2 files changed, 19 insertions, 18 deletions

diff --git a/src/js/wasm.js-post.js b/src/js/wasm.js-post.js
index 75a91d4d4..84ae825d9 100644
--- a/src/js/wasm.js-post.js
+++ b/src/js/wasm.js-post.js
@@ -55,14 +55,18 @@ function integrateWasmJS(Module) {
 
   function mergeMemory(newBuffer) {
     // The wasm instance creates its memory. But static init code might have written to
-    // buffer already, and we must copy it over in a proper merge.
+    // buffer already, including the mem init file, and we must copy it over in a proper merge.
     // TODO: avoid this copy, by avoiding such static init writes
     // TODO: in shorter term, just copy up to the last static init write
     var oldBuffer = Module['buffer'];
     assert(newBuffer.byteLength >= oldBuffer.byteLength, 'we might fail if we allocated more than TOTAL_MEMORY');
-    // the wasm module does write out the memory initialization, in range STATIC_BASE..STATIC_BUMP, so avoid that
-    (new Int8Array(newBuffer).subarray(0, STATIC_BASE)).set(new Int8Array(oldBuffer).subarray(0, STATIC_BASE));
-    (new Int8Array(newBuffer).subarray(STATIC_BASE + STATIC_BUMP)).set(new Int8Array(oldBuffer).subarray(STATIC_BASE + STATIC_BUMP));
+    var oldView = new Int8Array(oldBuffer);
+    var newView = new Int8Array(newBuffer);
+    if ({{{ WASM_BACKEND }}}) {
+      // memory segments arrived in the wast, do not trample them
+      oldView.set(newView.subarray(STATIC_BASE, STATIC_BASE + STATIC_BUMP), STATIC_BASE);
+    }
+    newView.set(oldView);
     updateGlobalBuffer(newBuffer);
     updateGlobalBufferViews();
     Module['reallocBuffer'] = function(size) {
@@ -175,6 +179,8 @@ function integrateWasmJS(Module) {
       return Module['buffer'] !== old ? Module['buffer'] : null; // if it was reallocated, it changed
     };
 
+    wasmJS['providedTotalMemory'] = Module['buffer'].byteLength;
+
     // Prepare to generate wasm, using either asm2wasm or wasm-s-parser
     var code = Module['read'](method == 'asm2wasm' ? Module['asmjsCodeFile'] : Module['wasmCodeFile']);
     var temp = wasmJS['_malloc'](code.length + 1);
@@ -186,8 +192,6 @@ function integrateWasmJS(Module) {
     }
     wasmJS['_free'](temp);
 
-    wasmJS['providedTotalMemory'] = Module['buffer'].byteLength;
-
     wasmJS['_instantiate'](temp);
 
     if (Module['newBuffer']) {
diff --git a/src/wasm-js.cpp b/src/wasm-js.cpp
index 9c9096ad8..7b9afab30 100644
--- a/src/wasm-js.cpp
+++ b/src/wasm-js.cpp
@@ -148,19 +148,16 @@ extern "C" void EMSCRIPTEN_KEEPALIVE instantiate() {
 
   struct JSExternalInterface : ModuleInstance::ExternalInterface {
     void init(Module& wasm) override {
-      // if we have memory segments, create a new buffer here, just like native wasm support would.
-      // otherwise, no need to.
-      if (wasm.memory.segments.size() > 0) {
+      // create a new buffer here, just like native wasm support would.
+      EM_ASM_({
+        Module['outside']['newBuffer'] = new ArrayBuffer($0);
+      }, wasm.memory.initial);
+      for (auto segment : wasm.memory.segments) {
         EM_ASM_({
-          Module['outside']['newBuffer'] = new ArrayBuffer($0);
-        }, wasm.memory.initial);
-        for (auto segment : wasm.memory.segments) {
-          EM_ASM_({
-            var source = Module['HEAP8'].subarray($1, $1 + $2);
-            var target = new Int8Array(Module['outside']['newBuffer']);
-            target.set(source, $0);
-          }, segment.offset, segment.data, segment.size);
-        }
+          var source = Module['HEAP8'].subarray($1, $1 + $2);
+          var target = new Int8Array(Module['outside']['newBuffer']);
+          target.set(source, $0);
+        }, segment.offset, segment.data, segment.size);
       }
     }