build: Allow non-strict aliasing and overflow optimizations (#1752)

Non-strict aliasing is found to resolve
#1737.

We also use signed integer overflow around ares, so we want to allow the
compiler to optimize for that if it wants to; essentially everywhere
ares runs is two's complement.

Couple quirks: Xcode has its own declaration for strict aliasing, so use
that. Clang-CL doesn't transparently accept `-fwrapv`, so it's passed
with `/clang:-fwrapv`.

`_ares_gcc_common_options` will eventually see more development to make
warnings consistent across platforms where possible.

(Fixes #1737)

cmake/common/compiler_common.cmake

@@ -43,6 +43,7 @@ set(CMAKE_VISIBILITY_INLINES_HIDDEN TRUE)
 # clang options for C, C++, ObjC, and ObjC++
 set(
   _ares_clang_common_options
+  -fno-strict-aliasing
   -Wblock-capture-autoreleasing
   # -Wswitch
   -Wdeprecated
@@ -99,6 +100,12 @@ set(
   -Wno-delete-non-abstract-non-virtual-dtor
 )
 
+set(
+  _ares_gcc_common_options
+  -fwrapv
+  -fno-strict-aliasing
+)
+
 if(NOT DEFINED CMAKE_COMPILE_WARNING_AS_ERROR)
   set(CMAKE_COMPILE_WARNING_AS_ERROR OFF)
 endif()

cmake/linux/compilerconfig.cmake

@@ -11,5 +11,10 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
   add_compile_options(
     "$<$<COMPILE_LANGUAGE:C>:${_ares_clang_c_options}>"
     "$<$<COMPILE_LANGUAGE:CXX>:${_ares_clang_cxx_options}>"
+    -fwrapv
+  )
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  add_compile_options(
+    ${_ares_gcc_common_options}
   )
 endif()

cmake/macos/compilerconfig.cmake

@@ -47,6 +47,7 @@ if(NOT XCODE)
   add_compile_options(
     "$<$<COMPILE_LANGUAGE:C>:${_ares_clang_c_options}>"
     "$<$<COMPILE_LANGUAGE:CXX>:${_ares_clang_cxx_options}>"
+    -fwrapv
     -mmacos-version-min=10.13
   )
 endif()

cmake/macos/xcode.cmake

@@ -122,6 +122,7 @@ set(CMAKE_XCODE_ATTRIBUTE_GCC_WARN_UNUSED_VALUE YES)
 set(CMAKE_XCODE_ATTRIBUTE_GCC_WARN_UNUSED_VARIABLE NO) # todo
 set(CMAKE_XCODE_ATTRIBUTE_GCC_NO_COMMON_BLOCKS YES)
 set(CMAKE_XCODE_ATTRIBUTE_GCC_WARN_64_TO_32_BIT_CONVERSION NO) # todo
+set(CMAKE_XCODE_ATTRIBUTE_GCC_STRICT_ALIASING NO)
 
 set(CMAKE_XCODE_ATTRIBUTE_GCC_WARN_ABOUT_MISSING_FIELD_INITIALIZERS NO)
 set(CMAKE_XCODE_ATTRIBUTE_GCC_WARN_SHADOW NO)

cmake/windows/compilerconfig.cmake

@@ -79,18 +79,21 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     "$<$<COMPILE_LANGUAGE:C,CXX>:${_ares_clang_cl_c_cxx_options}>"
   )
   if(NOT MSVC)
-    # statically link libstdc++ if compiling under msys2/mingw
+    # we are on msys2 clang
+    # statically link libc++
     add_link_options(-static-libstdc++)
     # msys2/mingw-specific invocations to make clang emit debug symbols
     set(_ares_mingw_clang_debug_compile_options -g -gcodeview)
     set(_ares_mingw_clang_debug_link_options -fuse-ld=lld -g -Wl,--pdb=)
     add_compile_options("$<$<CONFIG:Debug,RelWithDebInfo>:${_ares_mingw_clang_debug_compile_options}>")
     add_link_options("$<$<CONFIG:Debug,RelWithDebInfo>:${_ares_mingw_clang_debug_link_options}>")
+    # clang-cl does not understand -fwrapv, but we do want it on msys2 clang
+    add_compile_options(-fwrapv)
   else()
     # generate PDBs rather than embed debug symbols
     set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT ProgramDatabase)
     add_compile_options("$<$<COMPILE_LANGUAGE:CXX>:${_ares_msvc_cxx_options}>")
-    # kill
+    # work around https://gitlab.kitware.com/cmake/cmake/-/issues/26559
     add_compile_options($<$<AND:$<BOOL:${ENABLE_IPO}>,$<NOT:$<CONFIG:Debug>>>:-flto=thin>)
     add_link_options(
       $<$<AND:$<BOOL:${ENABLE_IPO}>,$<NOT:$<CONFIG:Debug>>>:-flto=thin>
@@ -99,6 +102,10 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
       $<$<NOT:$<CONFIG:Debug>>:/OPT:REF>
       $<$<NOT:$<CONFIG:Debug>>:/OPT:ICF>
     )
+    # add -fwrapv
+    add_compile_options(
+      "$<$<COMPILE_LANGUAGE:C,CXX>:/clang:-fwrapv>"
+    )
   endif()
 
   # optimizations
@@ -114,6 +121,10 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   if(CMAKE_COMPILE_WARNING_AS_ERROR)
     add_link_options(/WX)
   endif()
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  add_compile_options(
+    ${_ares_gcc_common_options}
+  )
 endif()
 
 if(NOT MINGW)