Richard Biener <richard.guent...@gmail.com> writes: > On Tue, Aug 16, 2022 at 10:14 AM Richard Sandiford > <richard.sandif...@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 <ro...@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