Tweak SmiSubConstant for X64

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 96fa4fc49630424093eaa29874c118e95c34a115..bd1933e4f190e3ac59866886844eefcbe1d887fe 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -1584,20 +1584,14 @@ void MacroAssembler::SmiSubConstant(Register dst,
     }
   } 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.
@@ -1605,8 +1599,7 @@ void MacroAssembler::SmiSubConstant(Register dst,
       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.