diff --git a/src/hx/gc/Immix.cpp b/src/hx/gc/Immix.cpp
index 00d2f2ff5..458e33a4f 100644
--- a/src/hx/gc/Immix.cpp
+++ b/src/hx/gc/Immix.cpp
@@ -7,6 +7,14 @@
 #include "GcRegCapture.h"
 #include <hx/Unordered.h>
 
+#ifdef EMSCRIPTEN
+   #include <emscripten/stack.h>
+   #ifdef HXCPP_SINGLE_THREADED_APP
+      // Use provided tools to measure stack extent
+      #define HXCPP_EXPLICIT_STACK_EXTENT
+   #endif
+#endif
+
 #include <string>
 #include <stdlib.h>
 
@@ -28,7 +36,6 @@ int gSlowPath = 0;
 using hx::gByteMarkID;
 using hx::gRememberedByteMarkID;
 
-// #define HXCPP_SINGLE_THREADED_APP
 
 namespace hx
 {
@@ -108,10 +115,6 @@ static size_t sgMaximumFreeSpace  = 1024*1024*1024;
 static size_t sgMaximumFreeSpace  = 1024*1024*1024;
 #endif
 
-#ifdef EMSCRIPTEN
-// #define HXCPP_STACK_UP
-#endif
-
 
 // #define HXCPP_GC_DEBUG_LEVEL 1
 
@@ -5762,8 +5765,10 @@ class LocalAllocator : public hx::StackContext
 
    bool           mMoreHoles;
 
+   #ifndef HXCPP_EXPLICIT_STACK_EXTENT
    int *mTopOfStack;
    int *mBottomOfStack;
+   #endif
 
    hx::RegisterCaptureBuffer mRegisterBuf;
    int                   mRegisterBufSize;
@@ -5801,7 +5806,9 @@ class LocalAllocator : public hx::StackContext
 
    void AttachThread(int *inTopOfStack)
    {
+      #ifndef HXCPP_EXPLICIT_STACK_EXTENT
       mTopOfStack = mBottomOfStack = inTopOfStack;
+      #endif
 
       mRegisterBufSize = 0;
       mStackLocks = 0;
@@ -5859,7 +5866,9 @@ class LocalAllocator : public hx::StackContext
       }
       #endif
 
+      #ifndef HXCPP_EXPLICIT_STACK_EXTENT
       mTopOfStack = mBottomOfStack = 0;
+      #endif
 
       sGlobalAlloc->RemoveLocalLocked(this);
 
@@ -5891,7 +5900,7 @@ class LocalAllocator : public hx::StackContext
    // Other places may call this to ensure the the current thread is registered
    //  indefinitely (until forcefully revoked)
    //
-   // Normally liraries/mains will then let this dangle.
+   // Normally libraries/mains will then let this dangle.
    //
    // However after the main, on android it calls SetTopOfStack(0,true), to unregister the thread,
    // because it is likely to be the ui thread, and the remaining call will be from
@@ -5916,13 +5925,28 @@ class LocalAllocator : public hx::StackContext
    //  SetTopOfStack(top,true) -> add stack lock
    //  SetTopOfStack(0,_) -> pop stack lock. If all gone, clear global stack lock
    //
+
+   #ifdef HXCPP_EXPLICIT_STACK_EXTENT // {
+
+   void SetTopOfStack(int *inTop,bool inPush) { }
+   void PushTopOfStack(void *inTop) { }
+   void PopTopOfStack() { }
+   void SetBottomOfStack(int *inBottom) { }
+   void PauseForCollect() { }
+   void EnterGCFreeZone() { }
+   bool TryGCFreeZone() { return true; }
+   bool TryExitGCFreeZone() { return false; }
+   void ExitGCFreeZoneLocked() { }
+
+
+   #else // } !HXCPP_EXPLICIT_STACK_EXTENT {
    void SetTopOfStack(int *inTop,bool inPush)
    {
       if (inTop)
       {
          if (!mTopOfStack)
             mTopOfStack = inTop;
-         // EMSCRIPTEN the stack grows upwards
+         // EMSCRIPTEN the stack grows upwards - not wasm.
          // It could be that the main routine was called from deep with in the stack,
          //  then some callback was called from a higher location on the stack
          #ifdef HXCPP_STACK_UP
@@ -5987,39 +6011,6 @@ class LocalAllocator : public hx::StackContext
       #endif
    }
 
