Clear out asm assignments in minimal runtime metadce (#10725)

Before this, if X was never used (via metadce), we would still have

 X = asm['X']

After this that doesn't exist.

Closure can do this (but doesn't always), so this mostly helps
non-closure builds.

tests/code_size/hello_webgl2_wasm2js.json

@@ -2,9 +2,9 @@
   "a.html": 588,
   "a.html.gz": 386,
   "a.js": 24518,
-  "a.js.gz": 9119,
+  "a.js.gz": 9124,
   "a.mem": 3168,
   "a.mem.gz": 2711,
   "total": 28274,
-  "total_gz": 12216
+  "total_gz": 12221
 }

tests/code_size/hello_webgl_wasm2js.json

@@ -2,9 +2,9 @@
   "a.html": 588,
   "a.html.gz": 386,
   "a.js": 24013,
-  "a.js.gz": 8955,
+  "a.js.gz": 8962,
   "a.mem": 3168,
   "a.mem.gz": 2711,
   "total": 27769,
-  "total_gz": 12052
+  "total_gz": 12059
 }

tests/code_size/hello_world_wasm2js.json

@@ -2,9 +2,9 @@
   "a.html": 697,
   "a.html.gz": 437,
   "a.js": 2026,
-  "a.js.gz": 911,
+  "a.js.gz": 913,
   "a.mem": 6,
   "a.mem.gz": 32,
   "total": 2729,
-  "total_gz": 1380
+  "total_gz": 1382
 }

tests/code_size/random_printf_wasm2js.json

@@ -1,6 +1,6 @@
 {
-  "a.html": 20054,
-  "a.html.gz": 8394,
-  "total": 20054,
-  "total_gz": 8394
+  "a.html": 20051,
+  "a.html.gz": 8371,
+  "total": 20051,
+  "total_gz": 8371
 }

tests/optimizer/minimal-runtime-applyDCEGraphRemovals-output.js

@@ -0,0 +1,22 @@
+var name;
+
+var asmLibraryArg = {
+ "save1": 1,
+ "save2": 2
+};
+
+WebAssembly.instantiate(Module["wasm"], imports).then(function(output) {
+ asm = output.instance.exports;
+ expD1 = asm["expD1"];
+ expD2 = asm["expD2"];
+ expD3 = asm["expD3"];
+
+ initRuntime(asm);
+ ready();
+});
+
+expD1;
+
+Module["expD2"];
+
+asm["expD3"];

tests/optimizer/minimal-runtime-applyDCEGraphRemovals.js

@@ -0,0 +1,20 @@
+var name;
+var asmLibraryArg = { 'save1': 1, 'number': 33, 'name': name, 'func': function() {}, 'save2': 2 };
+
+// exports gotten directly in the minimal runtime style
+WebAssembly.instantiate(Module["wasm"], imports).then(function(output) {
+ asm = output.instance.exports;
+ expD1 = asm['expD1'];
+ expD2 = asm['expD2'];
+ expD3 = asm['expD3'];
+ expD4 = asm['expD4'];
+ initRuntime(asm);
+ ready();
+});
+
+// add uses for some of them, leave *4 as non-roots
+expD1;
+Module['expD2'];
+asm['expD3'];
+
+// EXTRA_INFO: { "unused": ["emcc$import$number", "emcc$import$name", "emcc$import$func", "emcc$export$expD4", "emcc$export$expI4"] }

tests/test_other.py

@@ -2171,6 +2171,8 @@ int f() {
        ['emitDCEGraph', 'noPrint']),
       (path_from_root('tests', 'optimizer', 'emitDCEGraph5.js'), open(path_from_root('tests', 'optimizer', 'emitDCEGraph5-output.js')).read(),
        ['emitDCEGraph', 'noPrint']),
+      (path_from_root('tests', 'optimizer', 'minimal-runtime-applyDCEGraphRemovals.js'), open(path_from_root('tests', 'optimizer', 'minimal-runtime-applyDCEGraphRemovals-output.js')).read(),
+       ['applyDCEGraphRemovals']),
       (path_from_root('tests', 'optimizer', 'applyDCEGraphRemovals.js'), open(path_from_root('tests', 'optimizer', 'applyDCEGraphRemovals-output.js')).read(),
        ['applyDCEGraphRemovals']),
       (path_from_root('tests', 'optimizer', 'applyImportAndExportNameChanges.js'), open(path_from_root('tests', 'optimizer', 'applyImportAndExportNameChanges-output.js')).read(),

tools/acorn-optimizer.js

@@ -112,12 +112,19 @@ function nullify(node) {
   node.raw = 'null';
 }
 
-function undefinedify(node) {
+// This converts the node into something that terser will ignore in a var
+// declaration, that is, it is a way to get rid of initial values.
+function convertToNothingInVarInit(node) {
   node.type = 'Literal';
   node.value = undefined;
   node.raw = 'undefined';
 }
 
+function convertToNull(node) {
+  node.type = 'Identifier';
+  node.name = 'null';
+}
+
 function setLiteralValue(item, value) {
   item.value = value;
   item.raw = "'" + value + "'";
@@ -851,7 +858,24 @@ function applyDCEGraphRemovals(ast) {
         var value = node.right;
         if ((full in unused) &&
             (isAsmUse(value) || !hasSideEffects(value))) {
-          undefinedify(node);
+          // This will be in a var init, and we just remove that value.
+          convertToNothingInVarInit(node);
+        }
+      }
+    } else if (node.type === 'ExpressionStatement') {
+      var expr = node.expression;
+      // In the minimal runtime code pattern we have just
+      //   x = asm['x']
+      // and never in a var.
+      if (expr.operator === '=' &&
+          expr.left.type === 'Identifier' &&
+          isAsmUse(expr.right)) {
+        var name = expr.left.name;
+        if (name === getAsmOrModuleUseName(expr.right)) {
+          var full = 'emcc$export$' + name;
+          if (full in unused) {
+            emptyOut(node);
+          }
         }
       }
     }