I buy that, Acked-by: Jarno Rajahalme <jrajaha...@nicira.com>
> On Sep 30, 2014, at 5:03 PM, Ben Pfaff <b...@nicira.com> wrote: > > The C standard allows compilers to do type-based alias analysis, which > means that the compiler is allowed to assume that pointers to objects of > different types are pointers to different objects. For example, a compiler > may assume that "uint16_t *a" and "uint32_t *b" point to different and > nonoverlapping locations because the pointed-to types are different. This > can lead to surprising "optimizations" with compilers that by default do > this kind of analysis, which includes GCC and Clang. > > The one escape clause that the C standard gives us is that character types > must be assumed to alias any other object. We've always tried to use this > escape clause to avoid problems with type-based alias analysis in the past. > I think that we should continue to try to do this in the future. It's hard > to tell what compiler we might want to use in the future, and one never > knows what kind of control that compiler allows over alias analysis. > > However, recently I helped another developer debug a nasty and confusing > issue, which turned out to be the result of a surprising compiler > optimization due to alias analysis. I've seen enough of these that I don't > think it's worthwhile to risk more problems than we have to. Thus, this > commit turns off type-based alias analysis in GCC and Clang. > > Linus Torvalds thinks that type-base alias analysis is not sane, at least > as GCC implements it: https://lkml.org/lkml/2003/2/26/158 > > The GCC manual says that -Wstrict-aliasing is only effective without > -fno-strict-aliasing, otherwise I'd keep -Wstrict-aliasing also. > > Indications are that MSVC doesn't do type-based alias analysis by default. > > Signed-off-by: Ben Pfaff <b...@nicira.com> > --- > configure.ac | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/configure.ac b/configure.ac > index 35e884a..62224b9 100644 > --- a/configure.ac > +++ b/configure.ac > @@ -104,7 +104,6 @@ OVS_ENABLE_OPTION([-Wformat-security]) > OVS_ENABLE_OPTION([-Wno-format-zero-length]) > OVS_ENABLE_OPTION([-Wswitch-enum]) > OVS_ENABLE_OPTION([-Wunused-parameter]) > -OVS_ENABLE_OPTION([-Wstrict-aliasing]) > OVS_ENABLE_OPTION([-Wbad-function-cast]) > OVS_ENABLE_OPTION([-Wcast-align]) > OVS_ENABLE_OPTION([-Wstrict-prototypes]) > @@ -112,6 +111,7 @@ OVS_ENABLE_OPTION([-Wold-style-definition]) > OVS_ENABLE_OPTION([-Wmissing-prototypes]) > OVS_ENABLE_OPTION([-Wmissing-field-initializers]) > OVS_ENABLE_OPTION([-Wthread-safety]) > +OVS_ENABLE_OPTION([-fno-strict-aliasing]) > OVS_CONDITIONAL_CC_OPTION([-Wno-unused], [HAVE_WNO_UNUSED]) > OVS_CONDITIONAL_CC_OPTION([-Wno-unused-parameter], > [HAVE_WNO_UNUSED_PARAMETER]) > OVS_ENABLE_WERROR > -- > 1.7.10.4 > > _______________________________________________ > dev mailing list > dev@openvswitch.org > http://openvswitch.org/mailman/listinfo/dev _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
