Properly handle metadce of MINIMAL_RUNTIME exports (#10663)

We handled imports, but not exports. As a result, some things in
the wasm binary were not cleaned up by metadce. I noticed this
when comparing builds with minimal and normal runtime - the JS
was much smaller in minimal, but the binaries were a little bigger
due to stuff like unnecessary dynCalls, things like stackAlloc, etc.

This adds a metadce test for minimal runtime.

After this, minimal runtime has similar wasm sizes to the normal
runtime.

tests/code_size/hello_webgl2_fastcomp_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 377,
   "a.js": 5352,
   "a.js.gz": 2540,
-  "a.wasm": 8364,
-  "a.wasm.gz": 4520,
-  "total": 14293,
-  "total_gz": 7437
+  "a.wasm": 8350,
+  "a.wasm.gz": 4509,
+  "total": 14285,
+  "total_gz": 7426
 }

tests/code_size/hello_webgl2_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 377,
   "a.js": 5133,
   "a.js.gz": 2426,
-  "a.wasm": 11318,
-  "a.wasm.gz": 7100,
-  "total": 17014,
-  "total_gz": 9903
+  "a.wasm": 11262,
+  "a.wasm.gz": 7065,
+  "total": 16958,
+  "total_gz": 9868
 }

tests/code_size/hello_webgl2_wasm2js.json

@@ -1,10 +1,10 @@
 {
   "a.html": 588,
   "a.html.gz": 386,
-  "a.js": 24667,
-  "a.js.gz": 9175,
+  "a.js": 24518,
+  "a.js.gz": 9119,
   "a.mem": 3168,
   "a.mem.gz": 2711,
-  "total": 28423,
-  "total_gz": 12272
+  "total": 28274,
+  "total_gz": 12216
 }

tests/code_size/hello_webgl_fastcomp_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 377,
   "a.js": 4844,
   "a.js.gz": 2365,
-  "a.wasm": 8364,
-  "a.wasm.gz": 4520,
-  "total": 13785,
-  "total_gz": 7262
+  "a.wasm": 8350,
+  "a.wasm.gz": 4509,
+  "total": 13777,
+  "total_gz": 7251
 }

tests/code_size/hello_webgl_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 377,
   "a.js": 4624,
   "a.js.gz": 2254,
-  "a.wasm": 11318,
-  "a.wasm.gz": 7100,
-  "total": 16505,
-  "total_gz": 9731
+  "a.wasm": 11262,
+  "a.wasm.gz": 7065,
+  "total": 16449,
+  "total_gz": 9696
 }

tests/code_size/hello_webgl_wasm2js.json

@@ -1,10 +1,10 @@
 {
   "a.html": 588,
   "a.html.gz": 386,
-  "a.js": 24162,
-  "a.js.gz": 9012,
+  "a.js": 24013,
+  "a.js.gz": 8955,
   "a.mem": 3168,
   "a.mem.gz": 2711,
-  "total": 27918,
-  "total_gz": 12109
+  "total": 27769,
+  "total_gz": 12052
 }

tests/code_size/hello_world_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 427,
   "a.js": 450,
   "a.js.gz": 321,
-  "a.wasm": 172,
-  "a.wasm.gz": 166,
-  "total": 1287,
-  "total_gz": 914
+  "a.wasm": 96,
+  "a.wasm.gz": 109,
+  "total": 1211,
+  "total_gz": 857
 }

tests/code_size/hello_world_wasm2js.json

@@ -1,10 +1,10 @@
 {
   "a.html": 697,
   "a.html.gz": 437,
-  "a.js": 2179,
-  "a.js.gz": 979,
+  "a.js": 2026,
+  "a.js.gz": 911,
   "a.mem": 6,
   "a.mem.gz": 32,
-  "total": 2882,
-  "total_gz": 1448
+  "total": 2729,
+  "total_gz": 1380
 }

tests/code_size/random_printf_fastcomp_asmjs.json

@@ -1,6 +1,6 @@
 {
   "a.html": 2907,
   "a.html.gz": 1231,
-  "total": 2913,
+  "total": 2907,
   "total_gz": 1231
 }

tests/code_size/random_printf_fastcomp_wasm.json

@@ -1,6 +1,6 @@
 {
-  "a.html": 13704,
-  "a.html.gz": 7262,
-  "total": 13744,
-  "total_gz": 7262
+  "a.html": 13468,
+  "a.html.gz": 7113,
+  "total": 13468,
+  "total_gz": 7113
 }

tests/code_size/random_printf_wasm.json

@@ -1,6 +1,6 @@
 {
-  "a.html": 14100,
-  "a.html.gz": 7546,
-  "total": 14100,
-  "total_gz": 7546
+  "a.html": 13845,
+  "a.html.gz": 7353,
+  "total": 13845,
+  "total_gz": 7353
 }

tests/code_size/random_printf_wasm2js.json

@@ -1,6 +1,6 @@
 {
-  "a.html": 20529,
-  "a.html.gz": 8546,
-  "total": 20529,
-  "total_gz": 8546
+  "a.html": 20054,
+  "a.html.gz": 8394,
+  "total": 20054,
+  "total_gz": 8394
 }

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

@@ -36,8 +36,12 @@
  {
   "name": "emcc$export$_main",
   "export": "_main",
-  "reaches": [],
-  "root": true
+  "reaches": []
+ },
+ {
+  "name": "emcc$export$_unused",
+  "export": "c",
+  "reaches": []
  },
  {
   "name": "emcc$import$_emscripten_console_log",

tests/optimizer/minimal-runtime-emitDCEGraph.js

@@ -64,12 +64,13 @@ var imports = {
   })
  }
 };
-var _main;
+var _main, _unused;
 WebAssembly.instantiate(Module["wasm"], imports).then((function(output) {
  var asm = output.instance.exports;
  _main = asm["b"];
+ _unused = asm["c"];
  initRuntime();
  ready();
 }));
 
