Faster repeated append (particularly for short strings)

Summary:
https://phabricator.fb.com/D544159 reveals a large performance gap between
fbstring and std::string for repeated appends of short strings, which I
consider a relatively urgent matter (as much of our code uses such patterns).

This diff attempts to fix the issue in a principled manner by first appending
the first character with exponential reallocation, after which the rest of the
characters are appended normally.

With the proposed fix the benchmarks are much faster than the previous fbstring
and also than std::string (numbers to follow in comments).

Test Plan: unittested and benchmarked

Reviewed By: soren@fb.com

FB internal diff: D545416

folly/FBString.h

@@ -87,6 +87,7 @@
 #include <cassert>
 
 #include "folly/Traits.h"
+#include "folly/Likely.h"
 #include "folly/Malloc.h"
 #include "folly/Hash.h"
 
@@ -143,15 +144,17 @@ inline void pod_fill(Pod* b, Pod* e, T c) {
 
 /*
  * Lightly structured memcpy, simplifies copying PODs and introduces
- * some asserts
+ * some asserts. Unfortunately using this function may cause
+ * measurable overhead (presumably because it adjusts from a begin/end
+ * convention to a pointer/size convention, so it does some extra
+ * arithmetic even though the caller might have done the inverse
+ * adaptation outside).
  */
 template <class Pod>
-inline Pod* pod_copy(const Pod* b, const Pod* e, Pod* d) {
+inline void pod_copy(const Pod* b, const Pod* e, Pod* d) {
   assert(e >= b);
   assert(d >= e || d + (e - b) <= b);
-  const size_t s = e - b;
-  std::memcpy(d, b, s * sizeof(*b));
-  return d + s;
+  memcpy(d, b, (e - b) * sizeof(Pod));
 }
 
 /*
@@ -210,9 +213,10 @@ public:
   void shrink(size_t delta);
   // Expands the string by delta characters (i.e. after this call
   // size() will report the old size() plus delta) but without
-  // initializing the expanded region. The caller is expected to fill
-  // the expanded area appropriately.
-  void expand_noinit(size_t delta);
+  // initializing the expanded region. Returns a pointer to the memory
+  // to be initialized (the beginning of the expanded portion). The
+  // caller is expected to fill the expanded area appropriately.
+  Char* expand_noinit(size_t delta);
   // Expands the string by one character and sets the last character
   // to c.
   void push_back(Char c);
@@ -606,31 +610,33 @@ public:
     assert(capacity() >= minCapacity);
   }
 
-  void expand_noinit(const size_t delta) {
+  Char * expand_noinit(const size_t delta) {
     // Strategy is simple: make room, then change size
     assert(capacity() >= size());
-    size_t sz, newSz, cp;
+    size_t sz, newSz;
     if (category() == isSmall) {
       sz = smallSize();
       newSz = sz + delta;
       if (newSz <= maxSmallSize) {
         setSmallSize(newSz);
         writeTerminator();
-        return;
+        return small_ + sz;
       }
-      cp = maxSmallSize;
+      reserve(newSz);
     } else {
       sz = ml_.size_;
-      newSz = sz + delta;
-      cp = capacity();
+      newSz = ml_.size_ + delta;
+      if (newSz > ml_.capacity()) {
+        reserve(newSz);
+      }
     }
-    if (newSz > cp) reserve(newSz);
     assert(capacity() >= newSz);
     // Category can't be small - we took care of that above
     assert(category() == isMedium || category() == isLarge);
     ml_.size_ = newSz;
     writeTerminator();
     assert(size() == newSz);
+    return ml_.data_ + sz;
   }
 
   void push_back(Char c) {
@@ -644,7 +650,7 @@ public:
         writeTerminator();
         return;
       }
-      reserve(maxSmallSize * 3 / 2);
+      reserve(maxSmallSize * 2);
     } else {
       sz = ml_.size_;
       cp = ml_.capacity();
@@ -854,8 +860,10 @@ public:
     assert(delta <= size());
     backend_.resize(size() - delta);
   }
-  void expand_noinit(size_t delta) {
+  Char * expand_noinit(size_t delta) {
+    auto const sz = size();
     backend_.resize(size() + delta);
+    return backend_.data() + sz;
   }
   void push_back(Char c) {
     backend_.push_back(c);
@@ -1001,8 +1009,7 @@ public:
   }
 
   basic_fbstring(size_type n, value_type c, const A& a = A()) {
-    store_.expand_noinit(n);
-    auto const data = store_.mutable_data();
+    auto const data = store_.expand_noinit(n);
     fbstring_detail::pod_fill(data, data + n, c);
     store_.writeTerminator();
   }
@@ -1235,23 +1242,39 @@ public:
     return append(str.data() + pos, n);
   }
 
-  basic_fbstring& append(const value_type* s, const size_type n) {
+  basic_fbstring& append(const value_type* s, size_type n) {
 #ifndef NDEBUG
-    auto oldSize = size();
-#endif
     Invariant checker(*this);
     (void) checker;
-    static std::less_equal<const value_type*> le;
-    if (le(data(), s) && !le(data() + size(), s)) {// aliasing
-      assert(le(s + n, data() + size()));
-      const size_type offset = s - data();
-      store_.reserve(size() + n);
+#endif
+    if (UNLIKELY(!n)) {
+      // Unlikely but must be done
+      return *this;
+    }
+    auto const oldSize = size();
+    auto const oldData = data();
+    // Check for aliasing (rare). We could use "<=" here but in theory
+    // those do not work for pointers unless the pointers point to
+    // elements in the same array. For that reason we use
+    // std::less_equal, which is guaranteed to offer a total order
+    // over pointers. See discussion at http://goo.gl/Cy2ya for more
+    // info.
+    static const std::less_equal<const value_type*> le;
+    if (UNLIKELY(le(oldData, s) && !le(oldData + oldSize, s))) {
+      assert(le(s + n, oldData + oldSize));
+      const size_type offset = s - oldData;
+      store_.reserve(oldSize + n);
       // Restore the source
       s = data() + offset;
     }
-    store_.expand_noinit(n);
-    fbstring_detail::pod_copy(s, s + n, end() - n);
-    store_.writeTerminator();
+    // Warning! Repeated appends with short strings may actually incur
+    // practically quadratic performance. Avoid that by pushing back
+    // the first character (which ensures exponential growth) and then
+    // appending the rest normally. Worst case the append may incur a
+    // second allocation but that will be rare.
+    push_back(*s++);
+    --n;
+    memcpy(store_.expand_noinit(n), s, n * sizeof(value_type));
     assert(size() == oldSize + n);
     return *this;
   }