[PATCH] D119177: [demangler] Fix buffer growth

[Nathan Sidwell via Phabricator via llvm-commits]

urnathan created this revision.
urnathan added reviewers: ChuanqiXu, bruno, iains, aaron.ballman.
urnathan requested review of this revision.
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The output buffer growth algorithm had a couple of 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.


https://reviews.llvm.org/D119177

Files:
  libcxxabi/src/demangle/Utility.h
  llvm/include/llvm/Demangle/Utility.h


Index: llvm/include/llvm/Demangle/Utility.h
===================================================================
--- llvm/include/llvm/Demangle/Utility.h
+++ llvm/include/llvm/Demangle/Utility.h
@@ -33,12 +33,17 @@
   size_t CurrentPosition = 0;
   size_t BufferCapacity = 0;
 
-  // Ensure there is at least n more positions in buffer.
+  // Ensure there is at least N more positions in the buffer.
   void grow(size_t N) {
-    if (N + CurrentPosition >= BufferCapacity) {
+    size_t Need = N + CurrentPosition;
+    if (Need > BufferCapacity) {
       BufferCapacity *= 2;
-      if (BufferCapacity < N + CurrentPosition)
-        BufferCapacity = N + CurrentPosition;
+      if (Need > BufferCapacity) {
+        if (!BufferCapacity && Need < 100)
+          // Avoid many reallocations during startup
+          Need = 100;
+        BufferCapacity = Need;
+      }
       Buffer = static_cast<char *>(std::realloc(Buffer, BufferCapacity));
       if (Buffer == nullptr)
         std::terminate();
Index: libcxxabi/src/demangle/Utility.h
===================================================================
--- libcxxabi/src/demangle/Utility.h
+++ libcxxabi/src/demangle/Utility.h
@@ -33,12 +33,17 @@
   size_t CurrentPosition = 0;
   size_t BufferCapacity = 0;
 
-  // Ensure there is at least n more positions in buffer.
+  // Ensure there is at least N more positions in the buffer.
   void grow(size_t N) {
-    if (N + CurrentPosition >= BufferCapacity) {
+    size_t Need = N + CurrentPosition;
+    if (Need > BufferCapacity) {
       BufferCapacity *= 2;
-      if (BufferCapacity < N + CurrentPosition)
-        BufferCapacity = N + CurrentPosition;
+      if (Need > BufferCapacity) {
+        if (!BufferCapacity && Need < 100)
+          // Avoid many reallocations during startup
+          Need = 100;
+        BufferCapacity = Need;
+      }
       Buffer = static_cast<char *>(std::realloc(Buffer, BufferCapacity));
       if (Buffer == nullptr)
         std::terminate();


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