Re: [x86 PATCH] PR target/106577: force_reg may clobber operands during split.

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Tue, Aug 16, 2022 at 10:14 AM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> > On Fri, Aug 12, 2022 at 10:41 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>> >>
>> >>
>> >> This patch fixes PR target/106577 which is a recent ICE on valid regression
>> >> caused by my introduction of a *testti_doubleword pre-reload splitter in
>> >> i386.md.  During the split pass before reload, this converts the virtual
>> >> *testti_doubleword into an *andti3_doubleword and *cmpti_doubleword,
>> >> checking that any immediate operand is a valid "x86_64_hilo_general_operand"
>> >> and placing it into a TImode register using force_reg if it isn't.
>> >>
>> >> The unexpected behaviour (that caught me out) is that calling force_reg
>> >> may occasionally clobber the contents of the global operands array, or
>> >> more accurately recog_data.operand[0], which means that by the time
>> >> split_XXX calls gen_split_YYY the replacement insn's operands have been
>> >> corrupted.
>> >>
>> >> It's difficult to tell who (if anyone is at fault).  The re-entrant
>> >> stack trace (for the attached PR) looks like:
>> >>
>> >> gen_split_203 (*testti_doubleword) calls
>> >> force_reg calls
>> >> emit_move_insn calls
>> >> emit_move_insn_1 calls
>> >> gen_movti calls
>> >> ix86_expand_move calls
>> >> ix86_convert_const_wide_int_to_broadcast calls
>> >> ix86_vector_duplicate_value calls
>> >> recog_memoized calls
>> >> recog.
>> >>
>> >> By far the simplest and possibly correct fix is rather than attempt
>> >> to push and pop recog_data, to simply (in pre-reload splits) save a
>> >> copy of any operands that will be needed after force_reg, and use
>> >> these copies afterwards.  Many pre-reload splitters avoid this issue
>> >> using "[(clobber (const_int 0))]" and so avoid gen_split_YYY functions,
>> >> but in our case we still need to save a copy of operands[0] (even if we
>> >> call emit_insn or expand_* ourselves), so we might as well continue to
>> >> use the conveniently generated gen_split.
>> >>
>> >> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
>> >> and make -k check, both with and without --target_board=unix{-m32},
>> >> with no new failures. Ok for mainline?
>> >
>> > Why this obviously fixes the issue seen I wonder whether there's
>> > more of recog_data that might be used after control flow returns
>> > to recog_memoized and thus the fix would be there, not in any
>> > backend pattern triggering the issue like this?
>> >
>> > The "easiest" fix would maybe to add a in_recog flag and
>> > simply return FAIL from recog when recursing.  Not sure what
>> > the effect on this particular pattern would be though?
>> >
>> > The better(?) fix might be to push/pop recog_data in 'recog', but
>> > of course give that recog_data is currently a global leakage
>> > in intermediate code can still happen.
>> >
>> > That said - does anybody know of similar fixes for this issue in other
>> > backends patterns?
>>
>> I don't think it's valid for a simple query function like
>> ix86_vector_duplicate_value to clobber global state.  Doing that
>> could cause problems in other situations, not just splits.
>>
>> Ideally, it would be good to wean insn-recog.cc:recog off global state.
>> The only parts of recog_data it uses (if I didn't miss something)
>> are recog_data.operands and recog_data.insn (but only to nullify
>> it for recog_memoized, which wouldn't be necessary if recog didn't
>> clobber recog_data.operands).  But I guess some .md expand/insn
>> conditions probably rely on the operands array being in recog_data,
>> so that might not be easy.
>>
>> IMO the correct low-effort fix is to save and restore recog_data
>> in ix86_vector_duplicate_value.  It's a relatively big copy,
>> but the current code is pretty wasteful anyway (allocating at
>> least a new SET and INSN for every query).  Compared to the
>> overhead of doing that, a copy to and from the stack shouldn't
>> be too bad.
>
> I see.  I wonder if we should at least add some public API for
> save/restore of recog_data so the many places don't need to
> invent their own version and they are more easily to find later.

Plain assignment should work.  The structure isn't very fancy ;-)

> Maybe some RAII
>
> {
>   push_recog_data saved ();
>
> }
>
> ?

Maybe.  But if we're going to spend effort on something, moving away
from the global state seems better IMO.

> Shall we armor recog () for recursive invocation by adding a
> ->in_recog member to recog_data?

Yeah, we could do that, but it wouldn't catch the current bug.

Thanks,
Richard