On Wed, 29 Mar 2023, Nathaniel Shead via Gcc-patches wrote:
> This adds rudimentary lifetime tracking in C++ constexpr contexts,
> allowing the compiler to report errors with using values after their
> backing has gone out of scope. We don't yet handle other ways of ending
> lifetimes (e.g. explicit destructor calls).
Awesome!
>
> PR c++/96630
> PR c++/98675
> PR c++/70331
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (constexpr_global_ctx::put_value): Mark value as
> in lifetime.
> (constexpr_global_ctx::remove_value): Mark value as expired.
> (cxx_eval_call_expression): Remove comment that is no longer
> applicable.
> (non_const_var_error): Add check for expired values.
> (cxx_eval_constant_expression): Add checks for expired values. Forget
> local variables at end of bind expressions. Forget temporaries at end
> of cleanup points.
> * cp-tree.h (struct lang_decl_base): New flag to track expired values
> in constant evaluation.
> (DECL_EXPIRED_P): Access the new flag.
> (SET_DECL_EXPIRED_P): Modify the new flag.
> * module.cc (trees_out::lang_decl_bools): Write out the new flag.
> (trees_in::lang_decl_bools): Read in the new flag.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/constexpr-ice20.C: Update error raised by test.
> * g++.dg/cpp1y/constexpr-lifetime1.C: New test.
> * g++.dg/cpp1y/constexpr-lifetime2.C: New test.
> * g++.dg/cpp1y/constexpr-lifetime3.C: New test.
> * g++.dg/cpp1y/constexpr-lifetime4.C: New test.
> * g++.dg/cpp1y/constexpr-lifetime5.C: New test.
>
> Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
> ---
> gcc/cp/constexpr.cc | 69 +++++++++++++++----
> gcc/cp/cp-tree.h | 10 ++-
> gcc/cp/module.cc | 2 +
> gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C | 2 +-
> .../g++.dg/cpp1y/constexpr-lifetime1.C | 13 ++++
> .../g++.dg/cpp1y/constexpr-lifetime2.C | 20 ++++++
> .../g++.dg/cpp1y/constexpr-lifetime3.C | 13 ++++
> .../g++.dg/cpp1y/constexpr-lifetime4.C | 11 +++
> .../g++.dg/cpp1y/constexpr-lifetime5.C | 11 +++
> 9 files changed, 137 insertions(+), 14 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime1.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime2.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime3.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime4.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 3de60cfd0f8..bdbc12144a7 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -1185,10 +1185,22 @@ public:
> void put_value (tree t, tree v)
> {
> bool already_in_map = values.put (t, v);
> + if (!already_in_map && DECL_P (t))
> + {
> + if (!DECL_LANG_SPECIFIC (t))
> + retrofit_lang_decl (t);
> + if (DECL_LANG_SPECIFIC (t))
> + SET_DECL_EXPIRED_P (t, false);
> + }
Since this new flag would only be used only during constexpr evaluation,
could we instead use an on-the-side hash_set in constexpr_global_ctx for
tracking expired-ness? That way we won't have to allocate a
DECL_LANG_SPECIFIC structure for decls that lack it, and won't have to
worry about the flag in other parts of the compiler.
> if (!already_in_map && modifiable)
> modifiable->add (t);
> }
> - void remove_value (tree t) { values.remove (t); }
> + void remove_value (tree t)
> + {
> + if (DECL_P (t) && DECL_LANG_SPECIFIC (t))
> + SET_DECL_EXPIRED_P (t, true);
> + values.remove (t);
> + }
> };
>
> /* Helper class for constexpr_global_ctx. In some cases we want to avoid
> @@ -3157,10 +3169,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> for (tree save_expr : save_exprs)
> ctx->global->remove_value (save_expr);
>
> - /* Remove the parms/result from the values map. Is it worth
> - bothering to do this when the map itself is only live for
> - one constexpr evaluation? If so, maybe also clear out
> - other vars from call, maybe in BIND_EXPR handling? */
> + /* Remove the parms/result from the values map. */
> ctx->global->remove_value (res);
> for (tree parm = parms; parm; parm = TREE_CHAIN (parm))
> ctx->global->remove_value (parm);
> @@ -5708,6 +5717,13 @@ non_const_var_error (location_t loc, tree r, bool
> fundef_p)
> inform (DECL_SOURCE_LOCATION (r), "allocated here");
> return;
> }
> + if (DECL_EXPIRED_P (r))
> + {
> + if (constexpr_error (loc, fundef_p, "accessing object outside its "
> + "lifetime"))
> + inform (DECL_SOURCE_LOCATION (r), "declared here");
> + return;
> + }
> if (!constexpr_error (loc, fundef_p, "the value of %qD is not usable in "
> "a constant expression", r))
> return;
> @@ -7048,6 +7064,13 @@ cxx_eval_constant_expression (const constexpr_ctx
> *ctx, tree t,
> r = build_constructor (TREE_TYPE (t), NULL);
> TREE_CONSTANT (r) = true;
> }
> + else if (DECL_EXPIRED_P (t))
> + {
> + if (!ctx->quiet)
> + non_const_var_error (loc, r, /*fundef_p*/false);
> + *non_constant_p = true;
> + break;
> + }
> else if (ctx->strict)
> r = decl_really_constant_value (t, /*unshare_p=*/false);
