On Fri, 30 Oct 2020 at 04:22, Alexei Starovoitov
[off-list ref] wrote:

On Thu, Oct 29, 2020 at 05:28:11PM -0700, Nick Desaulniers wrote:

quoted

We already know that -fno-asynchronous-unwind-tables get dropped,
hence this patch.

On arm64 only. Not on x86

quoted

And we know -fomit-frame-pointer or
-fno-omit-frame-pointer I guess gets dropped, hence your ask.

yep. that one is bugged.

quoted

We might not know the full extent which other flags get dropped with the
optimize attribute, but I'd argue that my list above can all result in
pretty bad bugs when accidentally omitted (ok, maybe not -fshort-wchar
or -fmacro-prefix-map, idk what those do) or when mixed with code that

true.
Few month back I've checked that strict-aliasing and no-common flags
from your list are not dropped by this attr in gcc [6789].
I've also checked that no-red-zone and model=kernel preserved as well.

quoted

has different values those flags control.  Searching GCC's bug tracker
for `__attribute__((optimize` turns up plenty of reports to make me
think this attribute maybe doesn't work the way folks suspect or
intend: https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=__attribute__%28%28optimize&list_id=283390.

There is a risk.
Is it a footgun? Sure.
Yet. gcc testsuite is using __attribute__((optimize)).
And some of these tests were added _after_ offical gcc doc said that this
attribute is broken.
imo it's like 'beware of the dog' sign.

quoted

There's plenty of folks arguing against the use of the optimize
attribute in favor of the command line flag.  I urge you to please
reconsider the request.

ok. Applied this first patch to bpf tree and will get it to Linus soon.
Second patch that is splitting interpreter out because of this mess
is dropped. The effect of gcse on performance is questionable.
iirc some interpreters used to do -fno-gcse to gain performance.

That is absolutely fine. I only included the second patch to address
Daniel's concern that -fno-gcse could affect unrelated code living in
the same source file as __bpf_prog_run(), but if you don't care about
that, nor will I.