-   virtual void SetupStackAndCollect(bool inMajor, bool inForceCompact, bool inLocked=false,bool inFreeIsFragged=false)
-   {
-      #ifndef HXCPP_SINGLE_THREADED_APP
-        #if HXCPP_DEBUG
-        if (mGCFreeZone)
-           CriticalGCError("Collecting from a GC-free thread");
-        #endif
-      #endif
-
-      volatile int dummy = 1;
-      mBottomOfStack = (int *)&dummy;
-
-      CAPTURE_REGS;
-
-      if (!mTopOfStack)
-         mTopOfStack = mBottomOfStack;
-      // EMSCRIPTEN the stack grows upwards
-      #ifdef HXCPP_STACK_UP
-      if (mBottomOfStack < mTopOfStack)
-         mTopOfStack = mBottomOfStack;
-      #else
-      if (mBottomOfStack > mTopOfStack)
-         mTopOfStack = mBottomOfStack;
-      #endif
-
-      #ifdef VerifyStackRead
-      VerifyStackRead(mBottomOfStack, mTopOfStack)
-      #endif
-
-
-      sGlobalAlloc->Collect(inMajor, inForceCompact, inLocked, inFreeIsFragged);
-   }
-
 
    void PauseForCollect()
    {
@@ -6145,6 +6136,58 @@ class LocalAllocator : public hx::StackContext
       #endif
    }
 
+   #endif // }  HXCPP_EXPLICIT_STACK_EXTENT
+
+
+   void SetupStackAndCollect(bool inMajor, bool inForceCompact, bool inLocked=false,bool inFreeIsFragged=false)
+   {
+      #ifndef HXCPP_SINGLE_THREADED_APP
+        #if HXCPP_DEBUG
+        if (mGCFreeZone)
+           CriticalGCError("Collecting from a GC-free thread");
+        #endif
+      #endif
+
+      #ifndef HXCPP_EXPLICIT_STACK_EXTENT
+      volatile int dummy = 1;
+      mBottomOfStack = (int *)&dummy;
+
+      CAPTURE_REGS;
+
+      if (!mTopOfStack)
+         mTopOfStack = mBottomOfStack;
+
+      // EMSCRIPTEN the stack grows upwards
+      #ifdef HXCPP_STACK_UP
+      if (mBottomOfStack < mTopOfStack)
+         mTopOfStack = mBottomOfStack;
+      #else
+      if (mBottomOfStack > mTopOfStack)
+         mTopOfStack = mBottomOfStack;
+      #endif
+
+      #ifdef VerifyStackRead
+      VerifyStackRead(mBottomOfStack, mTopOfStack)
+      #endif
+
+      #endif
+
+
+      sGlobalAlloc->Collect(inMajor, inForceCompact, inLocked, inFreeIsFragged);
+   }
+
+
+
+
+
+
+
+
+
+
+
+
+
    void ExpandAlloc(int &ioSize)
    {
       #ifdef HXCPP_GC_NURSERY
@@ -6331,11 +6374,13 @@ class LocalAllocator : public hx::StackContext
 
    void Mark(hx::MarkContext *__inCtx)
    {
+      #ifndef HXCPP_SINGLE_THREADED_APP
       if (!mTopOfStack)
       {
          Reset();
          return;
       }
+      #endif
 
       #ifdef SHOW_MEM_EVENTS
       //int here = 0;
@@ -6345,19 +6390,33 @@ class LocalAllocator : public hx::StackContext
       #ifdef HXCPP_DEBUG
          MarkPushClass("Stack",__inCtx);
          MarkSetMember("Stack",__inCtx);
+
+         #ifdef HXCPP_EXPLICIT_STACK_EXTENT
+           hx::MarkConservative( (int *)emscripten_stack_get_current(),(int *)emscripten_stack_get_base(), __inCtx);
+         #else
          if (mTopOfStack && mBottomOfStack)
             hx::MarkConservative(mBottomOfStack, mTopOfStack , __inCtx);
+         #endif
+
          #ifdef HXCPP_SCRIPTABLE
             MarkSetMember("ScriptStack",__inCtx);
             hx::MarkConservative((int *)(stack), (int *)(pointer),__inCtx);
          #endif
          MarkSetMember("Registers",__inCtx);
          hx::MarkConservative(CAPTURE_REG_START, CAPTURE_REG_END, __inCtx);
+
+
          MarkPopClass(__inCtx);
       #else
-         if (mTopOfStack && mBottomOfStack)
-            hx::MarkConservative(mBottomOfStack, mTopOfStack , __inCtx);
-         hx::MarkConservative(CAPTURE_REG_START, CAPTURE_REG_END, __inCtx);
+
+         #ifdef HXCPP_EXPLICIT_STACK_EXTENT
+           hx::MarkConservative( (int *)emscripten_stack_get_current(), (int *) emscripten_stack_get_base(), __inCtx);
+         #else
+            if (mTopOfStack && mBottomOfStack)
+               hx::MarkConservative(mBottomOfStack, mTopOfStack , __inCtx);
+            hx::MarkConservative(CAPTURE_REG_START, CAPTURE_REG_END, __inCtx);
+         #endif
+
          #ifdef HXCPP_SCRIPTABLE
             hx::MarkConservative((int *)(stack), (int *)(pointer),__inCtx);
          #endif
@@ -6390,6 +6449,7 @@ inline LocalAllocator *GetLocalAlloc(bool inAllowEmpty=false)
    #endif
 }
 
