--- a/js/src/wasm/WasmCode.cpp
+++ b/js/src/wasm/WasmCode.cpp
@@ -794,21 +794,21 @@ void
 Code::setTier2(UniqueModuleSegment segment) const
 {
     MOZ_RELEASE_ASSERT(segment->tier() == Tier::Ion && segment1_->tier() != Tier::Ion);
     MOZ_RELEASE_ASSERT(!segment2_.get());
     segment2_ = takeOwnership(Move(segment));
 }
 
 uint32_t
-Code::lookupFuncIndex(JSFunction* fun) const
+Code::getFuncIndex(JSFunction* fun) const
 {
     if (fun->isAsmJSNative())
         return fun->asmJSFuncIndex();
-    return lookupRange(*fun->wasmJitEntry())->funcIndex();
+    return jumpTables_.funcIndexFromJitEntry(fun->wasmJitEntry());
 }
 
 Tiers
 Code::tiers() const
 {
     if (hasTier2())
         return Tiers(segment1_->tier(), segment2_->tier());
     return Tiers(segment1_->tier());

--- a/js/src/wasm/WasmCode.h
+++ b/js/src/wasm/WasmCode.h
@@ -506,16 +506,23 @@ class JumpTables
         jit_.get()[2 * i] = target;
         jit_.get()[2 * i + 1] = target;
     }
     void** getAddressOfJitEntry(size_t i) const {
         MOZ_ASSERT(i < numFuncs_);
         MOZ_ASSERT(jit_.get()[2 * i]);
         return &jit_.get()[2 * i];
     }
+    size_t funcIndexFromJitEntry(void** target) const {
+        MOZ_ASSERT(target >= &jit_.get()[0]);
+        MOZ_ASSERT(target <= &(jit_.get()[2 * numFuncs_ - 1]));
+        size_t index = (intptr_t*)target - (intptr_t*)&jit_.get()[0];
+        MOZ_ASSERT(index % 2 == 0);
+        return index / 2;
+    }
 
     void setTieringEntry(size_t i, void* target) const {
         MOZ_ASSERT(i < numFuncs_);
         // See comment in wasm::Module::finishTier2.
         if (mode_ == CompileMode::Tier1)
             tiering_.get()[i] = target;
     }
     void** tiering() const {
@@ -551,17 +558,17 @@ class Code : public ShareableBase<Code>
     Code();
     Code(UniqueModuleSegment tier, const Metadata& metadata, JumpTables&& maybeJumpTables);
 
     void setTieringEntry(size_t i, void* target) const { jumpTables_.setTieringEntry(i, target); }
     void** tieringJumpTable() const { return jumpTables_.tiering(); }
 
     void setJitEntry(size_t i, void* target) const { jumpTables_.setJitEntry(i, target); }
     void** getAddressOfJitEntry(size_t i) const { return jumpTables_.getAddressOfJitEntry(i); }
-    uint32_t lookupFuncIndex(JSFunction* fun) const;
+    uint32_t getFuncIndex(JSFunction* fun) const;
 
     bool hasTier2() const { return metadata_->hasTier2(); }
     void setTier2(UniqueModuleSegment segment) const;
     Tiers tiers() const;
     bool hasTier(Tier t) const;
 
     Tier stableTier() const;    // This is stable during a run
     Tier bestTier() const;      // This may transition from Baseline -> Ion at any time

--- a/js/src/wasm/WasmJS.cpp
+++ b/js/src/wasm/WasmJS.cpp
@@ -1273,17 +1273,17 @@ wasm::ExportedFunctionToInstanceObject(J
     return &v.toObject().as<WasmInstanceObject>();
 }
 
 uint32_t
 wasm::ExportedFunctionToFuncIndex(JSFunction* fun)
 {
     MOZ_ASSERT(IsExportedFunction(fun));
     Instance& instance = ExportedFunctionToInstanceObject(fun)->instance();
-    return instance.code().lookupFuncIndex(fun);
+    return instance.code().getFuncIndex(fun);
 }
 
 // ============================================================================
 // WebAssembly.Memory class and methods
 
 const ClassOps WasmMemoryObject::classOps_ =
 {
     nullptr, /* addProperty */