diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index c8cf32f36b1e..9ca1f4f79c9d 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -1551,7 +1551,7 @@ MacroAssembler::generateBailoutTail(Register scratch, Register bailoutInfo)
     {
         // Prepare a register set for use in this case.
         AllocatableGeneralRegisterSet regs(GeneralRegisterSet::All());
-        MOZ_ASSERT(!regs.has(getStackPointer()));
+        MOZ_ASSERT_IF(!IsHiddenSP(getStackPointer()), !regs.has(AsRegister(getStackPointer())));
         regs.take(bailoutInfo);
 
         // Reset SP to the point where clobbering starts.
@@ -3141,12 +3141,18 @@ MacroAssembler::wasmEmitOldTrapOutOfLineCode()
 }
 
 void
-MacroAssembler::wasmEmitStackCheck(Register sp, Register scratch, Label* onOverflow)
+MacroAssembler::wasmEmitStackCheck(RegisterOrSP sp, Register scratch, Label* onOverflow)
 {
-    branchPtr(Assembler::AboveOrEqual,
-              Address(WasmTlsReg, offsetof(wasm::TlsData, stackLimit)),
-              sp,
-              onOverflow);
+    if (IsHiddenSP(sp)) {
+        branchStackPtrRhs(Assembler::AboveOrEqual,
+                          Address(WasmTlsReg, offsetof(wasm::TlsData, stackLimit)),
+                          onOverflow);
+    } else {
+        branchPtr(Assembler::AboveOrEqual,
+                  Address(WasmTlsReg, offsetof(wasm::TlsData, stackLimit)),
+                  AsRegister(sp),
+                  onOverflow);
+    }
 }
 
 void
diff --git a/js/src/jit/MacroAssembler.h b/js/src/jit/MacroAssembler.h
index 6d586bb8b093..d84f9cfb1385 100644
--- a/js/src/jit/MacroAssembler.h
+++ b/js/src/jit/MacroAssembler.h
@@ -421,6 +421,10 @@ class MacroAssembler : public MacroAssemblerSpecific
         return size();
     }
 
+#ifdef JS_HAS_HIDDEN_SP
+    void Push(RegisterOrSP reg);
+#endif
+
     //{{{ check_macroassembler_decl_style
   public:
     // ===============================================================
@@ -1564,7 +1568,7 @@ class MacroAssembler : public MacroAssemblerSpecific
     void wasmEmitOldTrapOutOfLineCode();
 
     // Perform a stack-overflow test, branching to the given Label on overflow.
-    void wasmEmitStackCheck(Register sp, Register scratch, Label* onOverflow);
+    void wasmEmitStackCheck(RegisterOrSP sp, Register scratch, Label* onOverflow);
 
     void emitPreBarrierFastPath(JSRuntime* rt, MIRType type, Register temp1, Register temp2,
                                 Register temp3, Label* noBarrier);
diff --git a/js/src/jit/MoveResolver.cpp b/js/src/jit/MoveResolver.cpp
index 7af184fbce19..bd71d0acea44 100644
--- a/js/src/jit/MoveResolver.cpp
+++ b/js/src/jit/MoveResolver.cpp
@@ -35,7 +35,10 @@ MoveOperand::MoveOperand(MacroAssembler& masm, const ABIArg& arg)
         break;
       case ABIArg::Stack:
         kind_ = MEMORY;
-        code_ = masm.getStackPointer().code();
+        if (IsHiddenSP(masm.getStackPointer()))
+            MOZ_CRASH("Hidden SP cannot be represented as register code on this platform");
+        else
+            code_ = AsRegister(masm.getStackPointer()).code();
         disp_ = arg.offsetFromArgBase();
         break;
       case ABIArg::Uninitialized:
diff --git a/js/src/jit/Registers.h b/js/src/jit/Registers.h
index 1c02aacb1079..572554ffe160 100644
--- a/js/src/jit/Registers.h
+++ b/js/src/jit/Registers.h
@@ -117,6 +117,82 @@ struct Register {
     }
 };
 
