Do not turn off strict-aliasing optimizations

Issue #322 (bdwgc).

* CMakeLists.txt (HAVE_FLAG_F_NO_STRICT_ALIASING): Remove.
* CMakeLists.txt [HAVE_FLAG_F_NO_STRICT_ALIASING]
(add_compile_options): Do not add -fno-strict-aliasing.
* build.zig (build): Do not add "-fno-strict-aliasing" to flags; update
comment.
* configure.ac [$GCC=yes] (ac_cv_fno_strict_aliasing): Do not define.
* configure.ac [$GCC=yes]: Do not check if -fno-strict-aliasing is
supported.
* configure.ac [$GCC=yes && $ac_cv_fno_strict_aliasing=yes] (CFLAGS):
Do not append -fno-strict-aliasing.
* docs/platforms/README.win64: Do not mention -fno-strict-aliasing.
* extra/gc.c: Remove -fno-strict-aliasing from comment.

CMakeLists.txt

@@ -512,13 +512,6 @@ else()
   endif(WIN32)
 endif()
 
-# Disable strict aliasing optimizations.
-# It could re-enabled back by a flag passed in CFLAGS_EXTRA.
-check_c_compiler_flag(-fno-strict-aliasing HAVE_FLAG_F_NO_STRICT_ALIASING)
-if (HAVE_FLAG_F_NO_STRICT_ALIASING)
-  add_compile_options(-fno-strict-aliasing)
-endif()
-
 # Extra user-defined flags to pass both to C and C++ compilers.
 if (DEFINED CFLAGS_EXTRA)
   separate_arguments(CFLAGS_EXTRA_LIST UNIX_COMMAND "${CFLAGS_EXTRA}")

build.zig

@@ -374,12 +374,11 @@ pub fn build(b: *std.Build) void {
     // TODO: declare that the libraries do not refer to external symbols
     // of build_shared_libs.
 
-    // zig cc supports these flags.
+    // zig cc supports this flag.
     flags.appendSlice(&.{
         // TODO: -Wno-unused-command-line-argument
         // Prevent "__builtin_return_address with nonzero argument is unsafe".
         "-Wno-frame-address",
-        "-fno-strict-aliasing",
     }) catch unreachable;
 
     if (build_shared_libs) {

configure.ac

@@ -432,31 +432,6 @@ fi
 AM_CONDITIONAL([ASM_WITH_CPP_UNSUPPORTED],
     [test $compiler_xlc = yes -o $compiler_suncc = yes])
 
-if test "$GCC" = yes; then
-  # Disable aliasing optimization unless forced to.
-  AC_MSG_CHECKING([whether compiler supports -fno-strict-aliasing])
-  ac_cv_fno_strict_aliasing=no
-  for cflag in $CFLAGS; do
-    case "$cflag" in
-      -fstrict-aliasing)
-        # Opposite option already present
-        ac_cv_fno_strict_aliasing=skipped
-        break
-        ;;
-    esac
-  done
-  if test "$ac_cv_fno_strict_aliasing" != skipped; then
-    old_CFLAGS="$CFLAGS"
-    CFLAGS="$CFLAGS -fno-strict-aliasing"
-    AC_COMPILE_IFELSE([AC_LANG_SOURCE([])],
-                      [ac_cv_fno_strict_aliasing=yes])
-    CFLAGS="$old_CFLAGS"
-    AS_IF([test "$ac_cv_fno_strict_aliasing" = yes],
-          [CFLAGS="$CFLAGS -fno-strict-aliasing"])
-  fi
-  AC_MSG_RESULT($ac_cv_fno_strict_aliasing)
-fi
-
 # Check for getcontext (uClibc can be configured without it, for example)
 AC_CHECK_FUNC([getcontext], [],
     [AC_DEFINE([NO_GETCONTEXT], [1], [Missing getcontext function.])])

docs/platforms/README.win64

@@ -23,5 +23,3 @@ support - this could be done by passing "disable_threads=1" argument to nmake.
 Note that some warnings have been explicitly turned off in the makefile.
 
 VC++ note: to suppress warnings -D_CRT_SECURE_NO_DEPRECATE is used.
-
-gcc note: -fno-strict-aliasing should be used if optimizing.

extra/gc.c

@@ -22,7 +22,7 @@
 
 /* Tip: to get the highest level of compiler optimizations, the typical */
 /* compiler options (GCC) to use are:                                   */
-/* -O3 -fno-strict-aliasing -march=native -Wall -fprofile-generate/use  */
+/* -O3 -march=native -Wall -fprofile-generate/use                       */
 
 /* Warning: GCC for Linux (for C++ clients only): Use -fexceptions both */
 /* for GC and the client otherwise GC_thread_exit_proc() is not         */