[cfe-dev] (not) initializing assembly outputs with -ftrivial-auto-var-init

Tue Mar 26 08:34:28 PDT 2019

On Tue, Mar 26, 2019 at 3:38 PM Dmitry Vyukov <dvyukov at google.com> wrote:
>
> On Tue, Mar 26, 2019 at 3:30 PM JF Bastien <jfbastien at apple.com> wrote:
> > > On Mar 26, 2019, at 7:15 AM, Dmitry Vyukov <dvyukov at google.com> wrote:
> > >> On Tue, Mar 26, 2019 at 2:56 PM JF Bastien <jfbastien at apple.com> wrote:
> > >>>> On Mar 25, 2019, at 11:52 PM, Dmitry Vyukov <dvyukov at google.com> wrote:
> > >>>>> On Fri, Mar 22, 2019 at 3:22 AM Dmitry Vyukov <dvyukov at google.com> wrote:
> > >>>>> On Thu, Mar 21, 2019 at 7:15 PM James Y Knight <jyknight at google.com> wrote:
> > >>>>>
> > >>>>> If an asm's constraints claim that the variable is an output, but then don't actually write to it, that's a bug (at least if the value is actually used afterwards). An output-only constraint on inline asm definitely does _not_ mean "pass through the previous value unchanged, if the asm failed to actually write to it". If you need that behavior, it's spelled "+m", not "=m".
> > >>>>>
> > >>>>> We do seem to fail to take advantage of this for memory outputs (again, this is not just for ftrivial-auto-var-init -- we ought to eliminate manual initialization just the same), which I'd definitely consider an missing-optimization bug.
> > >>>>
> > >>>> You mean we assume C code is buggy and asm code is not buggy because
> > >>>> compiler fails to disprove that there is a bug?
> > >>>> Doing this optimization without -ftrivial-auto-var-init looks
> > >>>> reasonable, compilers do optimizations assuming absence of bugs
> > >>>> throughout. But -ftrivial-auto-var-init is specifically about assuming
> > >>>> these bugs are everywhere.
> > >>>>
> > >>>>>>> On Thu, Mar 21, 2019 at 10:16 AM Alexander Potapenko <glider at google.com> wrote:
> > >>>>>>>
> > >>>>>>> On Thu, Mar 21, 2019 at 2:58 PM James Y Knight <jyknight at google.com> wrote:
> > >>>>>>>
> > >>>>>>> Please be more specific about the problem, because your simplified example doesn't actually show an issue. If I write this function:
> > >>>>>>> int foo() {
> > >>>>>>> int retval;
> > >>>>>>> asm("# ..." : "=r"(retval));
> > >>>>>>> return retval;
> > >>>>>>> }
> > >>>>>>> it already does get treated as definitely writing retval, and optimizes away the initialization (whether you explicitly initialize retval, or use -ftrivial-auto-var-init).
> > >>>>>>> Example: https://godbolt.org/z/YYBCXL
> > >>>>>> This is probably because you're passing retval as a register output.
> > >>>>>> If you change "=r" to "=m" (https://godbolt.org/z/ulxSgx), it won't be
> > >>>>>> optimized away.
> > >>>>>> (I admit I didn't know about the difference)
> > >>>>>>>> On Thu, Mar 21, 2019 at 8:35 AM Alexander Potapenko via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> > >>>>>>>>
> > >>>>>>>> Hi JF et al.,
> > >>>>>>>>
> > >>>>>>>> In the Linux kernel we often encounter the following pattern:
> > >>>>>>>>
> > >>>>>>>> type op(...) {
> > >>>>>>>> type retval;
> > >>>>>>>> inline asm(... retval ...);
> > >>>>>>>> return retval;
> > >>>>>>>> }
> > >>>>>>>>
> > >>>>>>>> , which is used to implement low-level platform-dependent memory operations.
> > >>>>>>>>
> > >>>>>>>> Some of these operations turn out to be very hot, so we probably don't
> > >>>>>>>> want to initialize |retval| given that it's always initialized in the
> > >>>>>>>> assembly.
> > >>>>>>>>
> > >>>>>>>> However it's practically impossible to tell that a variable is being
> > >>>>>>>> written to by the inline assembly, or figure out the size of that
> > >>>>>>>> write.
> > >>>>>>>> Perhaps we could speculatively treat every scalar output of an inline
> > >>>>>>>> assembly routine as an initialized value (which is true for the Linux
> > >>>>>>>> kernel, but I'm not sure about other users of inline assembly, e.g.
> > >>>>>>>> video codecs).
> > >>>>>>>>
> > >>>>>>>> WDYT?
> > >>>>>>>>
> > >>>>>>>>
> > >>>>>>>> --
> > >>>>>>>> Alexander Potapenko