+// Architectures where the stack pointer is not a plain register with a standard
+// register encoding must define JS_HAS_HIDDEN_SP and HiddenSPEncoding.
+
+#ifdef JS_HAS_HIDDEN_SP
+struct RegisterOrSP
+{
+    // The register code -- but possibly one that cannot be represented as a bit
+    // position in a 32-bit vector.
+    const uint32_t code;
+
+    explicit RegisterOrSP(uint32_t code) : code(code) {}
+    explicit RegisterOrSP(Register r) : code(r.code()) {}
+};
+
+static inline bool
+IsHiddenSP(RegisterOrSP r)
+{
+    return r.code == HiddenSPEncoding;
+}
+
+static inline Register
+AsRegister(RegisterOrSP r)
+{
+    MOZ_ASSERT(!IsHiddenSP(r));
+    return Register::FromCode(r.code);
+}
+
+inline bool
+operator == (Register r, RegisterOrSP e) {
+    return r.code() == e.code;
+}
+
+inline bool
+operator != (Register r, RegisterOrSP e) {
+    return !(r == e);
+}
+
+inline bool
+operator == (RegisterOrSP e, Register r) {
+    return r == e;
+}
+
+inline bool
+operator != (RegisterOrSP e, Register r) {
+    return r != e;
+}
+
+inline bool
+operator == (RegisterOrSP lhs, RegisterOrSP rhs) {
+    return lhs.code == rhs.code;
+}
+
+inline bool
+operator != (RegisterOrSP lhs, RegisterOrSP rhs) {
+    return !(lhs == rhs);
+}
+#else
+// On platforms where there's nothing special about SP, make RegisterOrSP be
+// just Register, and return false for IsHiddenSP(r) for any r so that we use
+// "normal" code for handling the SP.  This reduces ifdeffery throughout the
+// jit.
+typedef Register RegisterOrSP;
+
+static inline bool
+IsHiddenSP(RegisterOrSP r)
+{
+    return false;
+}
+
+static inline Register
+AsRegister(RegisterOrSP r)
+{
+    return r;
+}
+#endif
+
 template <> inline Register::SetType
 Register::LiveAsIndexableSet<RegTypeName::GPR>(SetType set)
 {
diff --git a/js/src/jit/arm64/Architecture-arm64.h b/js/src/jit/arm64/Architecture-arm64.h
index 18a64b049200..05e72b54fcd8 100644
--- a/js/src/jit/arm64/Architecture-arm64.h
+++ b/js/src/jit/arm64/Architecture-arm64.h
@@ -10,10 +10,14 @@
 #include "mozilla/Assertions.h"
 #include "mozilla/MathAlgorithms.h"
 
+#include "jit/arm64/vixl/Instructions-vixl.h"
 #include "jit/shared/Architecture-shared.h"
 
 #include "js/Utility.h"
 
+#define JS_HAS_HIDDEN_SP
+static const uint32_t HiddenSPEncoding = vixl::kSPRegInternalCode;
+
 namespace js {
 namespace jit {
 
diff --git a/js/src/jit/arm64/MacroAssembler-arm64.cpp b/js/src/jit/arm64/MacroAssembler-arm64.cpp
index 1c7c9b018c2f..7e1f560f7bea 100644
--- a/js/src/jit/arm64/MacroAssembler-arm64.cpp
+++ b/js/src/jit/arm64/MacroAssembler-arm64.cpp
@@ -219,10 +219,19 @@ MacroAssemblerCompat::handleFailureWithHandlerTail(void* handler, Label* profile
 
 void
 MacroAssemblerCompat::profilerEnterFrame(Register framePtr, Register scratch)
+{
+    profilerEnterFrame(RegisterOrSP(framePtr), scratch);
+}
+
+void
+MacroAssemblerCompat::profilerEnterFrame(RegisterOrSP framePtr, Register scratch)
 {
     asMasm().loadJSContext(scratch);
     loadPtr(Address(scratch, offsetof(JSContext, profilingActivation_)), scratch);
-    storePtr(framePtr, Address(scratch, JitActivation::offsetOfLastProfilingFrame()));
+    if (IsHiddenSP(framePtr))
+        storeStackPtr(Address(scratch, JitActivation::offsetOfLastProfilingFrame()));
+    else
+        storePtr(AsRegister(framePtr), Address(scratch, JitActivation::offsetOfLastProfilingFrame()));
     storePtr(ImmPtr(nullptr), Address(scratch, JitActivation::offsetOfLastProfilingCallSite()));
 }
 
@@ -242,6 +251,16 @@ MacroAssembler::reserveStack(uint32_t amount)
     adjustFrame(amount);
 }
 
+void
+MacroAssembler::Push(RegisterOrSP reg)
+{
+    if (IsHiddenSP(reg))
+        push(sp);
+    else
+        push(AsRegister(reg));
+    adjustFrame(sizeof(intptr_t));
+}
+
 //{{{ check_macroassembler_style
 // ===============================================================
 // MacroAssembler high-level usage.
diff --git a/js/src/jit/arm64/MacroAssembler-arm64.h b/js/src/jit/arm64/MacroAssembler-arm64.h
index 1147a6ecca7c..0b59b1c74aef 100644
--- a/js/src/jit/arm64/MacroAssembler-arm64.h
+++ b/js/src/jit/arm64/MacroAssembler-arm64.h
@@ -72,11 +72,18 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
     bool oom() const {
         return Assembler::oom() || !enoughMemory_;
     }
+    static ARMRegister toARMRegister(RegisterOrSP r, size_t size) {
+        if (IsHiddenSP(r)) {
+            MOZ_ASSERT(size == 64);
+            return sp;
+        }
+        return ARMRegister(AsRegister(r), size);
+    }
     static MemOperand toMemOperand(const Address& a) {
-        return MemOperand(ARMRegister(a.base, 64), a.offset);
+        return MemOperand(toARMRegister(a.base, 64), a.offset);
     }
     void doBaseIndex(const vixl::CPURegister& rt, const BaseIndex& addr, vixl::LoadStoreOp op) {
-        const ARMRegister base = ARMRegister(addr.base, 64);
+        const ARMRegister base = toARMRegister(addr.base, 64);
         const ARMRegister index = ARMRegister(addr.index, 64);
         const unsigned scale = addr.scale;
 
@@ -175,6 +182,11 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
     void push(Register reg) {
         vixl::MacroAssembler::Push(ARMRegister(reg, 64));
     }
+    void push(RegisterOrSP reg) {
+        if (IsHiddenSP(reg))
+            vixl::MacroAssembler::Push(sp);
+        vixl::MacroAssembler::Push(toARMRegister(reg, 64));
+    }
     void push(Register r0, Register r1) {
         vixl::MacroAssembler::Push(ARMRegister(r0, 64), ARMRegister(r1, 64));
     }
@@ -761,7 +773,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
         Ldr(ARMRegister(dest, 64), MemOperand(address));
     }
     void loadPtr(const BaseIndex& src, Register dest) {
-        Register base = src.base;
+        ARMRegister base = toARMRegister(src.base, 64);
         uint32_t scale = Imm32::ShiftOf(src.scale).value;
         ARMRegister dest64(dest, 64);
         ARMRegister index64(src.index, 64);
@@ -769,16 +781,16 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
         if (src.offset) {
             vixl::UseScratchRegisterScope temps(this);
             const ARMRegister scratch = temps.AcquireX();
-            MOZ_ASSERT(!scratch.Is(ARMRegister(base, 64)));
+            MOZ_ASSERT(!scratch.Is(base));
             MOZ_ASSERT(!scratch.Is(dest64));
             MOZ_ASSERT(!scratch.Is(index64));
 
-            Add(scratch, ARMRegister(base, 64), Operand(int64_t(src.offset)));
+            Add(scratch, base, Operand(int64_t(src.offset)));
             Ldr(dest64, MemOperand(scratch, index64, vixl::LSL, scale));
             return;
         }
 
