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[69.118.112.187]) by smtp.gmail.com with ESMTPSA id 73-20020a370b4c000000b007259807a512sm2488632qkl.12.2023.02.09.17.32.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Feb 2023 17:32:51 -0800 (PST) From: Patrick Palka X-Google-Original-From: Patrick Palka Date: Thu, 9 Feb 2023 20:32:50 -0500 (EST) To: Jason Merrill cc: Patrick Palka , gcc-patches@gcc.gnu.org Subject: Re: [PATCH 2/2] c++: speculative constexpr and is_constant_evaluated [PR108243] In-Reply-To: Message-ID: References: <20230127220250.1896137-1-ppalka@redhat.com> <20230127220250.1896137-2-ppalka@redhat.com> <154813b4-b680-aad5-ce7a-8ea012626eda@redhat.com> <8ca008ee-d56c-52d9-4f3f-3f738e0e8af6@idea> <1ba53fb8-f688-b680-75f9-a158fa0d679b@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 Thu, 9 Feb 2023, Jason Merrill wrote: > On 2/9/23 09:36, Patrick Palka wrote: > > On Sun, 5 Feb 2023, Jason Merrill wrote: > > > > > On 2/3/23 15:51, Patrick Palka wrote: > > > > On Mon, 30 Jan 2023, Jason Merrill wrote: > > > > > > > > > On 1/27/23 17:02, Patrick Palka wrote: > > > > > > This PR illustrates that __builtin_is_constant_evaluated currently > > > > > > acts > > > > > > as an optimization barrier for our speculative constexpr evaluation, > > > > > > since we don't want to prematurely fold the builtin to false if the > > > > > > expression in question would be later manifestly constant evaluated > > > > > > (in > > > > > > which case it must be folded to true). > > > > > > > > > > > > This patch fixes this by permitting __builtin_is_constant_evaluated > > > > > > to get folded as false during cp_fold_function, since at that point > > > > > > we're sure we're doing manifestly constant evaluation. To that end > > > > > > we add a flags parameter to cp_fold that controls what mce_value the > > > > > > CALL_EXPR case passes to maybe_constant_value. > > > > > > > > > > > > bootstrapped and rgetsted no x86_64-pc-linux-gnu, does this look OK > > > > > > for > > > > > > trunk? > > > > > > > > > > > > PR c++/108243 > > > > > > > > > > > > gcc/cp/ChangeLog: > > > > > > > > > > > > * cp-gimplify.cc (enum fold_flags): Define. > > > > > > (cp_fold_data::genericize): Replace this data member with ... > > > > > > (cp_fold_data::fold_flags): ... this. > > > > > > (cp_fold_r): Adjust cp_fold_data use and cp_fold_calls. > > > > > > (cp_fold_function): Likewise. > > > > > > (cp_fold_maybe_rvalue): Likewise. > > > > > > (cp_fully_fold_init): Likewise. > > > > > > (cp_fold): Add fold_flags parameter. Don't cache if flags > > > > > > isn't empty. > > > > > > : Pass mce_false to maybe_constant_value > > > > > > if if ff_genericize is set. > > > > > > > > > > > > gcc/testsuite/ChangeLog: > > > > > > > > > > > > * g++.dg/opt/pr108243.C: New test. > > > > > > --- > > > > > > gcc/cp/cp-gimplify.cc | 76 > > > > > > ++++++++++++++++++----------- > > > > > > gcc/testsuite/g++.dg/opt/pr108243.C | 29 +++++++++++ > > > > > > 2 files changed, 76 insertions(+), 29 deletions(-) > > > > > > create mode 100644 gcc/testsuite/g++.dg/opt/pr108243.C > > > > > > > > > > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc > > > > > > index a35cedd05cc..d023a63768f 100644 > > > > > > --- a/gcc/cp/cp-gimplify.cc > > > > > > +++ b/gcc/cp/cp-gimplify.cc > > > > > > @@ -43,12 +43,20 @@ along with GCC; see the file COPYING3. If not > > > > > > see > > > > > > #include "omp-general.h" > > > > > > #include "opts.h" > > > > > > +/* Flags for cp_fold and cp_fold_r. */ > > > > > > + > > > > > > +enum fold_flags { > > > > > > + ff_none = 0, > > > > > > + /* Whether we're being called from cp_fold_function. */ > > > > > > + ff_genericize = 1 << 0, > > > > > > +}; > > > > > > + > > > > > > /* Forward declarations. */ > > > > > > static tree cp_genericize_r (tree *, int *, void *); > > > > > > static tree cp_fold_r (tree *, int *, void *); > > > > > > static void cp_genericize_tree (tree*, bool); > > > > > > -static tree cp_fold (tree); > > > > > > +static tree cp_fold (tree, fold_flags); > > > > > > /* Genericize a TRY_BLOCK. */ > > > > > > @@ -996,9 +1004,8 @@ struct cp_genericize_data > > > > > > struct cp_fold_data > > > > > > { > > > > > > hash_set