth <= loop_nesting_level) {
+    uint32_t loop_depth = Memory::uint32_at(back_edge_cursor + 2 * kIntSize);
+    if (static_cast<int>(loop_depth) <= loop_nesting_level) {
       uint32_t pc_offset = Memory::uint32_at(back_edge_cursor + kIntSize);
       Address pc_after = unoptimized_code->instruction_start() + pc_offset;
       RevertInterruptCodeAt(unoptimized_code,
@@ -2435,13 +2435,13 @@ void Deoptimizer::VerifyInterruptCode(Code* unoptimized_code,
   uint32_t table_length = Memory::uint32_at(back_edge_cursor);
   back_edge_cursor += kIntSize;
   for (uint32_t i = 0; i < table_length; ++i) {
-    uint8_t loop_depth = Memory::uint8_at(back_edge_cursor + 2 * kIntSize);
-    CHECK_LE(loop_depth, Code::kMaxLoopNestingMarker);
+    uint32_t loop_depth = Memory::uint32_at(back_edge_cursor + 2 * kIntSize);
+    CHECK_LE(static_cast<int>(loop_depth), Code::kMaxLoopNestingMarker);
     // Assert that all back edges for shallower loops (and only those)
     // have already been patched.
     uint32_t pc_offset = Memory::uint32_at(back_edge_cursor + kIntSize);
     Address pc_after = unoptimized_code->instruction_start() + pc_offset;
-    CHECK_EQ((loop_depth <= loop_nesting_level),
+    CHECK_EQ((static_cast<int>(loop_depth) <= loop_nesting_level),
              InterruptCodeIsPatched(unoptimized_code,
                                     pc_after,
                                     interrupt_code,

src/full-codegen.cc

@@ -379,7 +379,7 @@ unsigned FullCodeGenerator::EmitBackEdgeTable() {
   for (unsigned i = 0; i < length; ++i) {
     __ dd(back_edges_[i].id.ToInt());
     __ dd(back_edges_[i].pc);
-    __ db(back_edges_[i].loop_depth);
+    __ dd(back_edges_[i].loop_depth);
   }
   return offset;
 }

src/full-codegen.h

@@ -136,7 +136,7 @@ class FullCodeGenerator: public AstVisitor {
 #error Unsupported target architecture.
 #endif
 
-  static const int kBackEdgeEntrySize = 2 * kIntSize + kOneByteSize;
+  static const int kBackEdgeEntrySize = 3 * kIntSize;
 
  private:
   class Breakable;
@@ -648,7 +648,7 @@ class FullCodeGenerator: public AstVisitor {
   struct BackEdgeEntry {
     BailoutId id;
     unsigned pc;
-    uint8_t loop_depth;
+    uint32_t loop_depth;
   };
 
   struct TypeFeedbackCellEntry {

src/objects.cc

@@ -10799,7 +10799,8 @@ void Code::Disassemble(const char* name, FILE* out) {
       for (uint32_t i = 0; i < table_length; ++i) {
         uint32_t ast_id = Memory::uint32_at(back_edge_cursor);
         uint32_t pc_offset = Memory::uint32_at(back_edge_cursor + kIntSize);
-        uint8_t loop_depth = Memory::uint8_at(back_edge_cursor + 2 * kIntSize);
+        uint32_t loop_depth = Memory::uint32_at(back_edge_cursor +
+                                                2 * kIntSize);
         PrintF(out, "%6u  %9u  %10u\n", ast_id, pc_offset, loop_depth);
         back_edge_cursor += FullCodeGenerator::kBackEdgeEntrySize;
       }

src/runtime.cc

@@ -8539,13 +8539,13 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_CompileForOnStackReplacement) {
     Address table_cursor = start + unoptimized->back_edge_table_offset();
     uint32_t table_length = Memory::uint32_at(table_cursor);
     table_cursor += kIntSize;
-    uint8_t loop_depth = 0;
+    uint32_t loop_depth = 0;
     for (unsigned i = 0; i < table_length; ++i) {
       // Table entries are (AST id, pc offset) pairs.
       uint32_t pc_offset = Memory::uint32_at(table_cursor + kIntSize);
       if (pc_offset == target_pc_offset) {
         ast_id = BailoutId(static_cast<int>(Memory::uint32_at(table_cursor)));
-        loop_depth = Memory::uint8_at(table_cursor + 2 * kIntSize);
+        loop_depth = Memory::uint32_at(table_cursor + 2 * kIntSize);
         break;
       }
       table_cursor += FullCodeGenerator::kBackEdgeEntrySize;