-        Ldr(dest64, MemOperand(ARMRegister(base, 64), index64, vixl::LSL, scale));
+        Ldr(dest64, MemOperand(base, index64, vixl::LSL, scale));
     }
     void loadPrivate(const Address& src, Register dest);
 
@@ -1077,7 +1089,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
         Ldr(ARMFPRegister(dest, 64), MemOperand(src));
     }
     void loadDouble(const BaseIndex& src, FloatRegister dest) {
-        ARMRegister base(src.base, 64);
+        ARMRegister base = toARMRegister(src.base, 64);
         ARMRegister index(src.index, 64);
 
         if (src.offset == 0) {
@@ -1098,7 +1110,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
         fcvt(ARMFPRegister(dest, 64), ARMFPRegister(dest, 32));
     }
     void loadFloatAsDouble(const BaseIndex& src, FloatRegister dest) {
-        ARMRegister base(src.base, 64);
+        ARMRegister base = toARMRegister(src.base, 64);
         ARMRegister index(src.index, 64);
         if (src.offset == 0) {
             Ldr(ARMFPRegister(dest, 32), MemOperand(base, index, vixl::LSL, unsigned(src.scale)));
@@ -1118,7 +1130,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
         Ldr(ARMFPRegister(dest, 32), toMemOperand(addr));
     }
     void loadFloat32(const BaseIndex& src, FloatRegister dest) {
-        ARMRegister base(src.base, 64);
+        ARMRegister base = toARMRegister(src.base, 64);
         ARMRegister index(src.index, 64);
         if (src.offset == 0) {
             Ldr(ARMFPRegister(dest, 32), MemOperand(base, index, vixl::LSL, unsigned(src.scale)));
@@ -1875,11 +1887,14 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
     }
 
     void computeEffectiveAddress(const Address& address, Register dest) {
-        Add(ARMRegister(dest, 64), ARMRegister(address.base, 64), Operand(address.offset));
+        Add(ARMRegister(dest, 64), toARMRegister(address.base, 64), Operand(address.offset));
+    }
+    void computeEffectiveAddress(const Address& address, RegisterOrSP dest) {
+        Add(toARMRegister(dest, 64), toARMRegister(address.base, 64), Operand(address.offset));
     }
     void computeEffectiveAddress(const BaseIndex& address, Register dest) {
         ARMRegister dest64(dest, 64);
-        ARMRegister base64(address.base, 64);
+        ARMRegister base64 = toARMRegister(address.base, 64);
         ARMRegister index64(address.index, 64);
 
         Add(dest64, base64, Operand(index64, vixl::LSL, address.scale));
@@ -1895,6 +1910,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
     void handleFailureWithHandlerTail(void* handler, Label* profilerExitTail);
 
     void profilerEnterFrame(Register framePtr, Register scratch);
+    void profilerEnterFrame(RegisterOrSP framePtr, Register scratch);
     void profilerExitFrame() {
         jump(GetJitContext()->runtime->jitRuntime()->getProfilerExitFrameTail());
     }
@@ -1960,7 +1976,7 @@ class MacroAssemblerCompat : public vixl::MacroAssembler
 #ifdef DEBUG
         vixl::UseScratchRegisterScope temps(this);
         const ARMRegister scratch64 = temps.AcquireX();
-        MOZ_ASSERT(scratch64.asUnsized() != reg.asUnsized());
+        MOZ_ASSERT_IF(!reg.IsSP(), scratch64.asUnsized() != reg.asUnsized());
         Label aligned;
         Mov(scratch64, reg);
         Tst(scratch64, Operand(StackAlignment - 1));
diff --git a/js/src/jit/arm64/vixl/Assembler-vixl.h b/js/src/jit/arm64/vixl/Assembler-vixl.h
index a59eca6e22ff..3f1d659f83aa 100644
--- a/js/src/jit/arm64/vixl/Assembler-vixl.h
+++ b/js/src/jit/arm64/vixl/Assembler-vixl.h
@@ -265,8 +265,10 @@ class Register : public CPURegister {
   }
 
   js::jit::Register asUnsized() const {
-    if (code_ == kSPRegInternalCode)
-      return js::jit::Register::FromCode((js::jit::Register::Code)kZeroRegCode);
+    // asUnsized() is only ever used on temp registers or on registers that
+    // are known not to be SP, and there should be no risk of it being
+    // applied to SP.  Check anyway.
+    VIXL_ASSERT(code_ != kSPRegInternalCode);
     return js::jit::Register::FromCode((js::jit::Register::Code)code_);
   }
 