pset; > > > > > > - bool genericize; // called from cp_fold_function? > > > > > > - > > > > > > - cp_fold_data (bool g): genericize (g) {} > > > > > > + fold_flags flags; > > > > > > + cp_fold_data (fold_flags flags): flags (flags) {} > > > > > > }; > > > > > > static tree > > > > > > @@ -1039,7 +1046,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, > > > > > > void > > > > > > *data_) > > > > > > break; > > > > > > } > > > > > > - *stmt_p = stmt = cp_fold (*stmt_p); > > > > > > + *stmt_p = stmt = cp_fold (*stmt_p, data->flags); > > > > > > if (data->pset.add (stmt)) > > > > > > { > > > > > > @@ -1119,12 +1126,12 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, > > > > > > void > > > > > > *data_) > > > > > > here rather than in cp_genericize to avoid problems with the > > > > > > invisible > > > > > > reference transition. */ > > > > > > case INIT_EXPR: > > > > > > - if (data->genericize) > > > > > > + if (data->flags & ff_genericize) > > > > > > cp_genericize_init_expr (stmt_p); > > > > > > break; > > > > > > case TARGET_EXPR: > > > > > > - if (data->genericize) > > > > > > + if (data->flags & ff_genericize) > > > > > > cp_genericize_target_expr (stmt_p); > > > > > > /* Folding might replace e.g. a COND_EXPR with a > > > > > > TARGET_EXPR; > > > > > > in > > > > > > @@ -1157,7 +1164,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, > > > > > > void > > > > > > *data_) > > > > > > void > > > > > > cp_fold_function (tree fndecl) > > > > > > { > > > > > > - cp_fold_data data (/*genericize*/true); > > > > > > + cp_fold_data data (ff_genericize); > > > > > > cp_walk_tree (&DECL_SAVED_TREE (fndecl), cp_fold_r, &data, > > > > > > NULL); > > > > > > } > > > > > > @@ -2375,7 +2382,7 @@ cp_fold_maybe_rvalue (tree x, bool rval) > > > > > > { > > > > > > while (true) > > > > > > { > > > > > > - x = cp_fold (x); > > > > > > + x = cp_fold (x, ff_none); > > > > > > if (rval) > > > > > > x = mark_rvalue_use (x); > > > > > > if (rval && DECL_P (x) > > > > > > @@ -2434,7 +2441,7 @@ cp_fully_fold_init (tree x) > > > > > > if (processing_template_decl) > > > > > > return x; > > > > > > x = cp_fully_fold (x); > > > > > > - cp_fold_data data (/*genericize*/false); > > > > > > + cp_fold_data data (ff_none); > > > > > > cp_walk_tree (&x, cp_fold_r, &data, NULL); > > > > > > return x; > > > > > > } > > > > > > @@ -2469,7 +2476,7 @@ clear_fold_cache (void) > > > > > > Function returns X or its folded variant. */ > > > > > > static tree > > > > > > -cp_fold (tree x) > > > > > > +cp_fold (tree x, fold_flags flags) > > > > > > { > > > > > > tree op0, op1, op2, op3; > > > > > > tree org_x = x, r = NULL_TREE; > > > > > > @@ -2490,8 +2497,11 @@ cp_fold (tree x) > > > > > > if (fold_cache == NULL) > > > > > > fold_cache = hash_map::create_ggc (101); > > > > > > - if (tree *cached = fold_cache->get (x)) > > > > > > - return *cached; > > > > > > + bool cache_p = (flags == ff_none); > > > > > > + > > > > > > + if (cache_p) > > > > > > + if (tree *cached = fold_cache->get (x)) > > > > > > + return *cached; > > > > > > uid_sensitive_constexpr_evaluation_checker c; > > > > > > @@ -2526,7 +2536,7 @@ cp_fold (tree x) > > > > > > Don't create a new tree if op0 != TREE_OPERAND (x, 0), > > > > > > the > > > > > > folding of the operand should be in the caches and if in > > > > > > cp_fold_r > > > > > > it will modify it in place. */ > > > > > > - op0 = cp_fold (TREE_OPERAND (x, 0)); > > > > > > + op0 = cp_fold (TREE_OPERAND (x, 0), flags); > > > > > > if (op0 == error_mark_node) > > > > > > x = error_mark_node; > > > > > > break; > > > > > > @@ -2571,7 +2581,7 @@ cp_fold (tree x) > > > > > > { > > > > > > tree p = maybe_undo_parenthesized_ref (x); > > > > > > if (p != x) > > > > > > - return cp_fold (p); > > > > > > + return cp_fold (p, flags); > > > > > > } > > > > > > goto unary; > > > > > > @@ -2763,8 +2773,8 @@ cp_fold (tree x) > > > > > > case COND_EXPR: > > > > > > loc = EXPR_LOCATION (x); > > > > > > op0 = cp_fold_rvalue (TREE_OPERAND (x, 0)); > > > > > > - op1 = cp_fold (TREE_OPERAND (x, 1)); > > > > > > - op2 = cp_fold (TREE_OPERAND (x, 2)); > > > > > > + op1 = cp_fold (TREE_OPERAND (x, 1), flags); > > > > > > + op2 = cp_fold (TREE_OPERAND (x, 2), flags); > > > > > > if (TREE_CODE (TREE_TYPE (x)) == BOOLEAN_TYPE) > > > > > > { > > > > > > @@ -2854,7 +2864,7 @@ cp_fold (tree x) > > > > > > { > > > > > > if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r))) > > > > > > r = build_nop (TREE_TYPE (x), r); > > > > > > - x = cp_fold (r); > > > > > > + x = cp_fold (r, flags); > > > > > > break; > > > > > > } > > > > > > } > > > > > > @@ -2908,7 +2918,7 @@ cp_fold (tree x) > > > > > > int m = call_expr_nargs (x); > > > > > > for (int i = 0; i < m; i++) > > > > > > { > > > > > > - r = cp_fold (CALL_EXPR_ARG (x, i)); > > > > > > + r = cp_fold (CALL_EXPR_ARG (x, i), flags); > > > > > > if (r != CALL_EXPR_ARG (x, i)) > > > > > > { > > > > > > if (r == error_mark_node) > > > > > > @@ -2931,7 +2941,7 @@ cp_fold (tree x) > > > > > > if (TREE_CODE (r) != CALL_EXPR) > > > > > > { > > > > > > - x = cp_fold (r); > > > > > > + x = cp_fold (r, flags); > > > > > > break; > > > > > > } > > > > > > @@ -2944,7 +2954,15 @@ cp_fold (tree x) > > > > > > constant, but the call followed by an INDIRECT_REF is. */ > > > > > > if (callee && DECL_DECLARED_CONSTEXPR_P (callee) > > > > > > && !flag_no_inline) > > > > > > - r = maybe_constant_value (x); > > > > > > + { > > > > > > + mce_value manifestly_const_eval = mce_unknown; > > > > > > + if (flags & ff_genericize) > > > > > > + /* At genericization time it's safe to fold > > > > > > + __builtin_is_constant_evaluated to false. */ > > > > > > + manifestly_const_eval = mce_false; > > > > > > + r = maybe_constant_value (x, /*decl=*/NULL_TREE, > > > > > > + manifestly_const_eval); > > > > > > + } > > > > > > optimize = sv; > > > > > > if (TREE_CODE (r) != CALL_EXPR) > > > > > > @@ -2971,7 +2989,7 @@ cp_fold (tree x) > > > > > > vec *nelts = NULL; > > > > > > FOR_EACH_VEC_SAFE_ELT (elts, i, p) > > > > > > { > > > > > > - tree op = cp_fold (p->value); > > > > > > + tree op = cp_fold (p->value, flags); > > > > > > if (op != p->value) > > > > > > { > > > > > > if (op == error_mark_node) > > > > > > @@ -3002,7 +3020,7 @@ cp_fold (tree x) > > > > > > for (int i = 0; i < n; i++) > > > > > > { > > > > > > - tree op = cp_fold (TREE_VEC_ELT (x, i)); > > > > > > + tree op = cp_fold (TREE_VEC_ELT (x, i), flags); > > > > > > if (op != TREE_VEC_ELT (x, i)) > > > > > > { > > > > > > if (!changed) > > > > > > @@ -3019,10 +3037,10 @@ cp_fold (tree x) > > > > > > case ARRAY_RANGE_REF: > > > > > > loc = EXPR_LOCATION (x); > > > > > > - op0 = cp_fold (TREE_OPERAND (x, 0)); > > > > > > - op1 = cp_fold (TREE_OPERAND (x, 1)); > > > > > > - op2 = cp_fold (TREE_OPERAND (x, 2)); > > > > > > - op3 = cp_fold (TREE_OPERAND (x, 3)); > > > > > > + op0 = cp_fold (TREE_OPERAND (x, 0), flags); > > > > > > + op1 = cp_fold (TREE_OPERAND (x, 1), flags); > > > > > > + op2 = cp_fold (TREE_OPERAND (x, 2), flags); > > > > > > + op3 = cp_fold (TREE_OPERAND (x, 3), flags); > > > > > > if (op0 != TREE_OPERAND (x, 0) > > > > > > || op1 != TREE_OPERAND (x, 1) > > > > > > @@ -3050,7 +3068,7 @@ cp_fold (tree x) > > > > > > /* A SAVE_EXPR might contain e.g. (0 * i) + (0 * j), > > > > > > which, > > > > > > after > > > > > > folding, evaluates to an invariant. In that case no need to > > > > > > wrap > > > > > > this folded tree with a SAVE_EXPR. */ > > > > > > - r = cp_fold (TREE_OPERAND (x, 0)); > > > > > > + r = cp_fold (TREE_OPERAND (x, 0), flags); > > > > > > if (tree_invariant_p (r)) > > > > > > x = r; > > > > > > break; > > > > > > @@ -3069,7 +3087,7 @@ cp_fold (tree x) > > > > > > copy_warning (x, org_x); > > > > > > } > > > > > > - if (!c.evaluation_restricted_p ()) > > > > > > + if (cache_p && !c.evaluation_restricted_p ()) > > > > > > { > > > > > > fold_cache->put (org_x, x); > > > > > > /* Prevent that we try to fold an already folded result > > > > > > again. > > > > > > */ > > > > > > diff --git a/gcc/testsuite/g++.dg/opt/pr108243.C > > > > > > b/gcc/testsuite/g++.dg/opt/pr108243.C > > > > > > new file mode 100644 > > > > > > index 00000000000..4c45dbba13c > > > > > > --- /dev/null > > > > > > +++ b/gcc/testsuite/g++.dg/opt/pr108243.C > > > > > > @@ -0,0 +1,29 @@ > > > > > > +// PR c++/108243 > > > > > > +// { dg-do compile { target c++11 } } > > > > > > +// { dg-additional-options "-O -fdump-tree-original" } > > > > > > + > > > > > > +constexpr int foo() { > > > > > > + return __builtin_is_constant_evaluated() + 1; > > > > > > +} > > > > > > + > > > > > > +#if __cpp_if_consteval > > > > > > +constexpr int bar() { > > > > > > + if consteval { > > > > > > + return 5; > > > > > > + } else { > > > > > > + return 4; > > > > > > + } > > > > > > +} > > > > > > +#endif > > > > > > + > > > > > > +int p, q; > > > > > > + > > > > > > +int main() { > > > > > > + p = foo(); > > > > > > +#if __cpp_if_consteval > > > > > > + q = bar(); > > > > > > +#endif > > > > > > +} > > > > > > + > > > > > > +// { dg-final { scan-tree-dump-not "= foo" "original" } } > > > > > > +// { dg-final { scan-tree-dump-not "= bar" "original" } } > > > > > > > > > > Let's also test a static initializer that can't be fully > > > > > constant-evaluated. > > > > > > > > D'oh, doing so revealed that cp_fold_function doesn't reach static > > > > initializers; that's taken care of by cp_fully_fold_init. So it seems > > > > we need to make cp_fold when called from the latter entry point to also > > > > assume m_c_e is false. We can't re-use ff_genericize here because that > > > > flag has additional effects in cp_fold_r, so it seems we need another > > > > flag that that only affects the manifestly constant-eval stuff; I called > > > > it ff_mce_false. How does the following look? > > > > > > > > -- >8 -- > > > > > > > > Subject: [PATCH 2/2] c++: speculative constexpr and > > > > is_constant_evaluated > > > > [PR108243] > > > > > > > > This PR illustrates that __builtin_is_constant_evaluated currently acts > > > > as an optimization barrier for our speculative constexpr evaluation, > > > > since we don't want to prematurely fold the builtin to false if the > > > > expression in question would be later manifestly constant evaluated (in > > > > which case it must be folded to true). > > > > > > > > This patch fixes this by permitting __builtin_is_constant_evaluated > > > > to get folded as false during cp_fold_function and cp_fully_fold_init, > > > > since at these points we're sure we're done with manifestly constant > > > > evaluation. To that end we add a flags parameter to cp_fold that > > > > controls whether we pass mce_false or mce_unknown to > > > > maybe_constant_value > > > > when folding a CALL_EXPR. > > > > > > > > PR c++/108243 > > > > PR c++/97553 > > > > > > > > gcc/cp/ChangeLog: > > > > > > > > * cp-gimplify.cc (enum fold_flags): Define. > > > > (cp_fold_data::genericize): Replace this data member with ... > > > > (cp_fold_data::fold_flags): ... this. > > > > (cp_fold_r): Adjust use of cp_fold_data and calls to cp_fold. > > > > (cp_fold_function): Likewise. > > > > (cp_fold_maybe_rvalue): Likewise. > > > > (cp_fully_fold_init): Likewise. > > > > (cp_fold): Add fold_flags parameter. Don't cache if flags > > > > isn't empty. > > > > : If ff_genericize is set, fold > > > > __builtin_is_constant_evaluated to false and pass mce_false to > > > > maybe_constant_value. > > > > > > > > gcc/testsuite/ChangeLog: > > > > > > > > * g++.dg/opt/is_constant_evaluated1.C: New test. > > > > * g++.dg/opt/is_constant_evaluated2.C: New test. > > > > --- > > > > gcc/cp/cp-gimplify.cc | 88 > > > > ++++++++++++------- > > > > .../g++.dg/opt/is_constant_evaluated1.C | 14 +++ > > > > .../g++.dg/opt/is_constant_evaluated2.C | 32 +++++++ > > > > 3 files changed, 104 insertions(+), 30 deletions(-) > > > > create mode 100644 gcc/testsuite/g++.dg/opt/is_constant_evaluated1.C > > > > create mode 100644 gcc/testsuite/g++.dg/opt/is_constant_evaluated2.C > > > > > > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc > > > > index 9929d29981a..590ed787997 100644 > > > > --- a/gcc/cp/cp-gimplify.cc > > > > +++ b/gcc/cp/cp-gimplify.cc > > > > @@ -43,12 +43,26 @@ along with GCC; see the file COPYING3. If not see > > > > #include "omp-general.h" > > > > #include "opts.h" > > > > +/* Flags for cp_fold and cp_fold_r. */ > > > > + > > > > +enum fold_flags { > > > > + ff_none = 0, > > > > + /* Whether we're being called from cp_fold_function. */ > > > > + ff_genericize = 1 << 0, > > > > + /* Whether