Disable QTC inside the library by default (fixes #714)

This results in measurable performance improvements to packaged binary
libqpdf distributions. QTC remains available for library users and is
still selectively enabled in CI.

2 files changed, 26 insertions, 0 deletions

diff --git a/manual/installation.rst b/manual/installation.rst
index e02380ee..08c49765 100644
--- a/manual/installation.rst
+++ b/manual/installation.rst
@@ -257,6 +257,16 @@ CHECK_SIZES
   that ensures an exact match between classes in ``sizes.cc`` and
   classes in the library's public API. This option requires Python 3.
 
+ENABLE_QTC
+  This is off by default, except in maintainer mode. When off,
+  ``QTC::TC`` calls are compiled out by having ``QTC::TC`` be an empty
+  inline function. The underlying ``QTC::TC`` remains in the library,
+  so it is possible to build and package the qpdf library with
+  ``ENABLE_QTC`` turned off while still allowing developer code to use
+  ``QTC::TC`` if desired. If you are modifying qpdf code, it's a good
+  idea to have this on for more robust automated testing. Otherwise,
+  there's no reason to have it on.
+
 GENERATE_AUTO_JOB
   Some qpdf source files are automatically generated from
   :file:`job.yml` and the CLI documentation. If you are adding new
@@ -297,6 +307,8 @@ MAINTAINER_MODE
 
   - ``CHECK_SIZES``
 
+  - ``ENABLE_QTC``
+
   - ``GENERATE_AUTO_JOB``
 
   - ``WERROR``
diff --git a/manual/release-notes.rst b/manual/release-notes.rst
index ebbfd4f5..ab2c1d8e 100644
--- a/manual/release-notes.rst
+++ b/manual/release-notes.rst
@@ -7,6 +7,12 @@ For a detailed list of changes, please see the file
 :file:`ChangeLog` in the source distribution.
 
 11.0.0
+  - Performance improvements
+
+    - Many performance enhancements have been added. In developer
+      performance benchmarks, gains on the order of 20% have been
+      observed.
+
   - Replacement of ``PointerHolder`` with ``std::shared_ptr``
 
     - The qpdf-specific ``PointerHolder`` smart pointer implementation
@@ -231,6 +237,14 @@ For a detailed list of changes, please see the file
     - The qpdf source code is now formatted automatically with
       ``clang-format``. See :ref:`code-formatting` for information.
 
+    - Test coverage with ``QTC`` is enabled during development but
+      compiled out of distributed qpdf binaries by default. This
+      results in a significant performance improvement, especially on
+      Windows. ``QTC::TC`` is still available in the library and is
+      still usable by end user code even though calls to it made
+      internally by the library are turned off. Internally, there is
+      some additional caching to reduce the overhead of repeatedly
+      reading environment variables at runtime.
 
 10.6.3: March 8, 2022
   - Announcement of upcoming change: