SmallVector (#1912)

Trying to refactor the code to be simpler and less redundant, I ran into some perf issues that it seems like a small vector, with fixed-size storage and optional additional storage as needed, might help with. This implements that class and uses it in a few places.

This seems to help, I see some 1-2% fewer instructions and cycles in `perf stat`, but it's hard to tell if it really makes a noticeable difference.

1 files changed, 68 insertions, 0 deletions

diff --git a/test/example/small_vector.cpp b/test/example/small_vector.cpp
new file mode 100644
index 000000000..732cbfac3
--- /dev/null
+++ b/test/example/small_vector.cpp
@@ -0,0 +1,68 @@
+#include <iostream>
+#include <cassert>
+
+#include "support/small_vector.h"
+
+using namespace wasm;
+
+template<typename T>
+void test() {
+  {
+    T t;
+    // build up
+    assert(t.empty());
+    assert(t.size() == 0);
+    t.push_back(1);
+    assert(!t.empty());
+    assert(t.size() == 1);
+    t.push_back(2);
+    assert(!t.empty());
+    assert(t.size() == 2);
+    t.push_back(3);
+    assert(!t.empty());
+    // unwind
+    assert(t.size() == 3);
+    assert(t.back() == 3);
+    t.pop_back();
+    assert(t.size() == 2);
+    assert(t.back() == 2);
+    t.pop_back();
+    assert(t.size() == 1);
+    assert(t.back() == 1);
+    t.pop_back();
+    assert(t.size() == 0);
+    assert(t.empty());
+  }
+  {
+    T t;
+    // build up
+    t.push_back(1);
+    t.push_back(2);
+    t.push_back(3);
+    // unwind
+    t.clear();
+    assert(t.size() == 0);
+    assert(t.empty());
+  }
+  {
+    T t, u;
+    assert(t == u);
+    t.push_back(1);
+    assert(t != u);
+    u.push_back(1);
+    assert(t == u);
+    u.pop_back();
+    assert(t != u);
+    u.push_back(2);
+    assert(t != u);
+  }
+}
+
+int main() {
+  test<SmallVector<int, 0>>();
+  test<SmallVector<int, 1>>();
+  test<SmallVector<int, 2>>();
+  test<SmallVector<int, 10>>();
+  std::cout << "ok.\n";
+}
+