+#ifndef HXCPP_SINGLE_THREADED_APP
 void WaitForSafe(LocalAllocator *inAlloc)
 {
    inAlloc->WaitForSafe();
@@ -6399,6 +6459,7 @@ void ReleaseFromSafe(LocalAllocator *inAlloc)
 {
    inAlloc->ReleaseFromSafe();
 }
+#endif
 
 void MarkLocalAlloc(LocalAllocator *inAlloc,hx::MarkContext *__inCtx)
 {
diff --git a/toolchain/emscripten-toolchain.xml b/toolchain/emscripten-toolchain.xml
index f99d3a545..ab5316a57 100644
--- a/toolchain/emscripten-toolchain.xml
+++ b/toolchain/emscripten-toolchain.xml
@@ -1,17 +1,60 @@
 <xml>
 
+<!--
+  The Emscipten hxcpp compiler can be invoked from haxe with the 'emscripten' define.
+    haxe -main Main -cpp cpp -D emscripten
+
+  Setup:
+    Emscripten can be installed as per website, then EMSDK, EMSDK_PYTHON and optionally EMSDK_NODE should be set.
+
+  Hxcpp will now generate wasm, not asmjs.
+    Wasm output consists of a ".js" boot file and a ".wasm" file.  The .js file can be run from node directly or
+     via the hxcpp tool, eg: haxelib run hxcpp cpp/Test.js
+
+    If -D HXCPP_LINK_EMSCRIPTEN_EXT=".html" is passed or specified, then an additional html output,
+    eg, Test.html, will be generated.
+
+      Specifying HXCPP_LINK_EMRUN adds code ro redirect output secially for the emrun server.
+
+      The resulting can be used for running the code in a browser from a local server.
+        eg: $EMSDK/upstream/emscripten/emrun(.bat) cpp/Test.html
+
+  Exceptions:
+    You can specify HXCPP_LINK_NO_EXCEPTIONS to reduce overhead by removing exceptions,
+    for example if compiling non-haxe code.  Generally, haxe code needs exceptions.
+
+    -fwasm-exceptions is a newer addition and will be used by default.  If you need
+      greater compatibility, you can specify HXCPP_NO_WASM_EXEPTIONS
+
+  Garbage Collection:
+    Currently, the 'spill-pointers' options is needed for GC.  If you do not need gc,
+    for example compiling non-haxe code, you can disable by specifying HXCPP_NO_GC_LINK
+
+    emscripten target will default to single-threaded, which allows for some GC optimizations
+
+
+-->
+
 <setup name="emscripten" />
 <set name="HXCPP_LINK_EMSCRIPTEN_EXT" value=".js" unless="HXCPP_LINK_EMSCRIPTEN_EXT" />
 
-<path name="${EMSCRIPTEN_SDK}" if="EMSCRIPTEN_SDK" />
-<path name="${dir:EMSCRIPTEN_PYTHON}" if="EMSCRIPTEN_PYTHON" />
+<set name="EMSCRIPTEN_ROOT" value="${EMSDK}/upstream/emscripten" if="EMSDK" unless="EMSCRIPTEN_ROOT" />
+
+<set name="HXCPP_SINGLE_THREADED_APP" value="1" unless="HXCPP_MULTI_THREADED_APP" />
+
+<path name="${EMSDK}" if="EMSDK" />
+<path name="${EMSCRIPTEN_ROOT}" if="EMSCRIPTEN_ROOT" />
+<path name="${dir:EMSDK_PYTHON}" if="EMSDK_PYTHON" />
 <set name="HXCPP_RANLIB" value="emranlib" />
 <set name="HXCPP_AR" value="emar" />
+<set name="HXCPP_M32" value="1"/>
+<unset name="HXCPP_M64" />
 
