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[209.6.216.142]) by smtp.gmail.com with ESMTPSA id r78sm2580385qka.95.2020.09.30.08.36.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 30 Sep 2020 08:36:41 -0700 (PDT) Subject: Re: [PATCH] tree-optimization/97151 - improve PTA for C++ operator delete From: Jason Merrill To: Richard Biener Cc: gcc-patches@gcc.gnu.org, =?UTF-8?Q?Martin_Li=c5=a1ka?= , Jakub Jelinek , Jonathan Wakely References: <5878a705-6101-d7ac-fe11-13664a53b220@redhat.com> <0A92F1D5-5521-4F1A-B297-6637479FB278@suse.de> <5e4be9c0-3781-c983-ce05-09763bac67e5@redhat.com> <85ce021c-5730-b6e9-c7ad-3d9ae9afcba2@redhat.com> <3a2eb195-396e-4197-1857-8f24aeb58d7c@redhat.com> Message-ID: <30c8232a-99a8-ec78-a7ae-9357f48d1754@redhat.com> Date: Wed, 30 Sep 2020 11:36:40 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <3a2eb195-396e-4197-1857-8f24aeb58d7c@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/mixed; boundary="------------6A8CDEC7C94412C8608D7601" Content-Language: en-US X-Spam-Status: No, score=-14.6 required=5.0 tests=BAYES_00, BODY_8BITS, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, KAM_SHORT, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H5, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 30 Sep 2020 15:36:50 -0000 This is a multi-part message in MIME format. --------------6A8CDEC7C94412C8608D7601 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit On 9/28/20 3:09 PM, Jason Merrill wrote: > On 9/28/20 3:56 AM, Richard Biener wrote: >> On Fri, 25 Sep 2020, Jason Merrill wrote: >> >>> On 9/25/20 2:30 AM, Richard Biener wrote: >>>> On Thu, 24 Sep 2020, Jason Merrill wrote: >>>> >>>>> On 9/24/20 3:43 AM, Richard Biener wrote: >>>>>> On Wed, 23 Sep 2020, Jason Merrill wrote: >>>>>> >>>>>>> On 9/23/20 2:42 PM, Richard Biener wrote: >>>>>>>> On September 23, 2020 7:53:18 PM GMT+02:00, Jason Merrill >>>>>>>> >>>>>>>> wrote: >>>>>>>>> On 9/23/20 4:14 AM, Richard Biener wrote: >>>>>>>>>> C++ operator delete, when DECL_IS_REPLACEABLE_OPERATOR_DELETE_P, >>>>>>>>>> does not cause the deleted object to be escaped.  It also has no >>>>>>>>>> other interesting side-effects for PTA so skip it like we do >>>>>>>>>> for BUILT_IN_FREE. >>>>>>>>> >>>>>>>>> Hmm, this is true of the default implementation, but since the >>>>>>>>> function >>>>>>>>> >>>>>>>>> is replaceable, we don't know what a user definition might do >>>>>>>>> with the >>>>>>>>> pointer. >>>>>>>> >>>>>>>> But can the object still be 'used' after delete? Can delete fail / >>>>>>>> throw? >>>>>>>> >>>>>>>> What guarantee does the predicate give us? >>>>>>> >>>>>>> The deallocation function is called as part of a delete >>>>>>> expression in >>>>>>> order >>>>>>> to >>>>>>> release the storage for an object, ending its lifetime (if it was >>>>>>> not >>>>>>> ended >>>>>>> by >>>>>>> a destructor), so no, the object can't be used afterward. >>>>>> >>>>>> OK, but the delete operator can access the object contents if there >>>>>> wasn't a destructor ... >>>>> >>>>>>> A deallocation function that throws has undefined behavior. >>>>>> >>>>>> OK, so it seems the 'replaceable' operators are the global ones >>>>>> (for user-defined/class-specific placement variants I see arbitrary >>>>>> extra arguments that we'd possibly need to handle). >>>>>> >>>>>> I'm happy to revert but I'd like to have a testcase that FAILs >>>>>> with the patch ;) >>>>>> >>>>>> Now, the following aborts: >>>>>> >>>>>> struct X { >>>>>>      static struct X saved; >>>>>>      int *p; >>>>>>      X() { __builtin_memcpy (this, &saved, sizeof (X)); } >>>>>> }; >>>>>> void operator delete (void *p) >>>>>> { >>>>>>      __builtin_memcpy (&X::saved, p, sizeof (X)); >>>>>> } >>>>>> int main() >>>>>> { >>>>>>      int y = 1; >>>>>>      X *p = new X; >>>>>>      p->p = &y; >>>>>>      delete p; >>>>>>      X *q = new X; >>>>>>      *(q->p) = 2; >>>>>>      if (y != 2) >>>>>>        __builtin_abort (); >>>>>> } >>>>>> >>>>>> and I could fix this by not making *p but what *p points to escape. >>>>>> The testcase is of course maximally awkward, but hey ... ;) >>>>>> >>>>>> Now this would all be moot if operator delete may not access >>>>>> the object (or if the object contents are undefined at that point). >>>>>> >>>>>> Oh, and the testcase segfaults when compiled with GCC 10 because >>>>>> there we elide the new X / delete p pair ... which is invalid then? >>>>>> Hmm, we emit >>>>>> >>>>>>      MEM[(struct X *)_8] ={v} {CLOBBER}; >>>>>>      operator delete (_8, 8); >>>>>> >>>>>> so the object contents are undefined _before_ calling delete >>>>>> even when I do not have a DTOR?  That is, the above, >>>>>> w/o -fno-lifetime-dse, makes the PTA patch OK for the testcase. >>>>> >>>>> Yes, all classes have a destructor, even if it's trivial, so the >>>>> object's >>>>> lifetime definitely ends before the call to operator delete. This >>>>> is less >>>>> clear for scalar objects, but treating them similarly would be >>>>> consistent >>>>> with >>>>> other recent changes, so I think it's fine for us to assume that >>>>> scalar >>>>> objects are also invalidated before the call to operator delete. >>>>> But of >>>>> course this doesn't apply to explicit calls to operator delete >>>>> outside of a >>>>> delete expression. >>>> >>>> OK, so change the testcase main slightly to >>>> >>>> int main() >>>> { >>>>     int y = 1; >>>>     X *p = new X; >>>>     p->p = &y; >>>>     ::operator delete(p); >>>>     X *q = new X; >>>>     *(q->p) = 2; >>>>     if (y != 2) >>>>       __builtin_abort (); >>>> } >>>> >>>> in this case the lifetime of *p does not end before calling >>>> ::operator delete() and delete can stash the object contents >>>> somewhere before ending its lifetime.  For the very same reason >>>> we may not elide a new/delete pair like in >>>> >>>> int main() >>>> { >>>>     int *p = new int; >>>>     *p = 1; >>>>     ::operator delete (p); >>>> } >>> >>> Correct; the permission to elide new/delete pairs are for the >>> expressions, not >>> the functions. >>> >>>> which we before the change did not do only because calling >>>> operator delete made p escape.  Unfortunately points-to analysis >>>> cannot really reconstruct whether delete was called as part of >>>> a delete expression or directly (and thus whether object lifetime >>>> ended already), neither can DCE.  So I guess we need to mark >>>> the operator delete call in some way to make those transforms >>>> safe.  At least currently any operator delete call makes the >>>> alias guarantee of a operator new call moot by forcing the object >>>> to be aliased with all global and escaped memory ... >>>> >>>> Looks like there are some unallocated flags for CALL_EXPR we could >>>> pick but I wonder if we can recycle protected_flag which is >>>> >>>>          CALL_FROM_THUNK_P and >>>>          CALL_ALLOCA_FOR_VAR_P in >>>>              CALL_EXPR >>>> >>>> for calls to DECL_IS_OPERATOR_{NEW,DELETE}_P, thus whether >>>> we have CALL_FROM_THUNK_P for those operators.  Guess picking >>>> a new flag is safer. >>> >>> We won't ever call those operators from a thunk, so it should be OK >>> to reuse >>> it. >>> >>>> But, does it seem correct that we need to distinguish >>>> delete expressions from plain calls to operator delete? >>> >>> A reason for that distinction came up in the context of omitting >>> new/delete >>> pairs: we want to consider the operator first called by the new or >>> delete >>> expression, not a call from that first operator to another operator >>> new/delete >>> and exposed by inlining. >>> >>> https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543404.html >>> >>>> In this context I also wonder about non-replaceable operator delete, >>>> specifically operator delete in classes - are there any semantic >>>> differences between those or why did we choose to only mark >>>> the replaceable ones? >>> >>> The standard says that for omitting a 'new' allocation, the operator >>> new has >>> to be a replaceable one, but does not say the same about 'delete'; it >>> just >>> says that if the allocation was omitted, the delete-expression does >>> not call a >>> deallocation function.  