Backport hash collision workaround to 3.6.

src/x64/code-stubs-x64.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4524 matching lines @@
   // hash:         hash of two character string (32-bit int)
   // symbol_table: symbol table
   // mask:         capacity mask (32-bit int)
   // map:          -
   // scratch:      -
 
   // Perform a number of probes in the symbol table.
   static const int kProbes = 4;
   Label found_in_symbol_table;
   Label next_probe[kProbes];
+  Register candidate = scratch;  // Scratch register contains candidate.
   for (int i = 0; i < kProbes; i++) {
     // Calculate entry in symbol table.
     __ movl(scratch, hash);
     if (i > 0) {
       __ addl(scratch, Immediate(SymbolTable::GetProbeOffset(i)));
     }
     __ andl(scratch, mask);
 
     // Load the entry from the symbol table.
-    Register candidate = scratch;  // Scratch register contains candidate.
     STATIC_ASSERT(SymbolTable::kEntrySize == 1);
     __ movq(candidate,
             FieldOperand(symbol_table,
                          scratch,
                          times_pointer_size,
                          SymbolTable::kElementsStartOffset));
 
     // If entry is undefined no string with this hash can be found.
     Label is_string;
     __ CmpObjectType(candidate, ODDBALL_TYPE, map);
@@ skipping 25 matching lines @@
     __ andl(temp, Immediate(0x0000ffff));
     __ cmpl(chars, temp);
     __ j(equal, &found_in_symbol_table);
     __ bind(&next_probe[i]);
   }
 
   // No matching 2 character string found by probing.
   __ jmp(not_found);
 
   // Scratch register contains result when we fall through to here.
-  Register result = scratch;
+  Register result = candidate;
   __ bind(&found_in_symbol_table);
   if (!result.is(rax)) {
     __ movq(rax, result);
   }
 }
 
 
 void StringHelper::GenerateHashInit(MacroAssembler* masm,
                                     Register hash,
                                     Register character,
                                     Register scratch) {
-  // hash = character + (character << 10);
-  __ movl(hash, character);
-  __ shll(hash, Immediate(10));
-  __ addl(hash, character);
+  // hash = (seed + character) + ((seed + character) << 10);
+  __ LoadRoot(scratch, Heap::kStringHashSeedRootIndex);
+  __ SmiToInteger32(scratch, scratch);
+  __ addl(scratch, character);
+  __ movl(hash, scratch);
+  __ shll(scratch, Immediate(10));
+  __ addl(hash, scratch);
   // hash ^= hash >> 6;
   __ movl(scratch, hash);
-  __ sarl(scratch, Immediate(6));
+  __ shrl(scratch, Immediate(6));
   __ xorl(hash, scratch);
 }
 
 
 void StringHelper::GenerateHashAddCharacter(MacroAssembler* masm,
                                             Register hash,
                                             Register character,
                                             Register scratch) {
   // hash += character;
   __ addl(hash, character);
   // hash += hash << 10;
   __ movl(scratch, hash);
   __ shll(scratch, Immediate(10));
   __ addl(hash, scratch);
   // hash ^= hash >> 6;
   __ movl(scratch, hash);
-  __ sarl(scratch, Immediate(6));
+  __ shrl(scratch, Immediate(6));
   __ xorl(hash, scratch);
 }
 
 
 void StringHelper::GenerateHashGetHash(MacroAssembler* masm,
                                        Register hash,
                                        Register scratch) {
   // hash += hash << 3;
   __ leal(hash, Operand(hash, hash, times_8, 0));
   // hash ^= hash >> 11;
   __ movl(scratch, hash);
-  __ sarl(scratch, Immediate(11));
+  __ shrl(scratch, Immediate(11));
   __ xorl(hash, scratch);
   // hash += hash << 15;
   __ movl(scratch, hash);
   __ shll(scratch, Immediate(15));
   __ addl(hash, scratch);
 
+  uint32_t kHashShiftCutOffMask = (1 << (32 - String::kHashShift)) - 1;
+  __ andl(hash, Immediate(kHashShiftCutOffMask));
+
   // if (hash == 0) hash = 27;
   Label hash_not_zero;
   __ j(not_zero, &hash_not_zero);
   __ Set(hash, 27);
   __ bind(&hash_not_zero);
 }
 
 void SubStringStub::Generate(MacroAssembler* masm) {
   Label runtime;
 
@@ skipping 826 matching lines @@
   __ Drop(1);
   __ ret(2 * kPointerSize);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64