diff --git a/js/src/jit-test/tests/basic/testMathClz32.js b/js/src/jit-test/tests/basic/testMathClz32.js
new file mode 100644
index 000000000000..0cc4aae890cb
--- /dev/null
+++ b/js/src/jit-test/tests/basic/testMathClz32.js
@@ -0,0 +1,28 @@
+function f() {
+    var x = 0;
+    for (var i = 1; i < 1e6; i++) {
+        if (i > 0)
+            x += Math.clz32(i);
+    }
+    return x;
+}
+
+function g() {
+    var x = 0;
+    for (var i = 1; i < 1e6; i++) {
+        x += Math.clz32(i);
+    }
+    return x;
+}
+
+function h() {
+    var x = 0;
+    for (var i = 0; i < 1e6; i++) {
+        x += Math.clz32(i);
+    }
+    return x;
+}
+
+assertEq(f(), 13048543);
+assertEq(g(), 13048543);
+assertEq(h(), 13048575);
\ No newline at end of file
diff --git a/js/src/jit/IonBuilder.h b/js/src/jit/IonBuilder.h
index 558dd74b2047..894fdcf3388d 100644
--- a/js/src/jit/IonBuilder.h
+++ b/js/src/jit/IonBuilder.h
@@ -684,6 +684,7 @@ class IonBuilder : public MIRGenerator
     InliningStatus inlineMathAbs(CallInfo &callInfo);
     InliningStatus inlineMathFloor(CallInfo &callInfo);
     InliningStatus inlineMathCeil(CallInfo &callInfo);
+    InliningStatus inlineMathClz32(CallInfo &callInfo);
     InliningStatus inlineMathRound(CallInfo &callInfo);
     InliningStatus inlineMathSqrt(CallInfo &callInfo);
     InliningStatus inlineMathAtan2(CallInfo &callInfo);
diff --git a/js/src/jit/LIR-Common.h b/js/src/jit/LIR-Common.h
index 5b4a33aa7449..71afda7466ea 100644
--- a/js/src/jit/LIR-Common.h
+++ b/js/src/jit/LIR-Common.h
@@ -2677,6 +2677,20 @@ class LAbsF : public LInstructionHelper<1, 1, 0>
     }
 };
 
+// Count leading zeroes
+class LClzI : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(ClzI)
+    LClzI(const LAllocation &num) {
+        setOperand(0, num);
+    }
+
+    MClz *mir() const {
+        return mir_->toClz();
+    }
+};
+
 // Square root of a double.
 class LSqrtD : public LInstructionHelper<1, 1, 0>
 {
diff --git a/js/src/jit/LOpcodes.h b/js/src/jit/LOpcodes.h
index a2c9256de91c..6e4600820bbe 100644
--- a/js/src/jit/LOpcodes.h
+++ b/js/src/jit/LOpcodes.h
@@ -115,6 +115,7 @@
     _(AbsI)                         \
     _(AbsD)                         \
     _(AbsF)                         \
+    _(ClzI)                         \
     _(SqrtD)                        \
     _(SqrtF)                        \
     _(Atan2D)                       \
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index 33f443815cac..86ce64517d1b 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -1296,6 +1296,15 @@ LIRGenerator::visitAbs(MAbs *ins)
     return defineReuseInput(lir, ins, 0);
 }
 
+bool
+LIRGenerator::visitClz(MClz *ins)
+{
+    MDefinition *num = ins->num();
+
+    LClzI *lir = new(alloc()) LClzI(useRegisterAtStart(num));
+    return define(lir, ins);
+}
+
 bool
 LIRGenerator::visitSqrt(MSqrt *ins)
 {
diff --git a/js/src/jit/Lowering.h b/js/src/jit/Lowering.h
index e3320f92bede..71d8cc972926 100644
--- a/js/src/jit/Lowering.h
+++ b/js/src/jit/Lowering.h
@@ -125,6 +125,7 @@ class LIRGenerator : public LIRGeneratorSpecific
     bool visitRound(MRound *ins);
     bool visitMinMax(MMinMax *ins);
     bool visitAbs(MAbs *ins);
+    bool visitClz(MClz *ins);
     bool visitSqrt(MSqrt *ins);
     bool visitAtan2(MAtan2 *ins);
     bool visitHypot(MHypot *ins);
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index 4883045bcd0e..230a729772e5 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -54,6 +54,8 @@ IonBuilder::inlineNativeCall(CallInfo &callInfo, JSFunction *target)
         return inlineMathFloor(callInfo);
     if (native == js::math_ceil)
         return inlineMathCeil(callInfo);
+    if (native == js::math_clz32)
+        return inlineMathClz32(callInfo);
     if (native == js::math_round)
         return inlineMathRound(callInfo);
     if (native == js::math_sqrt)
@@ -791,6 +793,31 @@ IonBuilder::inlineMathCeil(CallInfo &callInfo)
     return InliningStatus_NotInlined;
 }
 