we're folding late enough that we could assume > > > > + we're definitely not in a manifestly constant-evaluated > > > > + context. */ > > > > > > It's not necessarily a matter of late enough; we could fold sooner and > > > still > > > know that, as in cp_fully_fold_init. We could do the same at other > > > full-expression points, but we don't because we want to delay folding as > > > much > > > as possible. So let's say "folding at a point where we know we're..." > > > > > > > + ff_mce_false = 1 << 1, > > > > +}; > > > > + > > > > +using fold_flags_t = int; > > > > + > > > > /* Forward declarations. */ > > > > static tree cp_genericize_r (tree *, int *, void *); > > > > static tree cp_fold_r (tree *, int *, void *); > > > > static void cp_genericize_tree (tree*, bool); > > > > -static tree cp_fold (tree); > > > > +static tree cp_fold (tree, fold_flags_t); > > > > /* Genericize a TRY_BLOCK. */ > > > > @@ -1012,9 +1026,8 @@ struct cp_genericize_data > > > > struct cp_fold_data > > > > { > > > > hash_set pset; > > > > - bool genericize; // called from cp_fold_function? > > > > - > > > > - cp_fold_data (bool g): genericize (g) {} > > > > + fold_flags_t flags; > > > > + cp_fold_data (fold_flags_t flags): flags (flags) {} > > > > }; > > > > static tree > > > > @@ -1055,7 +1068,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void > > > > *data_) > > > > break; > > > > } > > > > - *stmt_p = stmt = cp_fold (*stmt_p); > > > > + *stmt_p = stmt = cp_fold (*stmt_p, data->flags); > > > > if (data->pset.add (stmt)) > > > > { > > > > @@ -1135,12 +1148,12 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, > > > > void > > > > *data_) > > > > here rather than in cp_genericize to avoid problems with the > > > > invisible > > > > reference transition. */ > > > > case INIT_EXPR: > > > > - if (data->genericize) > > > > + if (data->flags & ff_genericize) > > > > cp_genericize_init_expr (stmt_p); > > > > break; > > > > case TARGET_EXPR: > > > > - if (data->genericize) > > > > + if (data->flags & ff_genericize) > > > > cp_genericize_target_expr (stmt_p); > > > > /* Folding might replace e.g. a COND_EXPR with a TARGET_EXPR; > > > > in > > > > @@ -1173,7 +1186,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void > > > > *data_) > > > > void > > > > cp_fold_function (tree fndecl) > > > > { > > > > - cp_fold_data data (/*genericize*/true); > > > > + cp_fold_data data (ff_genericize | ff_mce_false); > > > > > > Here would be a good place for a comment about passing mce_false because > > > all > > > manifestly-constant-evaluated expressions will have been > > > constant-evaluated > > > already if possible. > > > > > > > cp_walk_tree (&DECL_SAVED_TREE (fndecl), cp_fold_r, &data, NULL); > > > > } > > > > @@ -2391,7 +2404,7 @@ cp_fold_maybe_rvalue (tree x, bool rval) > > > > { > > > > while (true) > > > > { > > > > - x = cp_fold (x); > > > > + x = cp_fold (x, ff_none); > > > > if (rval) > > > > x = mark_rvalue_use (x); > > > > if (rval && DECL_P (x) > > > > @@ -2450,7 +2463,7 @@ cp_fully_fold_init (tree x) > > > > if (processing_template_decl) > > > > return x; > > > > x = cp_fully_fold (x); > > > > - cp_fold_data data (/*genericize*/false); > > > > + cp_fold_data data (ff_mce_false); > > > > cp_walk_tree (&x, cp_fold_r, &data, NULL); > > > > return x; > > > > } > > > > @@ -2485,7 +2498,7 @@ clear_fold_cache (void) > > > > Function returns X or its folded variant. */ > > > > static tree > > > > -cp_fold (tree x) > > > > +cp_fold (tree x, fold_flags_t flags) > > > > { > > > > tree op0, op1, op2, op3; > > > > tree org_x = x, r = NULL_TREE; > > > > @@ -2506,8 +2519,11 @@ cp_fold (tree x) > > > > if (fold_cache == NULL) > > > > fold_cache = hash_map::create_ggc (101); > > > > - if (tree *cached = fold_cache->get (x)) > > > > - return *cached; > > > > + bool cache_p = (flags == ff_none); > > > > + > > > > + if (cache_p) > > > > + if (tree *cached = fold_cache->get (x)) > > > > + return *cached; > > > > uid_sensitive_constexpr_evaluation_checker c; > > > > @@ -2542,7 +2558,7 @@ cp_fold (tree x) > > > > Don't create a new tree if op0 != TREE_OPERAND (x, 0), > > > > the > > > > folding of the operand should be in the caches and if