Additions from LG:

* Make cmake option OIIO_USE_HWY able to be overridden by end (by using
  `set_option()`)

* Let env variable OPENIMAGEIO_ENABLE_HWY override runtime hwy
  enabling. (It was documented to do so, but not yet implemented.)

* Clarify that hwy support will be added for 3.2, not 3.1. And that
  for right now, it defaults to OFF until we have worked out all the
  bugs and gain more confidence in it. The intention is for it to be
  enabled by default in time for the release of OIIO 3.2.

* When hwy is disabled at build time, don't allow attribute() to appear
  to enable it.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

CMakeLists.txt

@@ -112,7 +112,7 @@ else ()
 endif ()
 option (${PROJ_NAME}_BUILD_TOOLS "Build the command-line tools" ON)
 option (${PROJ_NAME}_BUILD_TESTS "Build the unit tests" ON)
-option (OIIO_USE_HWY "Enable experimental Google Highway SIMD optimizations (if Highway is available)" OFF)
+set_option (OIIO_USE_HWY "Enable experimental Google Highway SIMD optimizations (if Highway is available)" OFF)
 set (OIIO_LIBNAME_SUFFIX "" CACHE STRING
      "Optional name appended to ${PROJECT_NAME} libraries that are built")
 option (BUILD_OIIOUTIL_ONLY "If ON, will build *only* libOpenImageIO_Util" OFF)

src/doc/imageioapi.rst

@@ -378,7 +378,10 @@ inside the source code.
     image formats (uint8, uint16, int8, int16, uint32, int32, etc.) and provide
     additional speedup for scale-invariant operations (add, sub, min, max,
     absdiff) that can operate directly on integer data without float conversion.
-    (This was added in OpenImageIO 3.1.)
+
+    This was added in OpenImageIO 3.2.
+
+    NOTE: This is currently disabled by default while we are testing!
 
 .. cpp:var:: OPENIMAGEIO_PYTHON_LOAD_DLLS_FROM_PATH
 

src/libOpenImageIO/imageio.cpp

@@ -53,7 +53,14 @@ int png_linear_premult(0);
 int tiff_half(0);
 int tiff_multithread(1);
 int dds_bc5normal(0);
-int enable_hwy(1);  // Enable Google Highway SIMD optimizations by default
+#if OIIO_USE_HWY
+// hwy enabled at build time, but allow env var to override at runtime While
+// we're still testing, default to disabled. We intend to change the default
+// to be enabled in time for the release of 3.2.
+int enable_hwy = Strutil::stoi(Sysutil::getenv("OPENIMAGEIO_ENABLE_HWY", "0"));
+#else
+int enable_hwy = 0;  // Not enabled at build time
+#endif
 int limit_channels(1024);
 int limit_imagesize_MB(std::min(32 * 1024,
                                 int(Sysutil::physical_memory() >> 20)));
@@ -407,7 +414,9 @@ attribute(string_view name, TypeDesc type, const void* val)
         return true;
     }
     if (name == "enable_hwy" && type == TypeInt) {
+#if OIIO_USE_HWY
         enable_hwy = *(const int*)val;
+#endif
         return true;
     }
     if (name == "limits:channels" && type == TypeInt) {