+IonBuilder::InliningStatus
+IonBuilder::inlineMathClz32(CallInfo &callInfo)
+{
+    if (callInfo.constructing())
+        return InliningStatus_NotInlined;
+
+    if (callInfo.argc() != 1)
+        return InliningStatus_NotInlined;
+
+    MIRType returnType = getInlineReturnType();
+    if (returnType != MIRType_Int32)
+        return InliningStatus_NotInlined;
+
+    if (!IsNumberType(callInfo.getArg(0)->type()))
+        return InliningStatus_NotInlined;
+
+    callInfo.setImplicitlyUsedUnchecked();
+
+    MClz *ins = MClz::New(alloc(), callInfo.getArg(0));
+    current->add(ins);
+    current->push(ins);
+    return InliningStatus_Inlined;
+
+}
+
 IonBuilder::InliningStatus
 IonBuilder::inlineMathRound(CallInfo &callInfo)
 {
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 751c32e3de70..81110ffb20d7 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -3424,6 +3424,19 @@ MSqrt::trySpecializeFloat32(TempAllocator &alloc) {
     setPolicyType(MIRType_Float32);
 }
 
+MDefinition *
+MClz::foldsTo(TempAllocator &alloc)
+{
+    if (num()->isConstant()) {
+        int32_t n = num()->toConstant()->value().toInt32();
+        if (n == 0)
+            return MConstant::New(alloc, Int32Value(32));
+        return MConstant::New(alloc, Int32Value(mozilla::CountLeadingZeroes32(n)));
+    }
+
+    return this;
+}
+
 MDefinition *
 MBoundsCheck::foldsTo(TempAllocator &alloc)
 {
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 4080d0f29e72..611df7ac5090 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -4531,6 +4531,50 @@ class MAbs
     ALLOW_CLONE(MAbs)
 };
 
+class MClz
+    : public MUnaryInstruction
+    , public BitwisePolicy
+{
+    bool operandIsNeverZero_;
+
+    MClz(MDefinition *num)
+      : MUnaryInstruction(num),
+        operandIsNeverZero_(false)
+    {
+        JS_ASSERT(IsNumberType(num->type()));
+        specialization_ = MIRType_Int32;
+        setResultType(MIRType_Int32);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(Clz)
+    static MClz *New(TempAllocator &alloc, MDefinition *num) {
+        return new(alloc) MClz(num);
+    }
+    MDefinition *num() const {
+        return getOperand(0);
+    }
+    TypePolicy *typePolicy() {
+        return this;
+    }
+    bool congruentTo(const MDefinition *ins) const {
+        return congruentIfOperandsEqual(ins);
+    }
+
+    AliasSet getAliasSet() const {
+        return AliasSet::None();
+    }
+
+    bool operandIsNeverZero() const {
+        return operandIsNeverZero_;
+    }
+
+    MDefinition *foldsTo(TempAllocator &alloc);
+    void computeRange(TempAllocator &alloc);
+    void collectRangeInfoPreTrunc();
+};
+
 // Inline implementation of Math.sqrt().
 class MSqrt
   : public MUnaryInstruction,
diff --git a/js/src/jit/MOpcodes.h b/js/src/jit/MOpcodes.h
index 4becf6c00c76..342ab3d88867 100644
--- a/js/src/jit/MOpcodes.h
+++ b/js/src/jit/MOpcodes.h
@@ -63,6 +63,7 @@ namespace jit {
     _(Ursh)                                                                 \
     _(MinMax)                                                               \
     _(Abs)                                                                  \
+    _(Clz)                                                                  \
     _(Sqrt)                                                                 \
     _(Atan2)                                                                \
     _(Hypot)                                                                \
diff --git a/js/src/jit/ParallelSafetyAnalysis.cpp b/js/src/jit/ParallelSafetyAnalysis.cpp
index 573f283174f1..0db3b0015dbf 100644
--- a/js/src/jit/ParallelSafetyAnalysis.cpp
+++ b/js/src/jit/ParallelSafetyAnalysis.cpp
@@ -161,6 +161,7 @@ class ParallelSafetyVisitor : public MDefinitionVisitor
     SAFE_OP(Ursh)
     SPECIALIZED_OP(MinMax, PERMIT_NUMERIC)
     SAFE_OP(Abs)
+    SAFE_OP(Clz)
     SAFE_OP(Sqrt)
     UNSAFE_OP(Atan2)
     UNSAFE_OP(Hypot)
diff --git a/js/src/jit/RangeAnalysis.cpp b/js/src/jit/RangeAnalysis.cpp
index 57c030915c7c..6d5af745b836 100644
--- a/js/src/jit/RangeAnalysis.cpp
+++ b/js/src/jit/RangeAnalysis.cpp
@@ -1228,6 +1228,12 @@ MCeil::computeRange(TempAllocator &alloc)
     setRange(Range::ceil(alloc, &other));
 }
 
