[PATCH] D119972: [demangler] Improve buffer hysteresis

[Nathan Sidwell via Phabricator via llvm-commits]

urnathan created this revision.
urnathan added reviewers: ChuanqiXu, serge-sans-paille, bruno.
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In fixing the std:max problem with the earlier patch, I noticed we could do better here by adjusting how the allocator hysteresis is used.

Except in the initialization case, 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. Just add the hysteresis to what we need everytime we know we're going to allocate.  As the first request is probably a few bytes, avoid allocating just over 1K by dialing the hysteresis back a bit.


https://reviews.llvm.org/D119972

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
@@ -37,10 +37,10 @@
   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;
Index: libcxxabi/src/demangle/Utility.h
===================================================================
--- libcxxabi/src/demangle/Utility.h
+++ libcxxabi/src/demangle/Utility.h
@@ -37,10 +37,10 @@
   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;


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