[scudo] Optimize scudo test string allocation

When the underlying vector becomes full, it resizes, remaps, and then copies over the old data. To avoid thes excess allocations, allow reservation from the backing vector.

Differential Revision: https://reviews.llvm.org/D135119

Author: Dominic Chen

compiler-rt/lib/scudo/standalone/string_utils.h

@@ -28,6 +28,7 @@ class ScopedString {
   void append(const char *Format, va_list Args);
   void append(const char *Format, ...) FORMAT(2, 3);
   void output() const { outputRaw(String.data()); }
+  void reserve(size_t Size) { String.reserve(Size + 1); }
 
 private:
   Vector<char> String;

compiler-rt/lib/scudo/standalone/tests/strings_test.cpp

@@ -43,9 +43,11 @@ TEST(ScudoStringsTest, Clear) {
 }
 
 TEST(ScudoStringsTest, ClearLarge) {
+  constexpr char appendString[] = "123";
   scudo::ScopedString Str;
+  Str.reserve(sizeof(appendString) * 10000);
   for (int i = 0; i < 10000; ++i)
-    Str.append("123");
+    Str.append(appendString);
   Str.clear();
   EXPECT_EQ(0ul, Str.length());
   EXPECT_EQ('\0', *Str.data());
@@ -76,6 +78,7 @@ TEST(ScudoStringTest, PotentialOverflows) {
   // of it with variations of append. The expectation is for nothing to crash.
   const scudo::uptr PageSize = scudo::getPageSizeCached();
   scudo::ScopedString Str;
+  Str.reserve(2 * PageSize);
   Str.clear();
   fillString(Str, 2 * PageSize);
   Str.clear();