Optionally use 31-bits SMI value for 64-bit system

src/x64/code-stubs-x64.cc

 // Copyright 2013 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 706 matching lines @@
     MacroAssembler* masm,
     Label* slow,
     BinaryOpStub::SmiCodeGenerateHeapNumberResults allow_heapnumber_results,
     Token::Value op) {
 
   // Arguments to BinaryOpStub are in rdx and rax.
   const Register left = rdx;
   const Register right = rax;
 
   // We only generate heapnumber answers for overflowing calculations
-  // for the four basic arithmetic operations and logical right shift by 0.
+  // for the four basic arithmetic operations and logical right shift by 0
+  // for 32 bit SMI value, for 31 bit SMI value, we plus SHL and logical right
+  // shift 1.
   bool generate_inline_heapnumber_results =
       (allow_heapnumber_results == BinaryOpStub::ALLOW_HEAPNUMBER_RESULTS) &&
       (op == Token::ADD || op == Token::SUB ||
+       (kSmiValueSize == 31 && op == Token::SHL) ||

[danno, 2013/08/01 16:45:41]

Is think you should turn this into a predicate:

SmisAreUint32()

that does this test. 

[haitao.feng, 2013/08/02 09:35:51]

Done. Add IsUnsafeSmiOperator function in the macro assembler.

        op == Token::MUL || op == Token::DIV || op == Token::SHR);
 
   // Smi check of both operands.  If op is BIT_OR, the check is delayed
   // until after the OR operation.
   Label not_smis;
   Label use_fp_on_smis;
   Label fail;
 
   if (op != Token::BIT_OR) {
     Comment smi_check_comment(masm, "-- Smi check arguments");
@@ skipping 43 matching lines @@
       ASSERT(right.is(rax));
       __ SmiXor(right, right, left);  // BIT_XOR is commutative.
       break;
 
     case Token::BIT_AND:
       ASSERT(right.is(rax));
       __ SmiAnd(right, right, left);  // BIT_AND is commutative.
       break;
 
     case Token::SHL:
-      __ SmiShiftLeft(left, left, right);
+      if (kSmiValueSize == 31) {

[danno, 2013/08/01 16:45:41]

Why can't you put this extra stack manipulation in the 31-bit case inside the
SmiShiftLeft macro assembler instruction??

[haitao.feng, 2013/08/02 09:35:51]

Done.

+        __ push(left);
+        __ push(right);
+      }
+      __ SmiShiftLeft(left, left, right, &use_fp_on_smis);
       __ movq(rax, left);
+      if (kSmiValueSize == 31) {
+        __ addq(rsp, Immediate(2 * kRegisterSize));
+      }
       break;
 
     case Token::SAR:
       __ SmiShiftArithmeticRight(left, left, right);
       __ movq(rax, left);
       break;
 
     case Token::SHR:
+      if (kSmiValueSize == 31) {

[danno, 2013/08/01 16:45:41]

Same here.

[haitao.feng, 2013/08/02 09:35:51]

Done.

+        __ push(left);
+        __ push(right);
+      }
       __ SmiShiftLogicalRight(left, left, right, &use_fp_on_smis);
       __ movq(rax, left);
+      if (kSmiValueSize == 31) {
+        __ addq(rsp, Immediate(2 * kRegisterSize));
+      }
       break;
 
     default:
       UNREACHABLE();
   }
 
   // 5. Emit return of result in rax.  Some operations have registers pushed.
   __ ret(0);
 
   if (use_fp_on_smis.is_linked()) {

[danno, 2013/08/01 16:45:41]

All of this use_fp_on_smis code can be encapsulated in the macro assembler,
can't it? You shouldn't even have to pass use_fp_on_smi as a label, only the
fail label, since the fp-handling case only needs to jump to that code if it
overflow the smi range. I know that's a bigger change, but if you just pass
generate_inline_heapnumber_results to the macro assembler instructions for all
of the math operations and the fail label, I think you can handle everything
there.