> > >>>>>>>> Software Engineer
> > >>>>>>>>
> > >>>>>>>> Google Germany GmbH
> > >>>>>>>> Erika-Mann-Straße, 33
> > >>>>>>>> 80636 München
> > >>>>>>>>
> > >>>>>>>> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> > >>>>>>>> Registergericht und -nummer: Hamburg, HRB 86891
> > >>>>>>>> Sitz der Gesellschaft: Hamburg
> > >>>>>>>> _______________________________________________
> > >>>>>>>> cfe-dev mailing list
> > >>>>>>>> cfe-dev at lists.llvm.org
> > >>>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> --
> > >>>>>> Alexander Potapenko
> > >>>>>> Software Engineer
> > >>>>>>
> > >>>>>> Google Germany GmbH
> > >>>>>> Erika-Mann-Straße, 33
> > >>>>>> 80636 München
> > >>>>>>
> > >>>>>> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> > >>>>>> Registergericht und -nummer: Hamburg, HRB 86891
> > >>>>>> Sitz der Gesellschaft: Hamburg
> > >>>
> > >>> Does kernel asm use "+m" or "=m"?
> > >>>
> > >>> If asm _must_ write to that variable, then we could improve DSE in
> > >>> normal case (ftrivial-auto-var-init is not enabled). If
> > >>> ftrivial-auto-var-init is enabled, then strictly saying we should not
> > >>> remove initialization because we did not prove that asm actually
> > >>> writes. But we may remove initialization as well for practical
> > >>> reasons.
> > >>>
> > >>> Alex mentioned that in some cases we don't know actual address/size of
> > >>> asm writes. But we should know it if a local var is passed to the asm,
> > >>> which should be the case for kernel atomic asm blocks.
> > >>>
> > >>> Interestingly, ftrivial-auto-var-init DSE must not be stronger then
> > >>> non-ftrivial-auto-var-init DSE, unless we are talking about our own
> > >>> emitted initialization stores, in such case ftrivial-auto-var-init DSE
> > >>> may remove then more aggressively then what normal DSE would do, we
> > >>> don't actually have to _prove_ that the init store is dead.
> > >>
> > >>
> > >> IMO the auto var init mitigation shouldn’t change the DSE optimization at all. We shouldn’t treat the stores we add any different. We should just improve DSE and everything benefits (auto var init moreso).
> > >
> > > But you realize that this "just" improve involves fully understanding
> > > static and dynamic behavior of arbitrary assembly for any architecture
> > > without even using integrated asm? ;)
> >
> > If you want to solve every problem however unlikely, yes. If you narrow what you’re doing to a handful of cases that matter, no.
>
> How can we improve DSE to handle all main kernel patterns that matter?
> Can we? It's still unclear to me. Extending this optimization to
> generic DSE and all stores can make it much harder (unsolvable)
> problem...

Right now there's a handful of places in the kernel where we have to
use __attribute__((uninitialized)) just to avoid creating an extra
initializer: https://github.com/google/kmsan/commit/00387943691e6466659daac0312c8c5d8f9420b9
and https://github.com/google/kmsan/commit/2954f1c33a81c6f15c7331876f5b6e2fec0d631f
All those assembly directives are using local scalar variables of size
<= 8 bytes as "=qm" outputs, so we can narrow the problem down to "let
DSE remove redundant stores to local scalars that are used as asm()
"m" outputs"
False positives will sure be possible in theory, but hopefully rare in practice.

I would still love to know what's the main source of truth for the
semantics of asm() constraints.
For example, we've noticed that the BSF instruction, which can be used
as follows:

unsigned long ffs(unsigned long word) {
  unsigned long ret;
  asm("rep; bsf %1,%0" : "=r" (ret) : "rm" (word));
  return ret;
}

isn't guaranteed to initialize its output in the case |word| is 0
(according to unnamed Intel architect, it just zeroes out the top 32
bits of the return value).
Therefore the elimination of dead stores to |ret| done by both Clang
and GCC is correct only if the callers are careful enough.

-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg