Tweak SmiSubConstant for X64

src/x64/macro-assembler-x64.cc

 // Copyright 2012 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 1566 matching lines @@
                                     Register src,
                                     Smi* constant,
                                     Label* on_not_smi_result,
                                     Label::Distance near_jump) {
   if (constant->value() == 0) {
     if (!dst.is(src)) {
       movq(dst, src);
     }
   } else if (dst.is(src)) {
     ASSERT(!dst.is(kScratchRegister));
-    if (constant->value() == Smi::kMinValue) {
-      // Subtracting min-value from any non-negative value will overflow.
-      // We test the non-negativeness before doing the subtraction.
-      testq(src, src);
-      j(not_sign, on_not_smi_result, near_jump);
-      LoadSmiConstant(kScratchRegister, constant);
-      subq(dst, kScratchRegister);
-    } else {
-      // Subtract by adding the negation.
-      LoadSmiConstant(kScratchRegister, Smi::FromInt(-constant->value()));
-      addq(kScratchRegister, dst);
-      j(overflow, on_not_smi_result, near_jump);
-      movq(dst, kScratchRegister);
-    }
+    Label done;
+    LoadSmiConstant(kScratchRegister, constant);
+    subq(dst, kScratchRegister);
+    j(no_overflow, &done, Label::kNear);
+    // Restore src.
+    addq(dst, kScratchRegister);
+    jmp(on_not_smi_result, near_jump);
+    bind(&done);
   } else {
     if (constant->value() == Smi::kMinValue) {
       // Subtracting min-value from any non-negative value will overflow.
       // We test the non-negativeness before doing the subtraction.
       testq(src, src);
       j(not_sign, on_not_smi_result, near_jump);
       LoadSmiConstant(dst, constant);
-      // Adding and subtracting the min-value gives the same result, it only
-      // differs on the overflow bit, which we don't check here.
+      // No overflow.
       addq(dst, src);
     } else {
       // Subtract by adding the negation.
       LoadSmiConstant(dst, Smi::FromInt(-(constant->value())));
       addq(dst, src);
       j(overflow, on_not_smi_result, near_jump);
     }
   }
 }
 
@@ skipping 3297 matching lines @@
   j(greater, &no_memento_available);
   CompareRoot(MemOperand(scratch_reg, -AllocationMemento::kSize),
               Heap::kAllocationMementoMapRootIndex);
   bind(&no_memento_available);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64