[haitao.feng, 2013/08/02 09:35:51]

The X64 implementation utilizes a fact that when SHR 0 overflow, i.e., a
negative has to be represented as positive, the binary representation are the
same. If we do not use this optimization (we need to add an overhead of
movq(dst, src1) , I could get the code logic nearly the same for 31-bit SMI and
32-bit SMI. 

On 2013/08/01 16:45:41, danno wrote:

     // 6. For some operations emit inline code to perform floating point
     //    operations on known smis (e.g., if the result of the operation
     //    overflowed the smi range).
     __ bind(&use_fp_on_smis);
     if (op == Token::DIV || op == Token::MOD) {
       // Restore left and right to rdx and rax.
       __ movq(rdx, rcx);
       __ movq(rax, rbx);
     }
 
     if (generate_inline_heapnumber_results) {
-      __ AllocateHeapNumber(rcx, rbx, slow);
-      Comment perform_float(masm, "-- Perform float operation on smis");
-      if (op == Token::SHR) {
-        __ SmiToInteger32(left, left);
-        __ cvtqsi2sd(xmm0, left);
+      if (kSmiValueSize == 32) {
+        __ AllocateHeapNumber(rcx, rbx, slow);
+        Comment perform_float(masm, "-- Perform float operation on smis");
+        if (op == Token::SHR) {
+          __ SmiToInteger32(left, left);
+          __ cvtqsi2sd(xmm0, left);
+        } else {
+          FloatingPointHelper::LoadSSE2SmiOperands(masm);
+          switch (op) {
+            case Token::ADD: __ addsd(xmm0, xmm1); break;
+            case Token::SUB: __ subsd(xmm0, xmm1); break;
+            case Token::MUL: __ mulsd(xmm0, xmm1); break;
+            case Token::DIV: __ divsd(xmm0, xmm1); break;
+            default: UNREACHABLE();
+          }
+        }
+        __ movsd(FieldOperand(rcx, HeapNumber::kValueOffset), xmm0);
+        __ movq(rax, rcx);
+        __ ret(0);
       } else {
-        FloatingPointHelper::LoadSSE2SmiOperands(masm);
-        switch (op) {
-        case Token::ADD: __ addsd(xmm0, xmm1); break;
-        case Token::SUB: __ subsd(xmm0, xmm1); break;
-        case Token::MUL: __ mulsd(xmm0, xmm1); break;
-        case Token::DIV: __ divsd(xmm0, xmm1); break;
-        default: UNREACHABLE();
+        ASSERT(kSmiValueSize == 31);
+        Label goto_slow;
+        __ AllocateHeapNumber(rcx, rbx, &goto_slow);
+        Comment perform_float(masm, "-- Perform float operation on smis");
+        if (op == Token::SHL) {
+          __ cvtlsi2sd(xmm0, left);
+          // drop left and right on the stack.
+          __ addq(rsp, Immediate(2 * kRegisterSize));
+        } else if (op == Token::SHR) {
+          // The value of left is from MacroAssembler::SmiShiftLogicalRight
+          // We allow logical shift value:
+          // 0 : might turn a signed integer into unsigned integer
+          // 1 : the value might be above 2^30 - 1
+          __ cvtqsi2sd(xmm0, left);
+          // drop left and right on the stack.
+          __ addq(rsp, Immediate(2 * kRegisterSize));
+        } else {
+          FloatingPointHelper::LoadSSE2SmiOperands(masm);
+          switch (op) {
+            case Token::ADD: __ addsd(xmm0, xmm1); break;
+            case Token::SUB: __ subsd(xmm0, xmm1); break;
+            case Token::MUL: __ mulsd(xmm0, xmm1); break;
+            case Token::DIV: __ divsd(xmm0, xmm1); break;
+            default: UNREACHABLE();

[danno, 2013/08/01 16:45:41]

Also, I think you can share much of this code with the version in the 32-bit
version. Only the stack pop restore at the end needs to be added in the 31-bit
case.

