Optimise IntToString by avoiding string allocation.

base/strings/string_number_conversions.cc

Index: base/strings/string_number_conversions.cc
diff --git a/base/strings/string_number_conversions.cc b/base/strings/string_number_conversions.cc
index 16d98c8a3e9d4deb4655f75b31e052f2cacf459c..0f4f38133219521f2cd889d80281a88e8619b095 100644
--- a/base/strings/string_number_conversions.cc
+++ b/base/strings/string_number_conversions.cc
@@ -27,30 +27,32 @@ struct IntToStringT {
   static STR IntToString(INT value) {
     // log10(2) ~= 0.3 bytes needed per bit or per byte log10(2**8) ~= 2.4.
     // So round up to allocate 3 output characters per byte, plus 1 for '-'.
-    const int kOutputBufSize =
+    const size_t kOutputBufSize =
         3 * sizeof(INT) + std::numeric_limits<INT>::is_signed;
 
-    // Allocate the whole string right away, we will right back to front, and
+    // Create the string in a temporary buffer, write it back to front, and
     // then return the substr of what we ended up using.
-    STR outbuf(kOutputBufSize, 0);
+    using CHR = typename STR::value_type;
+    CHR outbuf[kOutputBufSize];
 
     // The ValueOrDie call below can never fail, because UnsignedAbs is valid
     // for all valid inputs.
     auto res = CheckedNumeric<INT>(value).UnsignedAbs().ValueOrDie();
 
-    typename STR::iterator it(outbuf.end());
+    CHR* end = outbuf + kOutputBufSize;
+    CHR* i = end;
     do {
-      --it;
-      DCHECK(it != outbuf.begin());
-      *it = static_cast<typename STR::value_type>((res % 10) + '0');
+      --i;
+      DCHECK(i != outbuf);
+      *i = static_cast<CHR>((res % 10) + '0');
       res /= 10;
     } while (res != 0);
     if (IsValueNegative(value)) {
-      --it;
-      DCHECK(it != outbuf.begin());
-      *it = static_cast<typename STR::value_type>('-');
+      --i;
+      DCHECK(i != outbuf);
+      *i = static_cast<CHR>('-');
     }
-    return STR(it, outbuf.end());
+    return STR(i, end);
   }
 };