It may not be necessary to make this >>> distinction for >>> delete.  And this distinction could be local to the front end. >>> >>> In the front end, we currently have cxx_replaceable_global_alloc_fn that >>> already ignores the replaceability of operator delete.  And we have >>> CALL_FROM_NEW_OR_DELETE_P, that would just need to move into the >>> middle end. >>> And perhaps get renamed to CALL_OMITTABLE_NEW_OR_DELETE_P, and not >>> get set for >>> calls to non-replaceable operator new. >> >> CALL_FROM_NEW_OR_DELETE_P indeed looks like the best fit - it's >> only evaluated when cxx_replaceable_global_alloc_fn matches in the C++ >> FE so could be made to cover only replaceable variants. >> >> CALL_REPLACEABLE_NEW_OR_DELETE_P () maybe, since we already use >> REPLACEABLE for the fndecl flags?  OMITTABLE is too specific >> for the PTA case where it really matters whether the object >> lifetime ends before the delete call, not whether it can be >> omitted (hmm, guess that's not 100% overlap then either...). > > That seems like good overlap to me, if we agree that object lifetime > ends before any delete call from a delete-expression, whether or not the > operator delete is replaceable. > >> Mind doing the C++ side of things recycling protected_flag as suggested? > > OK. --------------6A8CDEC7C94412C8608D7601 Content-Type: text/x-patch; charset=UTF-8; name="call-from-new.diff" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="call-from-new.diff" commit d4a8ce72c51e32c7577ca93a488028995fd45ebe Author: Jason Merrill Date: Mon Sep 28 15:33:54 2020 -0400 c++: Move CALL_FROM_NEW_OR_DELETE_P to tree.h. As discussed with richi, we should be able to use TREE_PROTECTED for this flag, since CALL_FROM_THUNK_P will never be set on a call to an operator new or delete. gcc/cp/ChangeLog: * lambda.c (call_from_lambda_thunk_p): New. * cp-gimplify.c (cp_genericize_r): Use it. * pt.c (tsubst_copy_and_build): Use it. * typeck.c (check_return_expr): Use it. * cp-tree.h: Declare it. (CALL_FROM_NEW_OR_DELETE_P): Move to gcc/tree.h. gcc/ChangeLog: * tree.h (CALL_FROM_NEW_OR_DELETE_P): Move from cp-tree.h. diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h index 42d0d76bf21..272ea88960b 100644 --- a/gcc/cp/cp-tree.h +++ b/gcc/cp/cp-tree.h @@ -464,7 +464,6 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX]; SWITCH_STMT_NO_BREAK_P (in SWITCH_STMT) LAMBDA_EXPR_CAPTURE_OPTIMIZED (in LAMBDA_EXPR) IMPLICIT_CONV_EXPR_BRACED_INIT (in IMPLICIT_CONV_EXPR) - CALL_FROM_NEW_OR_DELETE_P (in CALL_EXPR) 3: IMPLICIT_RVALUE_P (in NON_LVALUE_EXPR or STATIC_CAST_EXPR) ICS_BAD_FLAG (in _CONV) FN_TRY_BLOCK_P (in TRY_BLOCK) @@ -3840,11 +3839,6 @@ struct GTY(()) lang_decl { should be performed at instantiation time. */ #define KOENIG_LOOKUP_P(NODE) TREE_LANG_FLAG_0 (CALL_EXPR_CHECK (NODE)) -/* In a CALL_EXPR, true for allocator calls from new or delete - expressions. */ -#define CALL_FROM_NEW_OR_DELETE_P(NODE) \ - TREE_LANG_FLAG_2 (CALL_EXPR_CHECK (NODE)) - /* True if the arguments to NODE should be evaluated in left-to-right order regardless of PUSH_ARGS_REVERSED. */ #define CALL_EXPR_ORDERED_ARGS(NODE) \ @@ -7286,6 +7280,7 @@ extern bool lambda_fn_in_template_p (tree); extern void maybe_add_lambda_conv_op (tree); extern bool