+void
+MClz::computeRange(TempAllocator &alloc)
+{
+    setRange(Range::NewUInt32Range(alloc, 0, 32));
+}
+
 void
 MMinMax::computeRange(TempAllocator &alloc)
 {
@@ -2667,6 +2673,14 @@ MLoadElementHole::collectRangeInfoPreTrunc()
         needsNegativeIntCheck_ = false;
 }
 
+void
+MClz::collectRangeInfoPreTrunc()
+{
+    Range inputRange(input());
+    if (!inputRange.canBeZero())
+        operandIsNeverZero_ = true;
+}
+
 void
 MDiv::collectRangeInfoPreTrunc()
 {
diff --git a/js/src/jit/shared/Assembler-x86-shared.h b/js/src/jit/shared/Assembler-x86-shared.h
index 8254d881abf6..b56df8d2b013 100644
--- a/js/src/jit/shared/Assembler-x86-shared.h
+++ b/js/src/jit/shared/Assembler-x86-shared.h
@@ -1160,6 +1160,9 @@ class AssemblerX86Shared : public AssemblerShared
             MOZ_CRASH("unexpected operand kind");
         }
     }
+    void bsr(const Register &src, const Register &dest) {
+        masm.bsr_rr(src.code(), dest.code());
+    }
     void imull(Register multiplier) {
         masm.imull_r(multiplier.code());
     }
diff --git a/js/src/jit/shared/BaseAssembler-x86-shared.h b/js/src/jit/shared/BaseAssembler-x86-shared.h
index df7968b2c9b1..76ae77fb7d4a 100644
--- a/js/src/jit/shared/BaseAssembler-x86-shared.h
+++ b/js/src/jit/shared/BaseAssembler-x86-shared.h
@@ -324,6 +324,7 @@ private:
         OP_SETCC            = 0x90,
         OP2_IMUL_GvEv       = 0xAF,
         OP2_CMPXCHG_GvEw    = 0xB1,
+        OP2_BSR_GvEv        = 0xBD,
         OP2_MOVSX_GvEb      = 0xBE,
         OP2_MOVSX_GvEw      = 0xBF,
         OP2_MOVZX_GvEb      = 0xB6,
@@ -1301,6 +1302,12 @@ public:
     }
 #endif
 
+    void bsr_rr(RegisterID src, RegisterID dst)
+    {
+        spew("bsr        %s, %s", nameIReg(4, src), nameIReg(4, dst));
+        m_formatter.twoByteOp(OP2_BSR_GvEv, dst, src);
+    }
+
     void imull_rr(RegisterID src, RegisterID dst)
     {
         spew("imull      %s, %s", nameIReg(4,src), nameIReg(4, dst));
diff --git a/js/src/jit/shared/CodeGenerator-x86-shared.cpp b/js/src/jit/shared/CodeGenerator-x86-shared.cpp
index d72491711215..92c7bc8d0986 100644
--- a/js/src/jit/shared/CodeGenerator-x86-shared.cpp
+++ b/js/src/jit/shared/CodeGenerator-x86-shared.cpp
@@ -544,6 +544,28 @@ CodeGeneratorX86Shared::visitAbsF(LAbsF *ins)
     return true;
 }
 
+bool
+CodeGeneratorX86Shared::visitClzI(LClzI *ins)
+{
+    Register input = ToRegister(ins->input());
+    Register output = ToRegister(ins->output());
+
+    // bsr is undefined on 0
+    Label done, nonzero;
+    if (!ins->mir()->operandIsNeverZero()) {
+        masm.testl(input, input);
+        masm.j(Assembler::NonZero, &nonzero);
+        masm.move32(Imm32(32), output);
+        masm.jump(&done);
+    }
+
+    masm.bind(&nonzero);
+    masm.bsr(input, output);
+    masm.xor32(Imm32(0x1F), output);
+    masm.bind(&done);
+    return true;
+}
+
 bool
 CodeGeneratorX86Shared::visitSqrtD(LSqrtD *ins)
 {
diff --git a/js/src/jit/shared/CodeGenerator-x86-shared.h b/js/src/jit/shared/CodeGenerator-x86-shared.h
index 9b2bc6a74caf..9f37a6572335 100644
--- a/js/src/jit/shared/CodeGenerator-x86-shared.h
+++ b/js/src/jit/shared/CodeGenerator-x86-shared.h
@@ -153,6 +153,7 @@ class CodeGeneratorX86Shared : public CodeGeneratorShared
     virtual bool visitMinMaxD(LMinMaxD *ins);
     virtual bool visitAbsD(LAbsD *ins);
     virtual bool visitAbsF(LAbsF *ins);
+    virtual bool visitClzI(LClzI *ins);
     virtual bool visitSqrtD(LSqrtD *ins);
     virtual bool visitSqrtF(LSqrtF *ins);
     virtual bool visitPowHalfD(LPowHalfD *ins);