[wasm] Fix emitting element section in wasm-module-builder.js

We were emitting the table index once in the element section instead of
once per element segment. This did not cause failures because we never
had more than one element segment.
This CL fixes this, and adds a test for more than one segment.

R=rossberg@chromium.org

Change-Id: I13923baf3dae383c72760286e24242d0ad55c4db
Reviewed-on: https://chromium-review.googlesource.com/527155
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45857}

test/mjsunit/wasm/indirect-tables.js

@@ -523,3 +523,48 @@ function js_div(a, b) { return (a / b) | 0; }
   // Try to grow past imported maximum
   assertThrows(() => table.grow(21));
 })();
+
+(function MultipleElementSegments() {
+  let kTableSize = 10;
+  print("MultipleElementSegments...");
+
+  let mul = (a, b) => a * b;
+  let add = (a, b) => a + b;
+  let sub = (a, b) => a - b;
+
+  // Test 1 to 3 segments in the elements section.
+  // segment 1 sets [1, 2] to mul,
+  // segment 2 sets [2, 3, 4] to add,
+  // segment 3 sets [3, 4, 5, 6] to sub.
+  for (let num_segments = 1; num_segments < 4; ++num_segments) {
+    var builder = new WasmModuleBuilder();
+
+    builder.setFunctionTableLength(kTableSize);
+    builder.addExportOfKind("table", kExternalTable, 0);
+    let f = AddFunctions(builder);
+    let indexes = [f.mul.index, f.add.index, f.sub.index];
+    for (let i = 0; i < num_segments; ++i) {
+      let offset = i + 1;
+      let len = i + 2;
+      let index = indexes[i];
+      builder.addFunctionTableInit(offset, false, new Array(len).fill(index));
+    }
+
+    let instance = builder.instantiate();
+
+    let table = instance.exports.table;
+    assertEquals(kTableSize, table.length);
+
+    for (let i = 0; i < num_segments; ++i) {
+      let exp = i < 1 || i > 2 ? null : mul;
+      if (num_segments > 1 && i >= 2 && i <= 4) exp = add;
+      if (num_segments > 2 && i >= 3 && i <= 6) exp = sub;
+      if (!exp) {
+        assertSame(null, table.get(i));
+      } else {
+        assertEquals("function", typeof table.get(i));
+    assertEquals(exp(6, 3), table.get(i)(6, 3));
+      }
+    }
+  }
+})();

test/mjsunit/wasm/wasm-module-builder.js

@@ -463,9 +463,9 @@ class WasmModuleBuilder {
       binary.emit_section(kElementSectionCode, section => {
         var inits = wasm.function_table_inits;
         section.emit_u32v(inits.length);
-        section.emit_u8(0); // table index
 
         for (let init of inits) {
+          section.emit_u8(0); // table index
           if (init.is_global) {
             section.emit_u8(kExprGetGlobal);
           } else {