 <section if="windows_host">
-   <set name="HXCPP_RANLIB" value='python "${EMSCRIPTEN_SDK}/emranlib.py"' />
-   <set name="HXCPP_AR" value='python "${EMSCRIPTEN_SDK}/emar.py"' />
-   <set name="CXX" value='python "${EMSCRIPTEN_SDK}/emcc.py"' />
+   <error value="Please provide EMSDK" unless="EMSCRIPTEN_ROOT" />
+   <set name="HXCPP_RANLIB" value='python "${EMSCRIPTEN_ROOT}/emranlib.py"' />
+   <set name="HXCPP_AR" value='python "${EMSCRIPTEN_ROOT}/emar.py"' />
+   <set name="CXX" value='python "${EMSCRIPTEN_ROOT}/emcc.py"' />
 </section>
 
 <set name="nostrip" value="1" if="emscripten"/>
@@ -41,6 +84,10 @@
   <pchflag value="-x" />
   <pchflag value="c++-header" />
   <cppflag value="--std=c++11" />
+  <section unless="HXCPP_LINK_NO_EXCEPTIONS" >
+     <flag name="-s" value="DISABLE_EXCEPTION_CATCHING=0" if="HXCPP_NO_WASM_EXEPTIONS" />
+     <flag value="-fwasm-exceptions" unless="HXCPP_NO_WASM_EXEPTIONS" />
+  </section>
   <flag value="${HXCPP_OPTIM_LEVEL}" />
   <flag value="-fpic" if="HXCPP_EMSCRIPTEN_FPIC" />
   <flag value="-fPIC" if="HXCPP_EMSCRIPTEN_FPIC" />
@@ -100,9 +147,12 @@
     <flag value="${HXCPP_LINK_MEM_FILE}"/>
   </section>
 
-  <section if="WASM" >
-     <flag name="-s" value="WASM=1" />
-     <flag name="-s" value="BINARYEN_EXTRA_PASSES='--spill-pointers'" unless="HXCPP_NO_GC_LINK" />
+  <flag name="-s" value="WASM=1" />
+  <flag value="--Wno-limited-postlink-optimizations" unless="HXCPP_NO_GC_LINK" />
+  <flag name="-s" value="BINARYEN_EXTRA_PASSES='--spill-pointers'" unless="HXCPP_NO_GC_LINK" />
+  <section unless="HXCPP_LINK_NO_EXCEPTIONS" >
+     <flag name="-s" value="DISABLE_EXCEPTION_CATCHING=0" if="HXCPP_NO_WASM_EXEPTIONS" />
+     <flag value="-fwasm-exceptions" unless="HXCPP_NO_WASM_EXEPTIONS" />
   </section>
 
   <!-- <flag value="-DBUILD_AS_SHARED_LIBRARY" /> -->
@@ -119,18 +169,13 @@
   <fromfile value="" if="GCC_OLD" />
   <flag value="${HXCPP_LINK_OPTIM_LEVEL}" />
 
-  <section unless="HXCPP_LINK_NO_EXCEPTIONS" >
-     <flag value="-s" />
-     <flag value="DISABLE_EXCEPTION_CATCHING=0" />
-  </section>
-
   <section if="HXCPP_LINK_MEMORY_GROWTH" >
      <flag value="-s" />
      <flag value="ALLOW_MEMORY_GROWTH=1" />
   </section>
 
 
-  <section if="HXCPP_LINK_SAFE_HEAP" >
+  <section if="HXCPP_LINK_SAFE_HEAP">
      <flag value="-s" />
      <flag value="SAFE_HEAP=1" />
   </section>
@@ -140,10 +185,13 @@
      <flag value="TOTAL_MEMORY=${HXCPP_LINK_TOTAL_MEMORY}" />
   </section>
 
+
+  <!-- Only for asmjs
   <section if="HXCPP_LINK_MEM_FILE" >
     <flag value="--memory-init-file"/>
     <flag value="${HXCPP_LINK_MEM_FILE}"/>
   </section>
+  -->
 
   <section if="HXCPP_LINK_EMRUN" >
      <flag value="--emrun" />
@@ -153,9 +201,12 @@
      <flag value="-debug" />
   </section>
 
