11 files changed, 80 insertions, 4 deletions
diff --git a/src/tools/wasm-opt.cpp b/src/tools/wasm-opt.cpp
index 7d35efeab..421bcca59 100644
--- a/src/tools/wasm-opt.cpp
+++ b/src/tools/wasm-opt.cpp
@@ -137,6 +137,8 @@ int main(int argc, const char* argv[]) {
   // It should be safe to just always enable atomics in wasm-opt, because we
   // don't expect any passes to accidentally generate atomic ops
   FeatureSet features = Feature::Atomics;
+  // Same for MutableGlobals
+  features |= Feature::MutableGlobals;
 
   if (options.debug) std::cerr << "reading...\n";
 
diff --git a/src/wasm.h b/src/wasm.h
index a0f634d17..2e09ef088 100644
--- a/src/wasm.h
+++ b/src/wasm.h
@@ -40,6 +40,7 @@ namespace wasm {
 enum Feature : uint32_t {
   MVP = 0,
   Atomics = 1 << 0,
+  MutableGlobals = 1 << 1,
   All = 0xffffffff,
 };
 typedef uint32_t FeatureSet;
diff --git a/src/wasm/wasm-binary.cpp b/src/wasm/wasm-binary.cpp
index 2dc89ee90..eebf0bd63 100644
--- a/src/wasm/wasm-binary.cpp
+++ b/src/wasm/wasm-binary.cpp
@@ -199,7 +199,7 @@ void WasmBinaryWriter::writeImports() {
     writeImportHeader(global);
     o << U32LEB(int32_t(ExternalKind::Global));
     o << binaryType(global->type);
-    o << U32LEB(0); // Mutable globals can't be imported for now.
+    o << U32LEB(global->mutable_);
   });
   if (wasm->memory.imported()) {
     if (debug) std::cerr << "write one memory" << std::endl;
@@ -1009,7 +1009,6 @@ void WasmBinaryBuilder::readImports() {
         auto name = Name(std::string("gimport$") + std::to_string(i));
         auto type = getConcreteType();
         auto mutable_ = getU32LEB();
-        assert(!mutable_); // for now, until mutable globals
         auto* curr = builder.makeGlobal(name, type, nullptr, mutable_ ? Builder::Mutable : Builder::Immutable);
         curr->module = module;
         curr->base = base;
diff --git a/src/wasm/wasm-s-parser.cpp b/src/wasm/wasm-s-parser.cpp
index 34022b72b..42dd9df0d 100644
--- a/src/wasm/wasm-s-parser.cpp
+++ b/src/wasm/wasm-s-parser.cpp
@@ -1487,19 +1487,21 @@ void SExpressionWasmBuilder::parseImport(Element& s) {
     wasm.addFunction(func.release());
   } else if (kind == ExternalKind::Global) {
     Type type;
+    bool mutable_ = false;
     if (inner[j]->isStr()) {
       type = stringToType(inner[j]->str());
     } else {
       auto& inner2 = *inner[j];
       if (inner2[0]->str() != MUT) throw ParseException("expected mut");
       type = stringToType(inner2[1]->str());
-      throw ParseException("cannot import a mutable global", s.line, s.col);
+      mutable_ = true;
     }
     auto global = make_unique<Global>();
     global->name = name;
     global->module = module;
     global->base = base;
     global->type = type;
+    global->mutable_ = mutable_;
     wasm.addGlobal(global.release());
   } else if (kind == ExternalKind::Table) {
     wasm.table.module = module;
@@ -1571,12 +1573,12 @@ void SExpressionWasmBuilder::parseGlobal(Element& s, bool preParseImport) {
   if (importModule.is()) {
     // this is an import, actually
     if (!preParseImport) throw ParseException("!preParseImport in global");
-    if (mutable_) throw ParseException("cannot import a mutable global", s.line, s.col);
     auto im = make_unique<Global>();
     im->name = global->name;
     im->module = importModule;
     im->base = importBase;
     im->type = type;
+    im->mutable_ = mutable_;
     if (wasm.getGlobalOrNull(im->name)) throw ParseException("duplicate import", s.line, s.col);
     wasm.addGlobal(im.release());
     return;
diff --git a/src/wasm/wasm-validator.cpp b/src/wasm/wasm-validator.cpp
index a08c9a129..b88a94e6d 100644
--- a/src/wasm/wasm-validator.cpp
+++ b/src/wasm/wasm-validator.cpp
@@ -923,6 +923,11 @@ static void validateImports(Module& module, ValidationInfo& info) {
       }
     }
   });
+  if (!(info.features & Feature::MutableGlobals)) {
+    ModuleUtils::iterImportedGlobals(module, [&](Global* curr) {
+      info.shouldBeFalse(curr->mutable_, curr->name, "Imported global cannot be mutable");
+    });
+  }
 }
 
