From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2153) id 4BF543857018; Mon, 14 Mar 2022 09:49:33 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4BF543857018 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jakub Jelinek To: gcc-cvs@gcc.gnu.org Subject: [gcc r12-7641] c++: Reject __builtin_clear_padding on non-trivially-copyable types with one exception [PR102586] X-Act-Checkin: gcc X-Git-Author: Jakub Jelinek X-Git-Refname: refs/heads/master X-Git-Oldrev: a010954cc190e942994ed5d56fba503851b20b9a X-Git-Newrev: c879b92c30d088ff01eb0c3e912eb8a01f9fc6e3 Message-Id: <20220314094933.4BF543857018@sourceware.org> Date: Mon, 14 Mar 2022 09:49:33 +0000 (GMT) X-BeenThere: gcc-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 14 Mar 2022 09:49:33 -0000 https://gcc.gnu.org/g:c879b92c30d088ff01eb0c3e912eb8a01f9fc6e3 commit r12-7641-gc879b92c30d088ff01eb0c3e912eb8a01f9fc6e3 Author: Jakub Jelinek Date: Mon Mar 14 10:47:38 2022 +0100 c++: Reject __builtin_clear_padding on non-trivially-copyable types with one exception [PR102586] As mentioned by Jason in the PR, non-trivially-copyable types (or non-POD for purposes of layout?) types can be base classes of derived classes in which the padding in those non-trivially-copyable types can be reused for some real data members or even the layout can change and data members can be moved to other positions. __builtin_clear_padding is right now used for multiple purposes, in where it isn't used yet but was planned as the main spot it can be used for trivially copyable types only, ditto for std::bit_cast where we also use it. It is used for OpenMP long double atomics too but long double is trivially copyable, and lastly for -ftrivial-auto-var-init=. The following patch restricts the builtin to pointers to trivially-copyable types, with the exception when it is called directly on an address of a variable, in that case already the FE can verify it is the complete object type and so it is safe to clear all the paddings in it. 2022-03-14 Jakub Jelinek PR tree-optimization/102586 gcc/ * doc/extend.texi (__builtin_clear_padding): Clearify that for C++ argument type should be pointer to trivially-copyable type unless it is address of a variable or parameter. gcc/cp/ * call.cc (build_cxx_call): Diagnose __builtin_clear_padding where first argument's type is pointer to non-trivially-copyable type unless it is address of a variable or parameter. gcc/testsuite/ * g++.dg/cpp2a/builtin-clear-padding1.C: New test. Diff: --- gcc/cp/call.cc | 21 +++++++++ gcc/doc/extend.texi | 5 +++ .../g++.dg/cpp2a/builtin-clear-padding1.C | 50 ++++++++++++++++++++++ 3 files changed, 76 insertions(+) diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index 8fe8ef306ea..23d3fc496b8 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -10409,6 +10409,27 @@ build_cxx_call (tree fn, int nargs, tree *argarray, if (!check_builtin_function_arguments (EXPR_LOCATION (fn), vNULL, fndecl, orig_fndecl, nargs, argarray)) return error_mark_node; + else if (fndecl_built_in_p (fndecl, BUILT_IN_CLEAR_PADDING)) + { + tree arg0 = argarray[0]; + STRIP_NOPS (arg0); + if (TREE_CODE (arg0) == ADDR_EXPR + && DECL_P (TREE_OPERAND (arg0, 0)) + && same_type_ignoring_top_level_qualifiers_p + (TREE_TYPE (TREE_TYPE (argarray[0])), + TREE_TYPE (TREE_TYPE (arg0)))) + /* For __builtin_clear_padding (&var) we know the type + is for a complete object, so there is no risk in clearing + padding that is reused in some derived class member. */; + else if (!trivially_copyable_p (TREE_TYPE (TREE_TYPE (argarray[0])))) + { + error_at (EXPR_LOC_OR_LOC (argarray[0], input_location), + "argument %u in call to function %qE " + "has pointer to a non-trivially-copyable type (%qT)", + 1, fndecl, TREE_TYPE (argarray[0])); + return error_mark_node; + } + } } if (VOID_TYPE_P (TREE_TYPE (fn))) diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 0dc752e8aad..a4a25e86928 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -13993,6 +13993,11 @@ bits that are padding bits for all the union members. This built-in-function is useful if the padding bits of an object might have intederminate values and the object representation needs to be bitwise compared to some other object, for example for atomic operations. + +For C++, @var{ptr} argument type should be pointer to trivially-copyable +type, unless the argument is address of a variable or parameter, because +otherwise it isn't known if the type isn't just a base class whose padding +bits are reused or laid out differently in a derived class. @end deftypefn @deftypefn {Built-in Function} @var{type} __builtin_bit_cast (@var{type}, @var{arg}) diff --git a/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C b/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C new file mode 100644 index 00000000000..7b1282e7798 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C @@ -0,0 +1,50 @@ +// PR tree-optimization/102586 +// { dg-do compile } +// { dg-options "-Wno-inaccessible-base" } + +struct C0 {}; +struct C1 {}; +struct C2 : C1, virtual C0 {}; +struct C3 : virtual C2, C1 {}; +struct C4 : virtual C3, C1 {}; +struct C5 : C4 {}; +struct C6 { char c; }; +struct C7 : virtual C6, virtual C3, C1 {}; +struct C8 : C7 {}; + +void +foo (C0 *c0, C1 *c1, C2 *c2, C3 *c3, C4 *c4, C5 *c5, C6 *c6, C7 *c7, C8 *c8) +{ + __builtin_clear_padding (c0); + __builtin_clear_padding (c1); + __builtin_clear_padding (c2); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C2\\\*'\\\)" } + __builtin_clear_padding (c3); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C3\\\*'\\\)" } + __builtin_clear_padding (c4); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C4\\\*'\\\)" } + __builtin_clear_padding (c5); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C5\\\*'\\\)" } + __builtin_clear_padding (c6); + __builtin_clear_padding (c7); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C7\\\*'\\\)" } + __builtin_clear_padding (c8); // { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C8\\\*'\\\)" } +} + +void +bar () +{ + C0 c0; + C1 c1; + C2 c2; + C3 c3; + C4 c4; + C5 c5; + C6 c6; + C7 c7; + C8 c8; + __builtin_clear_padding (&c0); + __builtin_clear_padding (&c1); + __builtin_clear_padding (&c2); + __builtin_clear_padding (&c3); + __builtin_clear_padding (&c4); +// __builtin_clear_padding (&c5); + __builtin_clear_padding (&c6); + __builtin_clear_padding (&c7); + __builtin_clear_padding (&c8); +}