Runtime.stackAlloc should grow down for wasm (#1073)

* Runtime.stackAlloc should grow down for wasm

* stackAlloc should align properly; update tests

1 files changed, 4 insertions, 8 deletions

diff --git a/src/wasm-emscripten.cpp b/src/wasm-emscripten.cpp
index 43366e410..7463ec594 100644
--- a/src/wasm-emscripten.cpp
+++ b/src/wasm-emscripten.cpp
@@ -119,16 +119,12 @@ void generateStackAllocFunction(LinkerObject& linker) {
   SetLocal* setStackLocal = builder.makeSetLocal(1, loadStack);
   GetLocal* getStackLocal = builder.makeGetLocal(1, i32);
   GetLocal* getSizeArg = builder.makeGetLocal(0, i32);
-  Binary* add = builder.makeBinary(AddInt32, getStackLocal, getSizeArg);
+  Binary* sub = builder.makeBinary(SubInt32, getStackLocal, getSizeArg);
   const static uint32_t bitAlignment = 16;
   const static uint32_t bitMask = bitAlignment - 1;
-  Const* addConst = builder.makeConst(Literal(bitMask));
-  Binary* maskedAdd = builder.makeBinary(
-    AndInt32,
-    builder.makeBinary(AddInt32, add, addConst),
-    builder.makeConst(Literal(~bitMask))
-  );
-  Store* storeStack = generateStoreStackPointer(builder, linker, maskedAdd);
+  Const* subConst = builder.makeConst(Literal(~bitMask));
+  Binary* maskedSub = builder.makeBinary(AndInt32, sub, subConst);
+  Store* storeStack = generateStoreStackPointer(builder, linker, maskedSub);
 
   Block* block = builder.makeBlock();
   block->list.push_back(setStackLocal);