Add a CORRADE_BUILD_TESTS_FORCE_CPU_POINTER_DISPATCH option.

The tests will be implicitly configured to use function pointers for
CPU dispatch, but since it adds significant overhead on Emscripten,
there also needs to be a way to disable this.

Author: mosra

CMakeLists.txt

@@ -256,6 +256,11 @@ cmake_dependent_option(CORRADE_CPU_USE_IFUNC "Allow using GNU IFUNC for runtime
 # negative perf consequences, enable it by default only on platforms that have
 # IFUNC, and thus can avoid the function pointer indirection.
 option(CORRADE_BUILD_CPU_RUNTIME_DISPATCH "Build with runtime dispatch for CPU-dependent functionality" ${_CORRADE_CPU_CAN_USE_IFUNC})
+# Enabled by default and independent of CORRADE_BUILD_CPU_RUNTIME_DISPATCH as
+# correctness is important. It can however negatively affect performance
+# compared to IFUNC or compile-time dispatch, so make it possible to disable
+# this for benchmarks.
+cmake_dependent_option(CORRADE_BUILD_TESTS_FORCE_CPU_POINTER_DISPATCH "Force pointer-based dispatch for unit tests to verify all variants of CPU-dependent functionality" ON "CORRADE_BUILD_TESTS" OFF)
 
 # Backwards compatibility for unprefixed CMake options. If the user isn't
 # explicitly using prefixed options in the first run already, accept the

doc/building-corrade.dox

@@ -641,11 +641,21 @@ compatibility if `CORRADE_BUILD_DEPRECATED` isn't disabled.
 
 @subsection building-corrade-tests Building and running tests
 
-If you want to build also the tests (which are not built by default), enable
-`CORRADE_BUILD_TESTS` in CMake. The tests use Corrade's own
-@ref Corrade::TestSuite "TestSuite" framework and can be run either manually
-(the binaries are located in `Test/` subdirectories in the build directory) or
-using
+Building of tests is controlled by the following options:
+
+-   `CORRADE_BUILD_TESTS` --- Builds unit tests. Disabled by default.
+-   `CORRADE_BUILD_TESTS_FORCE_CPU_POINTER_DISPATCH` --- Force unit tests to be
+    built with function-pointer-based dispatch for CPU-dependent functionality,
+    independently of the `CORRADE_BUILD_CPU_RUNTIME_DISPATCH` and
+    `CORRADE_CPU_USE_IFUNC` options. This makes it possible for the tests to
+    verify all variants instead of just one, and is enabled by default. The
+    overhead from function pointers may however be significant on certain
+    platforms, skewing benchmark results. Disable it to make the tests use the
+    same dispatch method as the rest of the library.
+
+The tests use Corrade's own @relativeref{Corrade,TestSuite} framework and can
+be run either manually (the binaries are located in `Test/` subdirectories in
+the build directory) or using
 
 @code{.sh}
 ctest --output-on-failure