[demangler] Improve buffer hysteresis

Improve demangler buffer hysteresis.  If we needed more than double
the buffer, the original code would allocate exactly the amount
needed, and thus consequently the next request would also realloc.
We're very unlikely to get into wanting more than double, after the
first allocation, as it would require the user to have used an
identifier larger than the hysteresis.  With machine generated code
that's possible, but unlikely.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119972

libcxxabi/src/demangle/Utility.h

@@ -37,10 +37,10 @@ class OutputBuffer {
   void grow(size_t N) {
     size_t Need = N + CurrentPosition;
     if (Need > BufferCapacity) {
-      // Avoid many reallocations during startup, with a bit of hysteresis.
-      constexpr size_t MinInitAlloc = 1024;
-      if (Need < MinInitAlloc)
-        Need = MinInitAlloc;
+      // Reduce the number of reallocations, with a bit of hysteresis. The
+      // number here is chosen so the first allocation will more-than-likely not
+      // allocate more than 1K.
+      Need += 1024 - 32;
       BufferCapacity *= 2;
       if (BufferCapacity < Need)
         BufferCapacity = Need;

llvm/include/llvm/Demangle/Utility.h

@@ -37,10 +37,10 @@ class OutputBuffer {
   void grow(size_t N) {
     size_t Need = N + CurrentPosition;
     if (Need > BufferCapacity) {
-      // Avoid many reallocations during startup, with a bit of hysteresis.
-      constexpr size_t MinInitAlloc = 1024;
-      if (Need < MinInitAlloc)
-        Need = MinInitAlloc;
+      // Reduce the number of reallocations, with a bit of hysteresis. The
+      // number here is chosen so the first allocation will more-than-likely not
+      // allocate more than 1K.
+      Need += 1024 - 32;
       BufferCapacity *= 2;
       if (BufferCapacity < Need)
         BufferCapacity = Need;

llvm/unittests/Demangle/OutputBufferTest.cpp

@@ -78,3 +78,16 @@ TEST(OutputBufferTest, Prepend) {
 
   std::free(OB.getBuffer());
 }
+
+// Test when initial needed size is larger than the default.
+TEST(OutputBufferTest, Extend) {
+  OutputBuffer OB;
+
+  char Massive[2000];
+  std::memset(Massive, 'a', sizeof(Massive));
+  Massive[sizeof(Massive) - 1] = 0;
+  OB << Massive;
+  EXPECT_EQ(Massive, toString(OB));
+
+  std::free(OB.getBuffer());
+}