build: turn on -fno-strict-aliasing

Turn on `-fno-strict-aliasing` in libuv and add a note in the README for
downstream projects.

Fixes: https://github.com/libuv/libuv/issues/1230
PR-URL: https://github.com/libuv/libuv/pull/2588
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>

CMakeLists.txt

@@ -102,6 +102,9 @@ list(APPEND uv_cflags ${lint-no-conditional-assignment-msvc})
 list(APPEND uv_cflags ${lint-no-unsafe-msvc})
 list(APPEND uv_cflags ${lint-utf8-msvc} )
 
+check_c_compiler_flag(-fno-strict-aliasing UV_F_STRICT_ALIASING)
+list(APPEND uv_cflags $<$<BOOL:${UV_F_STRICT_ALIASING}>:-fno-strict-aliasing>)
+
 set(uv_sources
     src/fs-poll.c
     src/idna.c
@@ -325,7 +328,6 @@ if(CMAKE_SYSTEM_NAME STREQUAL "QNX")
     src/unix/bsd-ifaddrs.c
     src/unix/no-proctitle.c
     src/unix/no-fsevents.c)
-  list(APPEND uv_cflags -fno-strict-aliasing)
   list(APPEND uv_libraries socket)
 endif()
 

README.md

@@ -286,6 +286,16 @@ listed in `test/benchmark-list.h`.
 
 Check the [SUPPORTED_PLATFORMS file](SUPPORTED_PLATFORMS.md).
 
+### `-fno-strict-aliasing`
+
+It is recommended to turn on the `-fno-strict-aliasing` compiler flag in
+projects that use libuv. The use of ad hoc "inheritance" in the libuv API
+may not be safe in the presence of compiler optimizations that depend on
+strict aliasing.
+
+MSVC does not have an equivalent flag but it also does not appear to need it
+at the time of writing (December 2019.)
+
 ### AIX Notes
 
 AIX compilation using IBM XL C/C++ requires version 12.1 or greater.

configure.ac

@@ -25,6 +25,7 @@ AC_ENABLE_STATIC
 AC_PROG_CC
 AM_PROG_CC_C_O
 CC_FLAG_VISIBILITY #[-fvisibility=hidden]
+CC_CHECK_CFLAGS_APPEND([-fno-strict-aliasing])
 CC_CHECK_CFLAGS_APPEND([-g])
 CC_CHECK_CFLAGS_APPEND([-std=gnu89])
 CC_CHECK_CFLAGS_APPEND([-Wall])