is_lambda_ignored_entity (tree); extern bool lambda_static_thunk_p (tree); +extern bool call_from_lambda_thunk_p (tree); extern tree finish_builtin_launder (location_t, tree, tsubst_flags_t); extern tree cp_build_vec_convert (tree, location_t, tree, diff --git a/gcc/tree-core.h b/gcc/tree-core.h index 0e158784d0e..752bec31c3f 100644 --- a/gcc/tree-core.h +++ b/gcc/tree-core.h @@ -1220,7 +1220,8 @@ struct GTY(()) tree_base { all decls CALL_FROM_THUNK_P and - CALL_ALLOCA_FOR_VAR_P in + CALL_ALLOCA_FOR_VAR_P and + CALL_FROM_NEW_OR_DELETE_P in CALL_EXPR OMP_CLAUSE_LINEAR_VARIABLE_STRIDE in diff --git a/gcc/tree.h b/gcc/tree.h index 5bb6e7bc000..f27a7399a37 100644 --- a/gcc/tree.h +++ b/gcc/tree.h @@ -921,7 +921,8 @@ extern void omp_clause_range_check_failed (const_tree, const char *, int, (TREE_CHECK (NODE, PARM_DECL)->decl_common.decl_nonshareable_flag) /* In a CALL_EXPR, means that the call is the jump from a thunk to the - thunked-to function. */ + thunked-to function. Be careful to avoid using this macro when one of the + next two applies instead. */ #define CALL_FROM_THUNK_P(NODE) (CALL_EXPR_CHECK (NODE)->base.protected_flag) /* In a CALL_EXPR, if the function being called is BUILT_IN_ALLOCA, means that @@ -931,6 +932,12 @@ extern void omp_clause_range_check_failed (const_tree, const char *, int, #define CALL_ALLOCA_FOR_VAR_P(NODE) \ (CALL_EXPR_CHECK (NODE)->base.protected_flag) +/* In a CALL_EXPR, if the function being called is DECL_IS_OPERATOR_NEW_P or + DECL_IS_OPERATOR_DELETE_P, true for allocator calls from C++ new or delete + expressions. */ +#define CALL_FROM_NEW_OR_DELETE_P(NODE) \ + (CALL_EXPR_CHECK (NODE)->base.protected_flag) + /* Used in classes in C++. */ #define TREE_PRIVATE(NODE) ((NODE)->base.private_flag) /* Used in classes in C++. */ diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c index bc8a03c7b41..07549828dc9 100644 --- a/gcc/cp/cp-gimplify.c +++ b/gcc/cp/cp-gimplify.c @@ -962,7 +962,7 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data) omp_cxx_notice_variable (wtd->omp_ctx, stmt); /* Don't dereference parms in a thunk, pass the references through. */ - if ((TREE_CODE (stmt) == CALL_EXPR && CALL_FROM_THUNK_P (stmt)) + if ((TREE_CODE (stmt) == CALL_EXPR && call_from_lambda_thunk_p (stmt)) || (TREE_CODE (stmt) == AGGR_INIT_EXPR && AGGR_INIT_FROM_THUNK_P (stmt))) { *walk_subtrees = 0; diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c index 07a5401c97b..1a1647f465e 100644 --- a/gcc/cp/lambda.c +++ b/gcc/cp/lambda.c @@ -1325,6 +1325,13 @@ lambda_static_thunk_p (tree fn) && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fn))); } +bool +call_from_lambda_thunk_p (tree call) +{ + return (CALL_FROM_THUNK_P (call) + && lambda_static_thunk_p (current_function_decl)); +} + /* Returns true iff VAL is a lambda-related declaration which should be ignored by unqualified lookup. */ diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c index a09633751ca..b0c61cf31c6 100644 --- a/gcc/cp/pt.c +++ b/gcc/cp/pt.c @@ -19951,7 +19951,7 @@ tsubst_copy_and_build (tree t, /* Stripped-down processing for a call in a thunk. Specifically, in the thunk template for a generic lambda. */ - if (CALL_FROM_THUNK_P (t)) + if (call_from_lambda_thunk_p (t)) { /* Now that we've expanded any packs, the number of call args might be different. */ diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c index 9166156a5d5..95b36a92491 100644 --- a/gcc/cp/typeck.c +++ b/gcc/cp/typeck.c @@ -10171,7 +10171,7 @@ check_return_expr (tree retval, bool *no_warning) /* The call in a (lambda) thunk needs no conversions. */ if (TREE_CODE (retval) == CALL_EXPR - && CALL_FROM_THUNK_P (retval)) + && call_from_lambda_thunk_p (retval)) converted = true; /* First convert the value to the function's return type, then --------------6A8CDEC7C94412C8608D7601--