 static void validateExports(Module& module, ValidationInfo& info) {
@@ -935,6 +940,10 @@ static void validateExports(Module& module, ValidationInfo& info) {
           info.shouldBeUnequal(param, i64, f->name, "Exported function must not have i64 parameters");
         }
       }
+    } else if (curr->kind == ExternalKind::Global && !(info.features & Feature::MutableGlobals)) {
+      if (Global* g = module.getGlobalOrNull(curr->value)) {
+        info.shouldBeFalse(g->mutable_, g->name, "Exported global cannot be mutable");
+      }
     }
   }
   std::unordered_set<Name> exportNames;
diff --git a/test/mutable-global.wasm b/test/mutable-global.wasm
new file mode 100644
index 000000000..537a596fd
--- /dev/null
+++ b/test/mutable-global.wasm
diff --git a/test/mutable-global.wasm.fromBinary b/test/mutable-global.wasm.fromBinary
new file mode 100644
index 000000000..aba18cdc4
--- /dev/null
+++ b/test/mutable-global.wasm.fromBinary
@@ -0,0 +1,13 @@
+(module
+ (type $0 (func))
+ (import "env" "global-mut" (global $gimport$0 (mut i32)))
+ (func $0 (; 0 ;) (type $0)
+  (set_global $gimport$0
+   (i32.add
+    (get_global $gimport$0)
+    (i32.const 1)
+   )
+  )
+ )
+)
+
diff --git a/test/mutable-global.wast b/test/mutable-global.wast
new file mode 100644
index 000000000..71d333b17
--- /dev/null
+++ b/test/mutable-global.wast
@@ -0,0 +1,12 @@
+(module
+  (type $0 (func))
+  (import "env" "global-mut" (global $global-mut (mut i32)))
+  (func $foo (type $0)
+    (set_global $global-mut
+      (i32.add
+        (get_global $global-mut)
+        (i32.const 1)
+      )
+    )
+  )
+)
diff --git a/test/mutable-global.wast.from-wast b/test/mutable-global.wast.from-wast
new file mode 100644
index 000000000..f12fb3fa4
--- /dev/null
+++ b/test/mutable-global.wast.from-wast
@@ -0,0 +1,12 @@
+(module
+ (type $0 (func))
+ (import "env" "global-mut" (global $global-mut (mut i32)))
+ (func $foo (; 0 ;) (type $0)
+  (set_global $global-mut
+   (i32.add
+    (get_global $global-mut)
+    (i32.const 1)
+   )
+  )
+ )
+)
diff --git a/test/mutable-global.wast.fromBinary b/test/mutable-global.wast.fromBinary
new file mode 100644
index 000000000..3273575ef
--- /dev/null
+++ b/test/mutable-global.wast.fromBinary
@@ -0,0 +1,13 @@
+(module
+ (type $0 (func))
+ (import "env" "global-mut" (global $gimport$0 (mut i32)))
+ (func $foo (; 0 ;) (type $0)
+  (set_global $gimport$0
+   (i32.add
+    (get_global $gimport$0)
+    (i32.const 1)
+   )
+  )
+ )
+)
+
diff --git a/test/mutable-global.wast.fromBinary.noDebugInfo b/test/mutable-global.wast.fromBinary.noDebugInfo
new file mode 100644
index 000000000..aba18cdc4
--- /dev/null
+++ b/test/mutable-global.wast.fromBinary.noDebugInfo
@@ -0,0 +1,13 @@
+(module
+ (type $0 (func))
+ (import "env" "global-mut" (global $gimport$0 (mut i32)))
+ (func $0 (; 0 ;) (type $0)
+  (set_global $gimport$0
+   (i32.add
+    (get_global $gimport$0)
+    (i32.const 1)
+   )
+  )
+ )
+)
+