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[69.118.112.187]) by smtp.gmail.com with ESMTPSA id s16-20020a37a910000000b007415d02e9b5sm2247622qke.97.2023.02.21.12.18.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Feb 2023 12:18:16 -0800 (PST) From: Patrick Palka X-Google-Original-From: Patrick Palka Date: Tue, 21 Feb 2023 15:18:15 -0500 (EST) To: Patrick Palka cc: gcc-patches@gcc.gnu.org, jason@redhat.com Subject: Re: [PATCH] c++: constant non-copy-init is manifestly constant [PR108243] In-Reply-To: <20230220194641.4172416-1-ppalka@redhat.com> Message-ID: <44cf5ccc-e822-70c5-84dd-2fe5aefa9492@idea> References: <20230220194641.4172416-1-ppalka@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-13.9 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Mon, 20 Feb 2023, Patrick Palka wrote: > According to [basic.start.static]/2 and [expr.const]/2, a variable > with static storage duration initialized with a constant initializer > has constant initialization, and such an initializer is manifestly > constant-evaluated. > > We're already getting this right with copy initialization because in > that case check_initializer would consistently call store_init_value > (which for TREE_STATIC variables calls fold_non_dependent_init with > m_c_e=true). > > But for direct (or default) initialization, we don't always call > store_init_value. We instead however always call maybe_constant_init > from expand_default_init[1], albeit with m_c_e=false which means we > don't always get the "manifestly constant-evaluated" part right for > copy-init. > > This patch fixes this by simply passing m_c_e=true to this call to > maybe_constant_init for static storage duration variables, mirroring > what store_init_value basically does. > > [1]: this maybe_constant_init call isn't reached in the copy-init > case because there init is a CONSTRUCTOR rather than a TREE_LIST so > expand_default_init exits early returning an INIT_EXPR. This INIT_EXPR > is ultimately what causes us to consistently hit the store_init_value > code path from check_initializer in the copy-init case. > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for > trunk? Would it be suitable to backport this to the 12 branch since > it should only affect C++20 code? > > PR c++/108243 > > gcc/cp/ChangeLog: > > * init.cc (expand_default_init): Pass m_c_e=true instead of > =false to maybe_constant_init when initializing a variable > with static storage duration. > > gcc/testsuite/ChangeLog: > > * g++.dg/cpp2a/is-constant-evaluated14.C: New test. > --- > gcc/cp/init.cc | 5 +- > .../g++.dg/cpp2a/is-constant-evaluated14.C | 140 ++++++++++++++++++ > 2 files changed, 144 insertions(+), 1 deletion(-) > create mode 100644 gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C > > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc > index 52e96fbe590..705a5b3bdb6 100644 > --- a/gcc/cp/init.cc > +++ b/gcc/cp/init.cc > @@ -2203,7 +2203,10 @@ expand_default_init (tree binfo, tree true_exp, tree exp, tree init, int flags, > tree fn = get_callee_fndecl (rval); > if (fn && DECL_DECLARED_CONSTEXPR_P (fn)) > { > - tree e = maybe_constant_init (rval, exp); > + bool manifestly_const_eval = false; > + if (VAR_P (exp) && TREE_STATIC (exp)) > + manifestly_const_eval = true; > + tree e = maybe_constant_init (rval, exp, manifestly_const_eval); > if (TREE_CONSTANT (e)) > rval = cp_build_init_expr (exp, e); > } Hmm, alternatively we could just override manifestly_const_eval to true from maybe_constant_init for static storage duration variables, like so. I guess this approach much be preferable since it potentially benefits all maybe_constant_init callers? -- >8 -- gcc/cp/ChangeLog: * constexpr.cc (maybe_constant_init_1): Override manifestly_const_eval to true if is_static. diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index aa2c14355f8..8ae83a6eadf 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -8760,7 +8760,8 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant, bool is_static = (decl && DECL_P (decl) && (TREE_STATIC (decl) || DECL_EXTERNAL (decl))); t = cxx_eval_outermost_constant_expr (t, allow_non_constant, !is_static, - mce_value (manifestly_const_eval), + (is_static ? mce_true + : mce_value (manifestly_const_eval)), false, decl); } if (TREE_CODE (t) == TARGET_EXPR) > diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C > new file mode 100644 > index 00000000000..365bca3fd9a > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C > @@ -0,0 +1,140 @@ > +// PR c++/108243 > +// Verify a variable with static storage duration initialized with a > +// constant initializer has constant initialization, and the initializer > +// is manifestly constant-evaluated. > +// { dg-do run { target c++11 } } > +// { dg-additional-options "-fdump-tree-original" } > + > +#include > + > +struct A { > + constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { } > + constexpr A() : A(42) { } > + void verify_mce() const { > + if (m != 1) __builtin_abort(); > + } > + int n; > + int m; > +}; > + > +A a1 = {42}; > +A a2{42}; > +A a3(42); > +A a4; > +A a5{}; > + > +void f() { > + static A a1 = {42}; > + static A a2{42}; > + static A a3(42); > + static A a4; > + static A a5{}; > + for (auto& a : {a1, a2, a3, a4, a5}) > + a.verify_mce(); > +} > + > +template > +void g() { > + static A a1 = {42}; > + static A a2{42}; > + static A a3(42); > + static A a4; > + static A a5{}; > + static A a6 = {N...}; > + static A a7{N...}; > + static A a8(N...); > + for (auto& a : {a1, a2, a3, a4, a5, a6, a7, a8}) > + a.verify_mce(); > +} > + > +struct B { > + static A a1; > + static A a2; > + static A a3; > + static A a4; > + static A a5; > + static void verify_mce() { > + for (auto& a : {a1, a2, a3, a4, a5}) > + a.verify_mce(); > + } > +}; > + > +A B::a1 = {42}; > +A B::a2{42}; > +A B::a3(42); > +A B::a4; > +A B::a5{}; > + > +template > +struct BT { > + static A a1; > + static A a2; > + static A a3; > + static A a4; > + static A a5; > + static A a6; > + static A a7; > + static A a8; > + static void verify_mce() { > + for (auto& a : {a1, a2, a3, a4, a5}) > + a.verify_mce(); > + } > +}; > + > +template A BT::a1 = {42}; > +template A BT::a2{42}; > +template A BT::a3(42); > +template A BT::a4; > +template A BT::a5{}; > +template A BT::a6 = {N...}; > +template A BT::a7{N...}; > +template A BT::a8(N...); > + > +#if __cpp_inline_variables > +struct BI { > + static inline A a1 = {42}; > + static inline A a2{42}; > + static inline A a3; > + static inline A a4{}; > + static void verify_mce() { > + for (auto& a : {a1, a2, a3, a4}) > + a.verify_mce(); > + } > +}; > + > +template > +struct BIT { > + static inline A a1 = {42}; > + static inline A a2{42}; > + static inline A a3; > + static inline A a4{}; > + static inline A a5 = {N...}; > + static inline A a6{N...}; > + static void verify_mce() { > + for (auto& a : {a1, a2, a3, a4, a5, a6}) > + a.verify_mce(); > + } > +}; > +#endif > + > +int main() { > + for (auto& a : {a1, a2, a3, a4, a5}) > + a.verify_mce(); > + > + f(); > + g<42>(); > + g<>(); > + > + B::verify_mce(); > + BT<42>::verify_mce(); > + BT<>::verify_mce(); > + > +#if __cpp_inline_variables > + BI::verify_mce(); > + BIT<42>::verify_mce(); > + BIT<>::verify_mce(); > +#endif > +} > + > +// { dg-final { scan-tree-dump-not "static initializers for" "original" } } > +// { dg-final { scan-tree-dump-not "cxa_guard_acquire" "original" } } > -- > 2.39.2.501.gd9d677b2d8 > >