--- a/js/src/jit/x64/MacroAssembler-x64.h
+++ b/js/src/jit/x64/MacroAssembler-x64.h
@@ -1035,22 +1035,24 @@ class MacroAssemblerX64 : public MacroAs
     void int32ValueToFloat32(const ValueOperand &operand, const FloatRegister &dest) {
         convertInt32ToFloat32(operand.valueReg(), dest);
     }
 
     void loadConstantDouble(double d, const FloatRegister &dest);
     void loadConstantFloat32(float f, const FloatRegister &dest);
 
     void branchTruncateDouble(const FloatRegister &src, const Register &dest, Label *fail) {
-        const uint64_t IndefiniteIntegerValue = 0x8000000000000000;
-        JS_ASSERT(dest != ScratchReg);
         cvttsd2sq(src, dest);
-        movq(ImmWord(IndefiniteIntegerValue), ScratchReg);
-        cmpq(dest, ScratchReg);
-        j(Assembler::Equal, fail);
+
+        // cvttsd2sq returns 0x8000000000000000 on failure. Test for it by
+        // subtracting 1 and testing overflow (this avoids the need to
+        // materialize that value in a register).
+        cmpq(dest, Imm32(1));
+        j(Assembler::Overflow, fail);
+
         movl(dest, dest); // Zero upper 32-bits.
     }
 
     Condition testInt32Truthy(bool truthy, const ValueOperand &operand) {
         testl(operand.valueReg(), operand.valueReg());
         return truthy ? NonZero : Zero;
     }
     void branchTestBooleanTruthy(bool truthy, const ValueOperand &operand, Label *label) {

--- a/js/src/jit/x86/MacroAssembler-x86.h
+++ b/js/src/jit/x86/MacroAssembler-x86.h
@@ -874,26 +874,32 @@ class MacroAssemblerX86 : public MacroAs
     }
 
     void loadConstantDouble(double d, const FloatRegister &dest);
     void addConstantDouble(double d, const FloatRegister &dest);
     void loadConstantFloat32(float f, const FloatRegister &dest);
     void addConstantFloat32(float f, const FloatRegister &dest);
 
     void branchTruncateDouble(const FloatRegister &src, const Register &dest, Label *fail) {
-        const uint32_t IndefiniteIntegerValue = 0x80000000;
         cvttsd2si(src, dest);
-        cmpl(dest, Imm32(IndefiniteIntegerValue));
-        j(Assembler::Equal, fail);
+
+        // cvttsd2si returns 0x80000000 on failure. Test for it by
+        // subtracting 1 and testing overflow (this permits the use of a
+        // smaller immediate field).
+        cmpl(dest, Imm32(1));
+        j(Assembler::Overflow, fail);
     }
     void branchTruncateFloat32(const FloatRegister &src, const Register &dest, Label *fail) {
-        const uint32_t IndefiniteIntegerValue = 0x80000000;
         cvttss2si(src, dest);
-        cmpl(dest, Imm32(IndefiniteIntegerValue));
-        j(Assembler::Equal, fail);
+
+        // cvttss2si returns 0x80000000 on failure. Test for it by
+        // subtracting 1 and testing overflow (this permits the use of a
+        // smaller immediate field).
+        cmpl(dest, Imm32(1));
+        j(Assembler::Overflow, fail);
     }
 
     Condition testInt32Truthy(bool truthy, const ValueOperand &operand) {
         testl(operand.payloadReg(), operand.payloadReg());
         return truthy ? NonZero : Zero;
     }
     void branchTestBooleanTruthy(bool truthy, const ValueOperand &operand, Label *label) {
         testl(operand.payloadReg(), operand.payloadReg());