in > > > > cp_fold_r > > > > it will modify it in place. */ > > > > - op0 = cp_fold (TREE_OPERAND (x, 0)); > > > > + op0 = cp_fold (TREE_OPERAND (x, 0), flags); > > > > if (op0 == error_mark_node) > > > > x = error_mark_node; > > > > break; > > > > @@ -2587,7 +2603,7 @@ cp_fold (tree x) > > > > { > > > > tree p = maybe_undo_parenthesized_ref (x); > > > > if (p != x) > > > > - return cp_fold (p); > > > > + return cp_fold (p, flags); > > > > } > > > > goto unary; > > > > @@ -2779,8 +2795,8 @@ cp_fold (tree x) > > > > case COND_EXPR: > > > > loc = EXPR_LOCATION (x); > > > > op0 = cp_fold_rvalue (TREE_OPERAND (x, 0)); > > > > - op1 = cp_fold (TREE_OPERAND (x, 1)); > > > > - op2 = cp_fold (TREE_OPERAND (x, 2)); > > > > + op1 = cp_fold (TREE_OPERAND (x, 1), flags); > > > > + op2 = cp_fold (TREE_OPERAND (x, 2), flags); > > > > if (TREE_CODE (TREE_TYPE (x)) == BOOLEAN_TYPE) > > > > { > > > > @@ -2870,7 +2886,7 @@ cp_fold (tree x) > > > > { > > > > if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r))) > > > > r = build_nop (TREE_TYPE (x), r); > > > > - x = cp_fold (r); > > > > + x = cp_fold (r, flags); > > > > break; > > > > } > > > > } > > > > @@ -2890,8 +2906,12 @@ cp_fold (tree x) > > > > { > > > > switch (DECL_FE_FUNCTION_CODE (callee)) > > > > { > > > > - /* Defer folding __builtin_is_constant_evaluated. */ > > > > case CP_BUILT_IN_IS_CONSTANT_EVALUATED: > > > > + /* Defer folding __builtin_is_constant_evaluated unless > > > > + we can assume this isn't a manifestly constant-evaluated > > > > > > s/can assume/know/ > > > > > > OK with those comment changes. > > > > Thanks a lot. Unfortunately I think the patch has a significant problem > > that only just occurred to me -- disabling the cp_fold cache when the > > flag ff_mce_false is set effectively makes cp_fold_function and > > cp_fully_fold_init quadratic in the size of the expression (since > > cp_fold_r calls cp_fold on each subtree, and cp_fold when the cache is > > disabled will end up fully walking each subtree). Note that the reason > > we must disable the cache is because cp_fold with ff_mce_false might > > give a different folded result than without that flag if the expression > > contains a suitable CALL_EXPR subexpression. > > Good point. > > > One approach to fix this complexity issue would be to parameterize the > > cache according to the flags that were passed to cp_fold, which would > > allow us to keep the cache enabled when ff_mce_false is set. A downside > > to this approach is that the size of the cp_fold cache would essentially > > double since for each tree we'd now have two cache entries, one for > > flags=ff_none and another for flags=ff_mce_false. > > We could also clear the cache before cp_fold_function since the two folds > shouldn't overlap (much). Makes sense, but IIUC we'd also have to clear it before (and after) cp_fully_fold_init too, which unlike cp_fold_function may get called in the middle of a function body. > > > Another approach would be to split out the trial constexpr evaluation > > part of cp_fold's CALL_EXPR handling, parameterize that, and call it > > directly from cp_fold_r. With this approach we wouldn't perform as much > > folding, e.g. > > > > int n = 41 + !std::is_constant_evaluated(); > > > > would get folded to 1 + 41 rather than 42. But I suspect this would > > give us 95% of the reapable benefits of the above approach. > > > > I think I'm leaning towards this second approach, which the below patch > > implements instead. What do you think? Bootstrapped and regtested on > > x86_64-pc-linux-gnu. > > That sounds reasonable, but... > > > -- >8 -- > > > > Subject: [PATCH] c++: speculative constexpr and is_constant_evaluated > > [PR108243] > > > > This PR illustrates that __builtin_is_constant_evaluated currently acts > > as an optimization barrier for our speculative constexpr evaluation, > > since we don't want to prematurely fold the builtin to false before the > > expression in question undergoes constant evaluation as in a manifestly > > constant-evaluated context (in which case the builtin must instead be > > folded to true). > > > > This patch fixes this by permitting __builtin_is_constant_evaluated > > to get folded to false from cp_fold_r, where we know we're done with > > proper constant evaluation (of manifestly constant-evaluated contexts). > > > > PR c++/108243 > > PR c++/97553 > > > > gcc/cp/ChangeLog: > > > > * cp-gimplify.cc > > (cp_fold_r): Remove redundant *stmt_p assignments. After > > calling cp_fold, call maybe_fold_constexpr_call with mce_false. > > (cp_fold) : Split out trial constexpr evaluation > > into ... > > (maybe_fold_constexpr_call): ... here. > > > > gcc/testsuite/ChangeLog: > > > > * g++.dg/opt/is_constant_evaluated1.C: New test. > > * g++.dg/opt/is_constant_evaluated2.C: New test. > > --- > > gcc/cp/cp-gimplify.cc | 55 +++++++++++++++---- > > .../g++.dg/opt/is_constant_evaluated1.C | 20 +++++++ > > .../g++.dg/opt/is_constant_evaluated2.C | 32 +++++++++++ > > 3 files changed, 95 insertions(+), 12 deletions(-) > > create mode 100644 gcc/testsuite/g++.dg/opt/is_constant_evaluated1.C > > create mode 100644 gcc/testsuite/g++.dg/opt/is_constant_evaluated2.C > > > > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc > > index 9929d29981a..dca55056b2c 100644 > > --- a/gcc/cp/cp-gimplify.cc > > +++ b/gcc/cp/cp-gimplify.cc > > @@ -49,6 +49,7 @@ static tree cp_genericize_r (tree *, int *, void *); > > static tree cp_fold_r (tree *, int *, void *); > > static void cp_genericize_tree (tree*, bool); > > static tree cp_fold (tree); > > +static tree maybe_fold_constexpr_call (tree, mce_value); > > /* Genericize a TRY_BLOCK. */ > > @@ -1034,7 +1035,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void > > *data_) > > error_at (PTRMEM_CST_LOCATION (stmt), > > "taking address of an immediate function %qD", > > PTRMEM_CST_MEMBER (stmt)); > > - stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt)); > > + stmt = build_zero_cst (TREE_TYPE (stmt)); > > break; > > } > > break; > > @@ -1046,7 +1047,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void > > *data_) > > error_at (EXPR_LOCATION (stmt), > > "taking address of an immediate function %qD", > > TREE_OPERAND (stmt, 0)); > > - stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt)); > > + stmt = build_zero_cst (TREE_TYPE (stmt)); > > break; > > } > > break; > > @@ -1055,7 +1056,17 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void > > *data_) > > break; > > } > > - *stmt_p = stmt = cp_fold (*stmt_p); > > + stmt = cp_fold (stmt); > > + > > + if (TREE_CODE (stmt) == CALL_EXPR) > > + /* Since cp_fold_r is called (from cp_genericize, cp_fold_function > > + and cp_fully_fold_init) only after the overall expression has been > > + considered for constant-evaluation, we can by now safely fold any > > + remaining __builtin_is_constant_evaluated calls to false, so try > > + constexpr expansion with mce_false. */ > > + stmt = maybe_fold_constexpr_call (stmt, mce_false); > > + > > + *stmt_p = stmt; > > if (data->pset.add (stmt)) > > { > > @@ -2952,15 +2963,10 @@ cp_fold (tree x) > > } > > optimize = nw; > > - > > - /* Invoke maybe_constant_value for functions declared > > - constexpr and not called with AGGR_INIT_EXPRs. > > - TODO: > > - Do constexpr expansion of expressions where the call itself is not > > - constant, but the call followed by an INDIRECT_REF is. */ > > - if (callee && DECL_DECLARED_CONSTEXPR_P (callee) > > - && !flag_no_inline) > > - r = maybe_constant_value (x); > > + /* Pass mce_unknown to defer folding __builtin_is_constant_evaluated > > + since we don't know if we're in a manifestly constant-evaluated > > + context that hasn't yet been constant-evaluated. */ > > + r = maybe_fold_constexpr_call (x, mce_unknown); > > It seems unfortunate to try to fold both here and in cp_fold_r. Yes, though I'm afraid some duplication of work is pretty much unavoidable. Even if in cp_fold_r we did something like if (TREE_CODE (stmt) == CALL_EXPR) /* cp_fold_call_expr is the entire CALL_EXPR case of cp_fold. */ stmt = cp_fold_call_expr (stmt, mce_false); else stmt = cp_fold (stmt); instead of stmt = cp_fold (stmt); if (TREE_CODE (stmt) == CALL_EXPR) stmt = maybe_fold_constexpr_call (stmt, mce_false); we would still end up doing cp_fold on the CALL_EXPR if it's a subexpression of some larger expression (since cp_fold is recursive). And even if we went with the original approach of parameterizing cp_fold according to manifestly_const_eval totally, we would still end up trying to fold some constexpr calls twice, first with flags=ff_none (during some on-the-spot folding) and again with flags=ff_mce_false (during cp_fold_function), I think. > > Does this patch still fold __builtin_is_constant_evaluated() even though it no > longer touches the cp_fold builtin handling? Indeed it doesn't, instead __builtin_is_constant_evaluated() will continue to get folded during gimplification. I thought folding it might not be benefical with this approach, but on second thought we should do it for consistency at least. The following incremental patch seems to work: diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc index dca55056b2c..250147bde0f 100644 --- a/gcc/cp/cp-gimplify.cc +++ b/gcc/cp/cp-gimplify.cc @@ -3124,6 +3124,14 @@ maybe_fold_constexpr_call (tree x, mce_value manifestly_const_eval) if (TREE_CODE (r) != CALL_EXPR) return r; } + if (fndecl_built_in_p (callee, CP_BUILT_IN_IS_CONSTANT_EVALUATED, + BUILT_IN_FRONTEND)) + { + if (manifestly_const_eval == mce_true) + return boolean_true_node; + else if (manifestly_const_eval == mce_false) + return boolean_false_node; + } return x; } > > > optimize = sv; > > if (TREE_CODE (r) != CALL_EXPR) > > @@ -3096,6 +3102,31 @@ cp_fold (tree x) > > return x; > > } > > +/* If the CALL_EXPR X calls a constexpr function, try expanding it via > > + constexpr evaluation. Returns the expanded result or X if constexpr > > + evaluation wasn't possible. > > + > > + TODO: Do constexpr expansion of expressions where the call itself > > + is not constant, but the call followed by an INDIRECT_REF is. */ > > + > > +static tree > > +maybe_fold_constexpr_call (tree x, mce_value manifestly_const_eval) > > +{ > > + if (flag_no_inline) > > + return x; > > + tree callee = get_callee_fndecl (x); > > + if (!callee) > > + return x; > > + if (DECL_DECLARED_CONSTEXPR_P (callee)) > > + { > > + tree r = maybe_constant_value (x, /*decl=*/NULL_TREE, > > + manifestly_const_eval); > > + if (TREE_CODE (r) != CALL_EXPR) > > + return r; > > + } > > + return x; > > +} > > + > > /* Look up "hot", "cold", "likely" or "unlikely" in attribute list LIST. > > */ > > tree > > diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated1.C > > b/gcc/testsuite/g++.dg/opt/is_constant_evaluated1.C > > new file mode 100644 > > index 00000000000..2123f20e3e5 > > --- /dev/null > > +++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated1.C > > @@ -0,0 +1,20 @@ > > +// PR c++/108243 > > +// { dg-do compile { target c++11 } } > > +// { dg-additional-options "-O -fdump-tree-original" } > > + > > +struct A { > > + constexpr A(int n, int m) : n(n), m(m) { } > > + int n, m; > > +}; > > + > > +constexpr int foo(int n) { > > + return n + !__builtin_is_constant_evaluated(); > > +} > > + > > +A* f(int n) { > > + static A a = {n, foo(41)}; > > + return &a; > > +} > > + > > +// { dg-final { scan-tree-dump "42" "original" } } > > +// { dg-final { scan-tree-dump-not "foo \\(41\\)" "original" } } > > diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated2.C > > b/gcc/testsuite/g++.dg/opt/is_constant_evaluated2.C > > new file mode 100644 > > index 00000000000..ed964e20a7a > > --- /dev/null > > +++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated2.C > > @@ -0,0 +1,32 @@ > > +// PR c++/97553 > > +// { dg-do compile { target c++11 } } > > +// { dg-additional-options "-O -fdump-tree-original" } > > + > > +constexpr int foo() { > > + return __builtin_is_constant_evaluated() + 1; > > +} > > + > > +#if __cpp_if_consteval > > +constexpr int bar() { > > + if consteval { > > + return 5; > > + } else { > > + return 4; > > + } > > +} > > +#endif > > + > > +int p, q; > > + > > +int main() { > > + p = foo(); > > +#if __cpp_if_consteval > > + q = bar(); > > +#endif > > +} > > + > > +// { dg-final { scan-tree-dump "p = 1" "original" } } > > +// { dg-final { scan-tree-dump-not "= foo" "original" } } > > + > > +// { dg-final { scan-tree-dump "q = 4" "original" { target c++23 } } } > > +// { dg-final { scan-tree-dump-not "= bar" "original" { target c++23 } } } > >