[haitao.feng, 2013/08/02 09:35:51]

Done.

+          }
         }
+        __ movsd(FieldOperand(rcx, HeapNumber::kValueOffset), xmm0);
+        __ movq(rax, rcx);
+        __ ret(0);
+
+        __ bind(&goto_slow);
+        if (op == Token::SHL || op == Token::SHR) {
+          // Restore left and right from stack.
+          __ pop(right);
+          __ pop(left);
+        }
+        __ jmp(slow);
       }
-      __ movsd(FieldOperand(rcx, HeapNumber::kValueOffset), xmm0);
-      __ movq(rax, rcx);
-      __ ret(0);
     } else {
+      if (kSmiValueSize == 31 && (op == Token::SHL || op == Token::SHR)) {
+        // Restore left and right from stack.
+        __ pop(right);
+        __ pop(left);
+      }
       __ jmp(&fail);
     }
   }
 
   // 7. Non-smi operands reach the end of the code generated by
   //    GenerateSmiCode, and fall through to subsequent code,
   //    with the operands in rdx and rax.
   //    But first we check if non-smi values are HeapNumbers holding
   //    values that could be smi.
   __ bind(&not_smis);
@@ skipping 14 matching lines @@
   __ jmp(&smi_values);
   __ bind(&fail);
 }
 
 
 static void BinaryOpStub_GenerateHeapResultAllocation(MacroAssembler* masm,
                                                       Label* alloc_failure,
                                                       OverwriteMode mode);
 
 
 static void BinaryOpStub_GenerateFloatingPointCode(MacroAssembler* masm,

[danno, 2013/08/01 16:45:41]

start the parameter list on the next line with a 4-char indentations, it saves
you the weird formatting of result_type below.

[haitao.feng, 2013/08/02 09:35:51]

Done.

                                                    Label* allocation_failure,
                                                    Label* non_numeric_failure,
                                                    Token::Value op,
+                                                   BinaryOpIC::TypeInfo
+                                                       result_type,
+                                                   Label* non_int32_failure,
                                                    OverwriteMode mode) {
   switch (op) {
     case Token::ADD:
     case Token::SUB:
     case Token::MUL:
     case Token::DIV: {
       FloatingPointHelper::LoadSSE2UnknownOperands(masm, non_numeric_failure);
 