@@ -706,6 +708,9 @@ class Operand {
   explicit Operand(js::jit::Register, int32_t) {
     MOZ_CRASH("Operand with implicit Address");
   }
+  explicit Operand(js::jit::RegisterOrSP, int32_t) {
+    MOZ_CRASH("Operand with implicit Address");
+  }
 
   bool IsImmediate() const;
   bool IsShiftedRegister() const;
@@ -777,7 +782,7 @@ class MemOperand {
   // Adapter constructors using C++11 delegating.
   // TODO: If sp == kSPRegInternalCode, the xzr check isn't necessary.
   explicit MemOperand(js::jit::Address addr)
-    : MemOperand(addr.base.code() == 31 ? sp : Register(addr.base, 64),
+    : MemOperand(IsHiddenSP(addr.base) ? sp : Register(AsRegister(addr.base), 64),
                  (ptrdiff_t)addr.offset) {
   }
 
diff --git a/js/src/jit/arm64/vixl/MacroAssembler-vixl.h b/js/src/jit/arm64/vixl/MacroAssembler-vixl.h
index 352794432f9a..4fc0aa75a0d8 100644
--- a/js/src/jit/arm64/vixl/MacroAssembler-vixl.h
+++ b/js/src/jit/arm64/vixl/MacroAssembler-vixl.h
@@ -2224,12 +2224,8 @@ class MacroAssembler : public js::jit::Assembler {
     return sp_;
   }
 
-  const js::jit::Register getStackPointer() const {
-    int code = sp_.code();
-    if (code == kSPRegInternalCode) {
-      code = 31;
-    }
-    return js::jit::Register::FromCode(code);
+  const js::jit::RegisterOrSP getStackPointer() const {
+      return js::jit::RegisterOrSP(sp_.code());
   }
 
   CPURegList* TmpList() { return &tmp_list_; }
diff --git a/js/src/jit/shared/Assembler-shared.h b/js/src/jit/shared/Assembler-shared.h
index 7a5243975728..1a38d3a8c3ae 100644
--- a/js/src/jit/shared/Assembler-shared.h
+++ b/js/src/jit/shared/Assembler-shared.h
@@ -308,13 +308,18 @@ struct PatchedAbsoluteAddress
 // 32-bit offset.
 struct Address
 {
-    Register base;
+    RegisterOrSP base;
     int32_t offset;
 
-    Address(Register base, int32_t offset) : base(base), offset(offset)
+    Address(Register base, int32_t offset) : base(RegisterOrSP(base)), offset(offset)
     { }
 
-    Address() : base(Registers::Invalid), offset(0)
+#ifdef JS_HAS_HIDDEN_SP
+    Address(RegisterOrSP base, int32_t offset) : base(base), offset(offset)
+    { }
+#endif
+
+    Address() : base(RegisterOrSP(Registers::Invalid)), offset(0)
     { }
 };
 
@@ -340,17 +345,23 @@ HighWord(const Address& address) {
 // index with a scale, and a constant, 32-bit offset.
 struct BaseIndex
 {
-    Register base;
+    RegisterOrSP base;
     Register index;
     Scale scale;
     int32_t offset;
 
     BaseIndex(Register base, Register index, Scale scale, int32_t offset = 0)
-      : base(base), index(index), scale(scale), offset(offset)
+      : base(RegisterOrSP(base)), index(index), scale(scale), offset(offset)
     { }
 
