[demangler] Fix buffer growth

The output buffer growth algorithm had a few issues:

a) An off-by-one error in the initial size check, which uses
'>='. This error was safe, but could cause us to reallocate when there
was no need.

b) An inconsistency between the initial size check (>=) and the
post-doubling check (>).  The latter was somewhat obscured by the
swapped operands.

c) There would be many reallocs with an initially-small buffer.  Add a
little initialization hysteresis.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119177

1 files changed, 7 insertions, 5 deletions

diff --git a/libcxxabi/src/demangle/Utility.h b/libcxxabi/src/demangle/Utility.h
index 93ef2aae34ba..97272ae64fb7 100644
--- a/libcxxabi/src/demangle/Utility.h
+++ b/libcxxabi/src/demangle/Utility.h
@@ -33,12 +33,14 @@ class OutputBuffer {
   size_t CurrentPosition = 0;
   size_t BufferCapacity = 0;
 
-  // Ensure there is at least n more positions in buffer.
+  // Ensure there are at least N more positions in the buffer.
   void grow(size_t N) {
-    if (N + CurrentPosition >= BufferCapacity) {
-      BufferCapacity *= 2;
-      if (BufferCapacity < N + CurrentPosition)
-        BufferCapacity = N + CurrentPosition;
+    size_t Need = N + CurrentPosition;
+    if (Need > BufferCapacity) {
+      // Avoid many reallocations during startup, with a bit of hysteresis.
+      constexpr size_t MinInitAlloc = 1024;
+      Need = std::max(Need, MinInitAlloc);
+      BufferCapacity = std::max(Need, BufferCapacity * 2);
       Buffer = static_cast<char *>(std::realloc(Buffer, BufferCapacity));
       if (Buffer == nullptr)
         std::terminate();