       switch (op) {
         case Token::ADD: __ addsd(xmm0, xmm1); break;
         case Token::SUB: __ subsd(xmm0, xmm1); break;
         case Token::MUL: __ mulsd(xmm0, xmm1); break;
         case Token::DIV: __ divsd(xmm0, xmm1); break;
         default: UNREACHABLE();
       }
+
+      if (kSmiValueSize == 31 && non_int32_failure != NULL) {
+        if (result_type <= BinaryOpIC::INT32) {
+          __ cvttsd2si(kScratchRegister, xmm0);
+          __ cvtlsi2sd(xmm2, kScratchRegister);
+          __ pcmpeqd(xmm2, xmm0);
+          __ movmskpd(rcx, xmm2);
+          __ testl(rcx, Immediate(1));
+          __ j(zero, non_int32_failure);
+        }
+      }
+
       BinaryOpStub_GenerateHeapResultAllocation(
           masm, allocation_failure, mode);
       __ movsd(FieldOperand(rax, HeapNumber::kValueOffset), xmm0);
       __ ret(0);
       break;
     }
     case Token::MOD: {
       // For MOD we jump to the allocation_failure label, to call runtime.
       __ jmp(allocation_failure);
       break;
     }
     case Token::BIT_OR:
     case Token::BIT_AND:
     case Token::BIT_XOR:
     case Token::SAR:
     case Token::SHL:
     case Token::SHR: {
-      Label non_smi_shr_result;
+      Label non_smi_result;
+      Label goto_non_numeric_failure;
       Register heap_number_map = r9;
+      if (kSmiValueSize == 31) {
+        // Push arguments on stack
+        __ push(rdx);
+        __ push(rax);
+      }
       __ LoadRoot(heap_number_map, Heap::kHeapNumberMapRootIndex);
-      FloatingPointHelper::LoadAsIntegers(masm, non_numeric_failure,
-                                          heap_number_map);
+      if (kSmiValueSize == 32) {
+        FloatingPointHelper::LoadAsIntegers(masm, non_numeric_failure,
+                                            heap_number_map);
+      } else {
+        ASSERT(kSmiValueSize == 31);
+        FloatingPointHelper::LoadAsIntegers(masm, &goto_non_numeric_failure,
+                                            heap_number_map);
+      }
       switch (op) {
         case Token::BIT_OR:  __ orl(rax, rcx); break;
         case Token::BIT_AND: __ andl(rax, rcx); break;
         case Token::BIT_XOR: __ xorl(rax, rcx); break;
         case Token::SAR: __ sarl_cl(rax); break;
         case Token::SHL: __ shll_cl(rax); break;
         case Token::SHR: {
           __ shrl_cl(rax);
-          // Check if result is negative. This can only happen for a shift
-          // by zero.
-          __ testl(rax, rax);
-          __ j(negative, &non_smi_shr_result);
+          if (kSmiValueSize == 32) {
+            // Check if result is negative. This can only happen for a shift
+            // by zero.
+            __ testl(rax, rax);
+            __ j(negative, &non_smi_result);
+          }
           break;
         }
         default: UNREACHABLE();
       }
-      STATIC_ASSERT(kSmiValueSize == 32);
+
+      if (kSmiValueSize == 31) {
+        if (op == Token::SHR) {
+          __ JumpIfUIntNotValidSmiValue(rax, &non_smi_result, Label::kNear);
+        } else {
+          __ JumpIfNotValidSmiValue(rax, &non_smi_result, Label::kNear);
+        }
+        // Drop saved arguments.
+        __ addq(rsp, Immediate(2 * kRegisterSize));
+      }
+
       // Tag smi result and return.
       __ Integer32ToSmi(rax, rax);
       __ Ret();
 
