Initial implementation of float16 support (RFC100) (OSGeo#11180)

Author: eschnett

.github/workflows/icc/build.sh

@@ -5,11 +5,16 @@ set -eu
 # for precompiled headers
 ccache --set-config sloppiness=pch_defines,time_macros,include_file_mtime,include_file_ctime
 
+# Set C and C++ compiler flags to disable `_Float16`. This is
+# necessary because the system C and C++ compilers don't support it,
+# and Python's `build_ext` will use the system compiler to build GDAL
+# Python extensions.
 cmake ${GDAL_SOURCE_DIR:=..} \
     -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_C_COMPILER=icx \
     -DCMAKE_CXX_COMPILER=icx \
-    "-DUSE_PRECOMPILED_HEADERS=ON" \
+    -DCMAKE_C_FLAGS=-DGDAL_DISABLE_FLOAT16 \
+    -DCMAKE_CXX_FLAGS=-DGDAL_DISABLE_FLOAT16 \
+    -DUSE_PRECOMPILED_HEADERS=ON \
     -DUSE_CCACHE=ON
 make -j$(nproc)
-

CMakeLists.txt

@@ -111,6 +111,23 @@ else()
   cmake_pop_check_state()
 
 endif ()
+
+# Check whether std::float16_t is available and is working
+include(CheckCXXSourceCompiles)
+check_cxx_source_compiles(
+  "
+    #include <cmath>
+    #include <stdfloat>
+    int main() {
+      std::float16_t x = 0;
+      using std::nextafter;
+      std::float16_t y = nextafter(x, x);
+      return y == 0 ? 0 : 1;
+    }
+  "
+  HAVE_STD_FLOAT16_T
+)
+
 #
 option(CLANG_TIDY_ENABLED "Run clang-tidy with the compiler." OFF)
 set(CLANG_TIDY_CHECKS