[llvm] 995c4f3 - [demangler] Fix buffer growth
Nathan Sidwell via llvm-commits
llvm-commits at lists.llvm.org
Mon Feb 14 03:59:46 PST 2022
Author: Nathan Sidwell
Date: 2022-02-14T03:59:31-08:00
New Revision: 995c4f306890ad379de19106f053238ce89f1483
URL: https://github.com/llvm/llvm-project/commit/995c4f306890ad379de19106f053238ce89f1483
DIFF: https://github.com/llvm/llvm-project/commit/995c4f306890ad379de19106f053238ce89f1483.diff
LOG: [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
Added:
Modified:
libcxxabi/src/demangle/Utility.h
llvm/include/llvm/Demangle/Utility.h
Removed:
################################################################################
diff --git a/libcxxabi/src/demangle/Utility.h b/libcxxabi/src/demangle/Utility.h
index 93ef2aae34ba8..97272ae64fb75 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();
diff --git a/llvm/include/llvm/Demangle/Utility.h b/llvm/include/llvm/Demangle/Utility.h
index 58b1954ec82a5..df520f5cbc96d 100644
--- a/llvm/include/llvm/Demangle/Utility.h
+++ b/llvm/include/llvm/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();
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