-      // Logical shift right can produce an unsigned int32 that is not
-      // an int32, and so is not in the smi range.  Allocate a heap number
-      // in that case.
-      if (op == Token::SHR) {
-        __ bind(&non_smi_shr_result);
+      if (kSmiValueSize == 31) {
+        __ bind(&goto_non_numeric_failure);
+        // Restore arguments.
+        __ pop(rax);
+        __ pop(rdx);
+        __ jmp(non_numeric_failure);
+      }
+
+      if (kSmiValueSize == 32) {
+        if (op == Token::SHR) {
+          // Logical shift right can produce an unsigned int32 that is not
+          // an int32, and so is not in the smi range.  Allocate a heap number
+          // in that case.
+          __ bind(&non_smi_result);
+          Label allocation_failed;
+          __ movl(rbx, rax);  // rbx holds result value (uint32 value as int64).
+          // Allocate heap number in new space.
+          // Not using AllocateHeapNumber macro in order to reuse
+          // already loaded heap_number_map.
+          __ Allocate(HeapNumber::kSize, rax, rdx, no_reg, &allocation_failed,
+                      TAG_OBJECT);
+          // Set the map.
+          __ AssertRootValue(heap_number_map,
+                             Heap::kHeapNumberMapRootIndex,
+                             "HeapNumberMap register clobbered.");
+          __ movq(FieldOperand(rax, HeapObject::kMapOffset),
+                  heap_number_map);
+          __ cvtqsi2sd(xmm0, rbx);
+          __ movsd(FieldOperand(rax, HeapNumber::kValueOffset), xmm0);
+          __ Ret();
+
+          __ bind(&allocation_failed);
+          // We need tagged values in rdx and rax for the following code,
+          // not int32 in rax and rcx.
+          __ Integer32ToSmi(rax, rcx);
+          __ Integer32ToSmi(rdx, rbx);
+          __ jmp(allocation_failure);
+        }
+      } else {
+        ASSERT(kSmiValueSize == 31);
+        __ bind(&non_smi_result);
         Label allocation_failed;
         __ movl(rbx, rax);  // rbx holds result value (uint32 value as int64).
         // Allocate heap number in new space.
         // Not using AllocateHeapNumber macro in order to reuse
         // already loaded heap_number_map.
-        __ Allocate(HeapNumber::kSize, rax, rdx, no_reg, &allocation_failed,
-                    TAG_OBJECT);
+        Label skip_allocation;
+        switch (mode) {
+          case OVERWRITE_LEFT: {
+            __ movq(rax, Operand(rsp, 1 * kRegisterSize));
+            __ JumpIfNotSmi(rax, &skip_allocation);
+            __ Allocate(HeapNumber::kSize, rax, r8, no_reg, &allocation_failed,
+                        TAG_OBJECT);
+            __ movq(FieldOperand(rax, HeapObject::kMapOffset),
+                    heap_number_map);
+            __ bind(&skip_allocation);
+            break;
+          }
+          case OVERWRITE_RIGHT:
+            __ movq(rax, Operand(rsp, 0 * kRegisterSize));
+            __ JumpIfNotSmi(rax, &skip_allocation);
+            // Fall through!
+          case NO_OVERWRITE:
+            __ Allocate(HeapNumber::kSize, rax, r8, no_reg, &allocation_failed,
+                        TAG_OBJECT);
+            __ movq(FieldOperand(rax, HeapObject::kMapOffset),
+                    heap_number_map);
+            __ bind(&skip_allocation);
+            break;
+          default: UNREACHABLE();
+        }
         // Set the map.
         __ AssertRootValue(heap_number_map,
                            Heap::kHeapNumberMapRootIndex,
                            "HeapNumberMap register clobbered.");
-        __ movq(FieldOperand(rax, HeapObject::kMapOffset),
-                heap_number_map);
-        __ cvtqsi2sd(xmm0, rbx);
+        if (op == Token::SHR) {
+          __ cvtqsi2sd(xmm0, rbx);
+        } else {
+          // All other operations returns a signed int32, so we
+          // use lsi2sd here to retain the sign bit.
+          __ cvtlsi2sd(xmm0, rbx);
+        }
         __ movsd(FieldOperand(rax, HeapNumber::kValueOffset), xmm0);
+        // Drop saved arguments.
+        __ addq(rsp, Immediate(2 * kRegisterSize));
         __ Ret();
 
         __ bind(&allocation_failed);
-        // We need tagged values in rdx and rax for the following code,
-        // not int32 in rax and rcx.
-        __ Integer32ToSmi(rax, rcx);
-        __ Integer32ToSmi(rdx, rbx);
+        // Restore arguments from stack.
+        __ pop(rax);
+        __ pop(rdx);
         __ jmp(allocation_failure);
       }
       break;
     }
     default: UNREACHABLE(); break;
   }
   // No fall-through from this generated code.
   if (FLAG_debug_code) {
     __ Abort("Unexpected fall-through in "
              "BinaryStub_GenerateFloatingPointCode.");
@@ skipping 77 matching lines @@
       FrameScope scope(masm, StackFrame::INTERNAL);
       GenerateRegisterArgsPush(masm);
       GenerateCallRuntime(masm);
     }
     __ Ret();
   }
 }
 
 
 void BinaryOpStub::GenerateInt32Stub(MacroAssembler* masm) {
-  // The int32 case is identical to the Smi case.  We avoid creating this
-  // ic state on x64.
-  UNREACHABLE();
+  if (kSmiValueSize == 32) {
+    // The int32 case is identical to the Smi case.  We avoid creating this
+    // ic state on x64.
+    UNREACHABLE();
+  } else {
+    ASSERT(kSmiValueSize == 31);
+    ASSERT(Max(left_type_, right_type_) == BinaryOpIC::INT32);
+
+    Label gc_required, not_number, not_int32;
+    BinaryOpStub_GenerateFloatingPointCode(masm, &gc_required, &not_number,
+                                         op_, result_type_, &not_int32, mode_);

[danno, 2013/08/01 16:45:41]

strange indentation

[haitao.feng, 2013/08/02 09:35:51]

Done.