-// EXTRA_INFO: { "exports": [["_main","_main"]]}
+// EXTRA_INFO: { "exports": [["_main","_main"]]}

tests/other/metadce/minimal_Os_MINIMAL_RUNTIME.exports

@@ -0,0 +1,2 @@
+a
+b

tests/other/metadce/minimal_Os_MINIMAL_RUNTIME.funcs

@@ -0,0 +1,2 @@
+$__wasm_call_ctors
+$add

tests/other/metadce/minimal_Os_MINIMAL_RUNTIME.imports

@@ -0,0 +1 @@
+

tests/other/metadce/minimal_Os_MINIMAL_RUNTIME.sent

@@ -0,0 +1,2 @@
+memory
+table

tests/test_other.py

@@ -8257,6 +8257,7 @@ int main() {
     'O3': (['-O3'], [], [],          85), # noqa
     'Os': (['-Os'], [], [],          85), # noqa
     'Oz': (['-Oz'], [], [],          85), # noqa
+    'Os_mr': (['-Os', '-s', 'MINIMAL_RUNTIME'], [], [], 85), # noqa
   })
   @no_fastcomp()
   def test_metadce_minimal(self, *args):

tools/acorn-optimizer.js

@@ -550,7 +550,7 @@ function emitDCEGraph(ast) {
   //
   // The imports that wasm receives look like this:
   //
-  //  Module.asmLibraryArg = { "abort": abort, "assert": assert, [..] };
+  //  var asmLibraryArg = { "abort": abort, "assert": assert, [..] };
   //
   // The exports are trickier, as they have a different form whether or not
   // async compilation is enabled. It can be either:
@@ -563,6 +563,14 @@ function emitDCEGraph(ast) {
   //   return Module["asm"]["_malloc"].apply(null, arguments);
   //  });
   //
+  // or, in the minimal runtime, it looks like
+  //
+  //  WebAssembly.instantiate(Module["wasm"], imports).then(function(output) {
+  //   var asm = output.instance.exports;
+  //   ..
+  //   _malloc = asm["malloc"];
+  //   ..
+  //  });
   var imports = [];
   var defuns = [];
   var dynCallNames = [];
@@ -570,6 +578,7 @@ function emitDCEGraph(ast) {
   var modulePropertyToGraphName = {};
   var exportNameToGraphName = {}; // identical to asm['..'] nameToGraphName
   var foundAsmLibraryArgAssign = false;
+  var foundMinimalRuntimeExports = false;
   var graph = [];
 
   function saveAsmExport(name, asmName) {
@@ -628,15 +637,68 @@ function emitDCEGraph(ast) {
               // name may be slightly different (extra "_" in wasm backend)
               saveAsmExport(name, asmName);
               emptyOut(node); // ignore this in the second pass; this does not root
+              return;
             }
           }
         }
       }
+      // A variable declaration that has no initial values can be ignored in
+      // the second pass, these are just declarations, not roots - an actual
+      // use must be found in order to root.
+      if (!node.declarations.reduce(function(hasInit, decl) {
+            return hasInit || !!decl.init
+          }, false)) {
+        emptyOut(node);
+      }
     } else if (node.type === 'FunctionDeclaration') {
       defuns.push(node);
       var name = node.id.name;
       nameToGraphName[name] = getGraphName(name, 'defun');
       emptyOut(node); // ignore this in the second pass; we scan defuns separately
+    } else if (node.type === 'FunctionExpression') {
+      // Check if this is the minimal runtime exports function, which looks like
+      //   function(output) { var asm = output.instance.exports;
+      if (node.params.length === 1 && node.params[0].type === 'Identifier' &&
+          node.params[0].name === 'output' && node.body.type === 'BlockStatement') {
+        var body = node.body.body;
+        if (body.length >= 1) {
+          var first = body[0];
+          if (first.type === 'VariableDeclaration' && first.declarations.length === 1) {
+            var decl = first.declarations[0];
+            if (decl.id.type === 'Identifier' && decl.id.name === 'asm') {
+              var value = decl.init;
+              if (value.type === 'MemberExpression' &&
+                  value.object.type === 'MemberExpression' &&
+                  value.object.object.type === 'Identifier' &&
+                  value.object.object.name === 'output' &&
+                  value.object.property.type === 'Identifier' &&
+                  value.object.property.name === 'instance' &&
+                  value.property.type === 'Identifier' &&
+                  value.property.name === 'exports') {
+                // This looks very much like what we are looking for.
+                assert(!foundMinimalRuntimeExports);
+                for (var i = 1; i < body.length; i++) {
+                  var item = body[i];
+                  if (item.type === 'ExpressionStatement' &&
+                      item.expression.type === 'AssignmentExpression' &&
+                      item.expression.operator === '=' &&
+                      item.expression.left.type === 'Identifier' &&
+                      item.expression.right.type === 'MemberExpression' &&
+                      item.expression.right.object.type === 'Identifier' &&
+                      item.expression.right.object.name === 'asm' &&
+                      item.expression.right.property.type === 'Literal') {
+                    var name = item.expression.left.name;
+                    var asmName = item.expression.right.property.value;
+                    saveAsmExport(name, asmName);
+                    emptyOut(item); // ignore all this in the second pass; this does not root
+                  }
+                }
+                foundMinimalRuntimeExports = true;
+              }
+            }
+          }
+        }
+      }
     }
   });
   // must find the info we need