-  <section if="WASM" >
-     <flag name="-s" value="WASM=1" />
-     <flag name="-s" value="BINARYEN_EXTRA_PASSES='--spill-pointers'" unless="HXCPP_NO_GC_LINK" />
+  <flag name="-s" value="WASM=1" />
+  <flag value="--Wno-limited-postlink-optimizations" unless="HXCPP_NO_GC_LINK" />
+  <flag name="-s" value="BINARYEN_EXTRA_PASSES='--spill-pointers'" unless="HXCPP_NO_GC_LINK" />
+  <section unless="HXCPP_LINK_NO_EXCEPTIONS" >
+     <flag name="-s" value="DISABLE_EXCEPTION_CATCHING=0" if="HXCPP_NO_WASM_EXEPTIONS" />
+     <flag value="-fwasm-exceptions" unless="HXCPP_NO_WASM_EXEPTIONS" />
   </section>
 
   <section if="BINARYEN_METHOD" >
diff --git a/toolchain/haxe-target.xml b/toolchain/haxe-target.xml
index cfff7ceeb..a795596ab 100644
--- a/toolchain/haxe-target.xml
+++ b/toolchain/haxe-target.xml
@@ -1,7 +1,7 @@
 <xml>
 
 <!-- make sure hxcpp.n is up to date -->
-<pleaseUpdateHxcppTool version="5" />
+<pleaseUpdateHxcppTool version="6" />
 
 <pragma once="true" />
 
diff --git a/tools/hxcpp/BuildTool.hx b/tools/hxcpp/BuildTool.hx
index 92be6063c..22f85d898 100644
--- a/tools/hxcpp/BuildTool.hx
+++ b/tools/hxcpp/BuildTool.hx
@@ -1612,8 +1612,8 @@ class BuildTool
       {
          Setup.initHXCPPConfig(defines);
          Setup.setupEmscripten(defines);
-         var node = defines.get("EMSCRIPTEN_NODE_JS");
-         Log.v( node==null ? "EMSCRIPTEN_NODE_JS undefined, using 'node'" : 'Using $node from EMSCRIPTEN_NODE_JS');
+         var node = defines.get("EMSDK_NODE");
+         Log.v( node==null ? "EMSDK_NODE undefined, using 'node'" : 'Using $node from EMSDK_NODE');
          if (node=="" || node==null)
             node = "node";
 
@@ -2347,7 +2347,7 @@ class BuildTool
    public function checkToolVersion(inVersion:String)
    {
       var ver = Std.parseInt(inVersion);
-      if (ver>5)
+      if (ver>6)
          Log.error("Your version of hxcpp.n is out-of-date.  Please update by compiling 'haxe compile.hxml' in hxcpp/tools/hxcpp.");
    }
 
diff --git a/tools/hxcpp/Setup.hx b/tools/hxcpp/Setup.hx
index 546fc7efa..1b6eb36ff 100644
--- a/tools/hxcpp/Setup.hx
+++ b/tools/hxcpp/Setup.hx
@@ -267,11 +267,12 @@ class Setup
 
    public static function setupEmscripten(ioDefines:Hash<String>)
    {
-      // Setup EMSCRIPTEN_SDK if possible - else assume developer has it in path
-      if (!ioDefines.exists("EMSCRIPTEN_SDK"))
+      // Setup EMSDK if possible - else assume developer has it in path
+      if (!ioDefines.exists("EMSDK") )
       {
          var home = ioDefines.get("HXCPP_HOME");
          var file = home + "/.emscripten";
+         Log.v('No EMSDK provided, checking $file');
          if (FileSystem.exists(file))
          {
             var content = sys.io.File.getContent(file);
@@ -285,16 +286,33 @@ class Setup
                   var val= value.matched(2);
                   if (name=="EMSCRIPTEN_ROOT")
                   {
-                     ioDefines.set("EMSCRIPTEN_SDK", val);
+                     ioDefines.set("EMSDK", val);
                   }
                   if (name=="PYTHON")
-                     ioDefines.set("EMSCRIPTEN_PYTHON", val);
+                     ioDefines.set("EMSDK_PYTHON", val);
                   if (name=="NODE_JS")
-                     ioDefines.set("EMSCRIPTEN_NODE_JS", val);
+                     ioDefines.set("EMSDK_NODE", val);
                }
             }
          }
       }
+      else
+      {
+         Log.v('Using provided EMSDK ${ioDefines.get("EMSDK")}');
+      }
+
+      if (!ioDefines.exists("EMSDK_PYTHON"))
+      {
+         Log.v("No EMSDK_PYTHON provided, using 'python'");
+      }
+      else
+         Log.v('Using provided EMSDK_PYTHON ${ioDefines.get("EMSDK_PYTHON")}');
+
+      if (!ioDefines.exists("EMSDK_NODE"))
+      {
+         Log.v("No EMSDK_NODE provided, using 'node'");
+         ioDefines.set("EMSDK_NODE", "node");
+      }
    }