+#ifdef JS_HAS_HIDDEN_SP
+    BaseIndex(RegisterOrSP base, Register index, Scale scale, int32_t offset = 0)
+      : base(base), index(index), scale(scale), offset(offset)
+    { }
+#endif
+
     BaseIndex()
-      : base(Registers::Invalid)
+      : base(RegisterOrSP(Registers::Invalid))
       , index(Registers::Invalid)
       , scale(TimesOne)
       , offset(0)
@@ -384,8 +395,14 @@ HighWord(const BaseIndex& address) {
 struct BaseValueIndex : BaseIndex
 {
     BaseValueIndex(Register base, Register index, int32_t offset = 0)
+      : BaseIndex(RegisterOrSP(base), index, ValueScale, offset)
+    { }
+
+#ifdef JS_HAS_HIDDEN_SP
+    BaseValueIndex(RegisterOrSP base, Register index, int32_t offset = 0)
       : BaseIndex(base, index, ValueScale, offset)
     { }
+#endif
 };
 
 // Specifies the address of an indexed Value within object elements from a
@@ -397,6 +414,14 @@ struct BaseObjectElementIndex : BaseValueIndex
     {
         NativeObject::elementsSizeMustNotOverflow();
     }
+
+#ifdef JS_HAS_HIDDEN_SP
+    BaseObjectElementIndex(RegisterOrSP base, Register index, int32_t offset = 0)
+      : BaseValueIndex(base, index, offset)
+    {
+        NativeObject::elementsSizeMustNotOverflow();
+    }
+#endif
 };
 
 // Like BaseObjectElementIndex, except for object slots.
@@ -407,6 +432,14 @@ struct BaseObjectSlotIndex : BaseValueIndex
     {
         NativeObject::slotsSizeMustNotOverflow();
     }
+
+#ifdef JS_HAS_HIDDEN_SP
+    BaseObjectSlotIndex(RegisterOrSP base, Register index)
+      : BaseValueIndex(base, index)
+    {
+        NativeObject::slotsSizeMustNotOverflow();
+    }
+#endif
 };
 
 class Relocation {
diff --git a/js/src/wasm/WasmBaselineCompile.cpp b/js/src/wasm/WasmBaselineCompile.cpp
index 0cacb4a9e202..0a3fd2ac13ce 100644
--- a/js/src/wasm/WasmBaselineCompile.cpp
+++ b/js/src/wasm/WasmBaselineCompile.cpp
@@ -985,7 +985,7 @@ class BaseStackFrame
     CodeOffset stackAddOffset_;
 
     // The stack pointer, cached for brevity.
-    Register sp_;
+    RegisterOrSP sp_;
 
   public:
 
@@ -1178,7 +1178,7 @@ class BaseStackFrame
 
     void allocStack(Register tmp0, Register tmp1, Label* stackOverflowLabel) {
         stackAddOffset_ = masm.sub32FromStackPtrWithPatch(tmp0);
-        masm.wasmEmitStackCheck(tmp0, tmp1, stackOverflowLabel);
+        masm.wasmEmitStackCheck(RegisterOrSP(tmp0), tmp1, stackOverflowLabel);
     }
 
     void patchAllocStack() {
diff --git a/js/src/wasm/WasmStubs.cpp b/js/src/wasm/WasmStubs.cpp
index b8c2e4612846..7485d3f074d8 100644
--- a/js/src/wasm/WasmStubs.cpp
+++ b/js/src/wasm/WasmStubs.cpp
@@ -137,7 +137,7 @@ SetupABIArguments(MacroAssembler& masm, const FuncExport& fe, Register argv, Reg
                 masm.storePtr(scratch, Address(masm.getStackPointer(), iter->offsetFromArgBase()));
                 break;
               case MIRType::Int64: {
-                Register sp = masm.getStackPointer();
+                RegisterOrSP sp = masm.getStackPointer();
 #if JS_BITS_PER_WORD == 32
                 masm.load32(LowWord(src), scratch);
                 masm.store32(scratch, LowWord(Address(sp, iter->offsetFromArgBase())));