+
+    __ bind(&not_number);
+    __ bind(&not_int32);
+    GenerateTypeTransition(masm);
+
+    __ bind(&gc_required);
+    {
+      FrameScope scope(masm, StackFrame::INTERNAL);
+      GenerateRegisterArgsPush(masm);
+      GenerateCallRuntime(masm);
+    }
+    __ Ret();
+  }
 }
 
 
 void BinaryOpStub::GenerateBothStringStub(MacroAssembler* masm) {
   Label call_runtime;
   ASSERT(left_type_ == BinaryOpIC::STRING && right_type_ == BinaryOpIC::STRING);
   ASSERT(op_ == Token::ADD);
   // If both arguments are strings, call the string add stub.
   // Otherwise, do a transition.
 
@@ skipping 85 matching lines @@
   // or the right operand. For precise type feedback, patch the IC
   // again if this changes.
   if (left_type_ == BinaryOpIC::SMI) {
     BinaryOpStub_CheckSmiInput(masm, rdx, &not_number);
   }
   if (right_type_ == BinaryOpIC::SMI) {
     BinaryOpStub_CheckSmiInput(masm, rax, &not_number);
   }
 
   BinaryOpStub_GenerateFloatingPointCode(
-      masm, &gc_required, &not_number, op_, mode_);
+      masm, &gc_required, &not_number, op_, result_type_, NULL, mode_);
 
   __ bind(&not_number);
   GenerateTypeTransition(masm);
 
   __ bind(&gc_required);
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
     GenerateRegisterArgsPush(masm);
     GenerateCallRuntime(masm);
   }
   __ Ret();
 }
 
 
 void BinaryOpStub::GenerateGeneric(MacroAssembler* masm) {
   Label call_runtime, call_string_add_or_runtime;
 
   BinaryOpStub_GenerateSmiCode(
       masm, &call_runtime, ALLOW_HEAPNUMBER_RESULTS, op_);
 
   BinaryOpStub_GenerateFloatingPointCode(
-      masm, &call_runtime, &call_string_add_or_runtime, op_, mode_);
+      masm, &call_runtime, &call_string_add_or_runtime, op_,
+      result_type_, NULL, mode_);
 
   __ bind(&call_string_add_or_runtime);
   if (op_ == Token::ADD) {
     GenerateAddStrings(masm);
   }
 
   __ bind(&call_runtime);
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
     GenerateRegisterArgsPush(masm);
@@ skipping 530 matching lines @@
   // Convert HeapNumber to smi if possible.
   __ movsd(xmm0, FieldOperand(first, HeapNumber::kValueOffset));
   __ movq(scratch2, xmm0);
   __ cvttsd2siq(smi_result, xmm0);
   // Check if conversion was successful by converting back and
   // comparing to the original double's bits.
   __ cvtlsi2sd(xmm1, smi_result);
   __ movq(kScratchRegister, xmm1);
   __ cmpq(scratch2, kScratchRegister);
   __ j(not_equal, on_not_smis);
+  __ JumpIfNotValidSmiValue(smi_result, on_not_smis);
   __ Integer32ToSmi(first, smi_result);
 
   __ bind(&first_done);
   __ JumpIfSmi(second, (on_success != NULL) ? on_success : &done);
   __ bind(&first_smi);
   __ AssertNotSmi(second);
   __ cmpq(FieldOperand(second, HeapObject::kMapOffset), heap_number_map);
   __ j(not_equal,
        (convert_undefined == CONVERT_UNDEFINED_TO_ZERO)
            ? &maybe_undefined_second
            : on_not_smis);
   // Convert second to smi, if possible.
   __ movsd(xmm0, FieldOperand(second, HeapNumber::kValueOffset));
   __ movq(scratch2, xmm0);
   __ cvttsd2siq(smi_result, xmm0);
   __ cvtlsi2sd(xmm1, smi_result);
   __ movq(kScratchRegister, xmm1);
   __ cmpq(scratch2, kScratchRegister);
   __ j(not_equal, on_not_smis);
+  __ JumpIfNotValidSmiValue(smi_result, on_not_smis);
   __ Integer32ToSmi(second, smi_result);
   if (on_success != NULL) {
     __ jmp(on_success);
   } else {
     __ jmp(&done);
   }
 
