Fix code for anything-to-string space estimation

Summary:
When looking at the benchmark for 64-bit integer-to-string conversion,
I noticed something strange:

  ===================================================
  folly/test/ConvBenchmark.cpp              time/iter
  ===================================================
  u64ToStringFollyMeasure(12)                 26.59ns
  u64ToStringFollyMeasure(13)                 26.89ns
  u64ToStringFollyMeasure(14)                 28.26ns  <---
  u64ToStringFollyMeasure(15)                 52.06ns  <---
  u64ToStringFollyMeasure(16)                 54.44ns
  u64ToStringFollyMeasure(17)                 55.96ns
  ===================================================

There was a sudden, unexpected jump in latency going from 14 digits to
15 digits. Profiling showed that this was due to malloc() and free()
calls for the 15 digit benchmark that didn't occur when converting
only 14 digit numbers. This was surprising, knowing that fbstrings
should be able to store up to 23 digits inline.

Even though the code to estimate the number of digits is correct, the
code to estimate the space needed within the string was off by 9 bytes.

The reason for that is that reserveInTarget() and reserveInTargetDelim()
are called with the target string as the last parameter. However, the
parameter processing in estimateSpaceToReserve() didn't consider this,
and so reserved space for the size of the pointer + 1, which explains
the wrap at 15 digits.

The fix is to make all overloads of estimateSpaceToReserve() consider
the target parameter correctly.

The benchmark shows there's no jump in latency with the fix:

  ==============================================================
  folly/test/ConvBenchmark.cpp             time/iter   time/iter
  ==============================================================
  preallocateTestNoFloat                    590.12ns    599.20ns
  preallocateTestFloat                      580.25ns    581.72ns
  preallocateTestInt8                       116.27ns    119.08ns
  preallocateTestInt16                      130.03ns    131.89ns
  preallocateTestInt32                      156.24ns    154.91ns
  preallocateTestInt64                      210.66ns    207.04ns
  preallocateTestInt128                       4.56us      4.54us
  preallocateTestNoFloatWithInt128            4.27us      4.26us
  --------------------------------------------------------------
  u64ToStringFollyMeasure(1)                 15.49ns     15.19ns
  u64ToStringFollyMeasure(2)                 16.10ns     15.80ns
  u64ToStringFollyMeasure(3)                 17.32ns     17.01ns
  u64ToStringFollyMeasure(4)                 18.53ns     18.23ns
  u64ToStringFollyMeasure(5)                 18.84ns     18.53ns
  u64ToStringFollyMeasure(6)                 20.19ns     19.83ns
  u64ToStringFollyMeasure(7)                 21.42ns     21.11ns
  u64ToStringFollyMeasure(8)                 22.48ns     22.33ns
  u64ToStringFollyMeasure(9)                 22.94ns     22.63ns
  u64ToStringFollyMeasure(10)                24.12ns     23.82ns
  u64ToStringFollyMeasure(11)                25.53ns     25.25ns
  u64ToStringFollyMeasure(12)                26.59ns     26.36ns
  u64ToStringFollyMeasure(13)                26.89ns     26.67ns
  u64ToStringFollyMeasure(14)                28.26ns     28.01ns
  u64ToStringFollyMeasure(15)                52.06ns     29.44ns
  u64ToStringFollyMeasure(16)                54.44ns     31.05ns
  u64ToStringFollyMeasure(17)                55.96ns     34.64ns
  u64ToStringFollyMeasure(18)                57.69ns     35.10ns
  u64ToStringFollyMeasure(19)                59.45ns     36.46ns
  u64ToStringFollyMeasure(20)                60.91ns     38.17ns
  ==============================================================

Reviewed By: meyering

Differential Revision: D3455825

fbshipit-source-id: 0146cbfc0105f0d709b64bcf1ed297c4e27d1129

folly/Conv.h

@@ -719,7 +719,9 @@ estimateSpaceNeeded(const Src&) {
 
 namespace detail {
 
-inline size_t estimateSpaceToReserve(size_t sofar) {
+template <class Tgt>
+typename std::enable_if<IsSomeString<Tgt>::value, size_t>::type
+estimateSpaceToReserve(size_t sofar, Tgt*) {
   return sofar;
 }
 
@@ -728,11 +730,6 @@ size_t estimateSpaceToReserve(size_t sofar, const T& v, const Ts&... vs) {
   return estimateSpaceToReserve(sofar + estimateSpaceNeeded(v), vs...);
 }
 
-template<class T>
-size_t estimateSpaceToReserve(size_t sofar, const T& v) {
-  return sofar + estimateSpaceNeeded(v);
-}
-
 template<class...Ts>
 void reserveInTarget(const Ts&...vs) {
   getLastElement(vs...)->reserve(estimateSpaceToReserve(0, vs...));
@@ -741,7 +738,8 @@ void reserveInTarget(const Ts&...vs) {
 template<class Delimiter, class...Ts>
 void reserveInTargetDelim(const Delimiter& d, const Ts&...vs) {
   static_assert(sizeof...(vs) >= 2, "Needs at least 2 args");
-  size_t fordelim = (sizeof...(vs) - 2) * estimateSpaceToReserve(0, d);
+  size_t fordelim = (sizeof...(vs) - 2) *
+      estimateSpaceToReserve(0, d, static_cast<std::string*>(nullptr));
   getLastElement(vs...)->reserve(estimateSpaceToReserve(fordelim, vs...));
 }