   __ bind(&maybe_undefined_first);
   __ CompareRoot(first, Heap::kUndefinedValueRootIndex);
   __ j(not_equal, on_not_smis);
@@ skipping 2789 matching lines @@
   // by the code above.
   if ((flags_ & STRING_ADD_CHECK_BOTH) != STRING_ADD_CHECK_BOTH) {
     __ movq(r8, FieldOperand(rax, HeapObject::kMapOffset));
     __ movq(r9, FieldOperand(rdx, HeapObject::kMapOffset));
   }
   // Get the instance types of the two strings as they will be needed soon.
   __ movzxbl(r8, FieldOperand(r8, Map::kInstanceTypeOffset));
   __ movzxbl(r9, FieldOperand(r9, Map::kInstanceTypeOffset));
 
   // Look at the length of the result of adding the two strings.
-  STATIC_ASSERT(String::kMaxLength <= Smi::kMaxValue / 2);
-  __ SmiAdd(rbx, rbx, rcx);
+  if (kSmiValueSize == 32) {
+    ASSERT(String::kMaxLength <= Smi::kMaxValue / 2);
+    __ SmiAdd(rbx, rbx, rcx);

[danno, 2013/08/01 16:45:41]

As noted above, I think you should change SmiAdd to handle the 31-bit cases in
the macro-assmebler and have a single interface 

[haitao.feng, 2013/08/02 09:35:51]

There are two SmiAdd interfaces. SmiAdd (with no fail) is used only when it's
known that overflowing is impossible. I have handled the interface with fail in
macro-assembler. Here it is safe to use __ SmiAdd(rbx, rbx, rcx, &call_runtime),
but we have un-necessary runtime overhead for the 32-bit SMI.

+  } else {
+    ASSERT(kSmiValueSize == 31);
+    __ SmiAdd(rbx, rbx, rcx, &call_runtime);
+  }
+
   // Use the string table when adding two one character strings, as it
   // helps later optimizations to return an internalized string here.
   __ SmiCompare(rbx, Smi::FromInt(2));
   __ j(not_equal, &longer_than_two);
 
   // Check that both strings are non-external ASCII strings.
   __ JumpIfBothInstanceTypesAreNotSequentialAscii(r8, r9, rbx, rcx,
                                                   &call_runtime);
 
   // Get the two characters forming the sub string.
@@ skipping 934 matching lines @@
   __ IncrementCounter(counters->string_compare_native(), 1);
   __ ret(2 * kPointerSize);
 
   __ bind(&not_same);
 
   // Check that both are sequential ASCII strings.
   __ JumpIfNotBothSequentialAsciiStrings(rdx, rax, rcx, rbx, &runtime);
 
   // Inline comparison of ASCII strings.
   __ IncrementCounter(counters->string_compare_native(), 1);
-  // Drop arguments from the stack
+  // Drop saved arguments.
   __ pop(rcx);
   __ addq(rsp, Immediate(2 * kPointerSize));
   __ push(rcx);
   GenerateCompareFlatAsciiStrings(masm, rdx, rax, rcx, rbx, rdi, r8);
 
   // Call the runtime; it returns -1 (less), 0 (equal), or 1 (greater)
   // tagged as a small integer.
   __ bind(&runtime);
   __ TailCallRuntime(Runtime::kStringCompare, 2, 1);
 }
@@ skipping 1274 matching lines @@
   __ bind(&fast_elements_case);
   GenerateCase(masm, FAST_ELEMENTS);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64