> else
> @@ -7093,7 +7116,15 @@ cxx_eval_constant_expression (const constexpr_ctx
> *ctx, tree t,
> else
> {
> if (!ctx->quiet)
> - error ("%qE is not a constant expression", t);
> + {
> + if (DECL_EXPIRED_P (r))
> + {
> + error_at (loc, "accessing object outside its lifetime");
> + inform (DECL_SOURCE_LOCATION (r), "declared here");
> + }
> + else
> + error_at (loc, "%qE is not a constant expression", t);
> + }
> *non_constant_p = true;
> }
> break;
> @@ -7315,17 +7346,28 @@ cxx_eval_constant_expression (const constexpr_ctx
> *ctx, tree t,
> auto_vec<tree, 2> cleanups;
> vec<tree> *prev_cleanups = ctx->global->cleanups;
> ctx->global->cleanups = &cleanups;
> - r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0),
> +
> + auto_vec<tree, 10> save_exprs;
> + constexpr_ctx new_ctx = *ctx;
> + new_ctx.save_exprs = &save_exprs;
> +
> + r = cxx_eval_constant_expression (&new_ctx, TREE_OPERAND (t, 0),
> lval,
> non_constant_p, overflow_p,
> jump_target);
> +
> ctx->global->cleanups = prev_cleanups;
> unsigned int i;
> tree cleanup;
> /* Evaluate the cleanups. */
> FOR_EACH_VEC_ELT_REVERSE (cleanups, i, cleanup)
> - cxx_eval_constant_expression (ctx, cleanup, vc_discard,
> + cxx_eval_constant_expression (&new_ctx, cleanup, vc_discard,
> non_constant_p, overflow_p);
> +
> + /* Forget SAVE_EXPRs and TARGET_EXPRs created by this
> + full-expression. */
> + for (tree save_expr : save_exprs)
> + ctx->global->remove_value (save_expr);
> }
> break;
>
> @@ -7831,10 +7873,13 @@ cxx_eval_constant_expression (const constexpr_ctx
> *ctx, tree t,
> non_constant_p, overflow_p, jump_target);
>
> case BIND_EXPR:
> - return cxx_eval_constant_expression (ctx, BIND_EXPR_BODY (t),
> - lval,
> - non_constant_p, overflow_p,
> - jump_target);
> + r = cxx_eval_constant_expression (ctx, BIND_EXPR_BODY (t),
> + lval,
> + non_constant_p, overflow_p,
> + jump_target);
> + for (tree decl = BIND_EXPR_VARS (t); decl; decl = DECL_CHAIN (decl))
> + ctx->global->remove_value (decl);
> + return r;
>
> case PREINCREMENT_EXPR:
> case POSTINCREMENT_EXPR:
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index b74c18b03ad..3cc08da816f 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -2860,6 +2860,7 @@ struct GTY(()) lang_decl_base {
> unsigned concept_p : 1; /* applies to vars and functions
> */
> unsigned var_declared_inline_p : 1; /* var */
> unsigned dependent_init_p : 1; /* var */
> + unsigned expired_p : 1; /* var or parm */
>
> /* The following apply to VAR, FUNCTION, TYPE, CONCEPT, & NAMESPACE
> decls. */
> @@ -2871,7 +2872,7 @@ struct GTY(()) lang_decl_base {
> /* VAR_DECL or FUNCTION_DECL has keyed decls. */
> unsigned module_keyed_decls_p : 1;
>
> - /* 12 spare bits. */
> + /* 11 spare bits. */
> };
>
> /* True for DECL codes which have template info and access. */
> @@ -4366,6 +4367,13 @@ get_vec_init_expr (tree t)
> #define SET_DECL_DEPENDENT_INIT_P(NODE, X) \
> (DECL_LANG_SPECIFIC (VAR_DECL_CHECK (NODE))->u.base.dependent_init_p = (X))
>
> +/* Nonzero if NODE is a VAR_DECL, PARM_DECL, or FIELD_DECL that is within
> + its lifetime for constant evaluation purposes. */
> +#define DECL_EXPIRED_P(NODE) \
> + (DECL_LANG_SPECIFIC (NODE) && DECL_LANG_SPECIFIC (NODE)->u.base.expired_p)
> +#define SET_DECL_EXPIRED_P(NODE, X) \
> + (DECL_LANG_SPECIFIC (NODE)->u.base.expired_p = (X))
> +
> /* Nonzero if NODE is an artificial VAR_DECL for a C++17 structured binding
> declaration or one of VAR_DECLs for the user identifiers in it. */
> #define DECL_DECOMPOSITION_P(NODE) \
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index ac2fe66b080..7af43b5736d 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -5654,6 +5654,7 @@ trees_out::lang_decl_bools (tree t)
> WB (lang->u.base.concept_p);
> WB (lang->u.base.var_declared_inline_p);
> WB (lang->u.base.dependent_init_p);
> + WB (lang->u.base.expired_p);
> /* When building a header unit, everthing is marked as purview, (so
> we know which decls to write). But when we import them we do not
> want to mark them as in module purview. */
> @@ -5728,6 +5729,7 @@ trees_in::lang_decl_bools (tree t)
> RB (lang->u.base.concept_p);
> RB (lang->u.base.var_declared_inline_p);
> RB (lang->u.base.dependent_init_p);
> + RB (lang->u.base.expired_p);
> RB (lang->u.base.module_purview_p);
> RB (lang->u.base.module_attach_p);
> if (VAR_OR_FUNCTION_DECL_P (t))
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C
> index e2d4853a284..ebaa95e5324 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C
> @@ -4,4 +4,4 @@
> typedef bool (*Function)(int);
> constexpr bool check(int x, Function p) { return p(x); } // { dg-message
> "in .constexpr. expansion of" }
>
> -static_assert(check(2, check), ""); // { dg-error "conversion|constant|in
> .constexpr. expansion of" }
> +static_assert(check(2, check), ""); // { dg-error
> "conversion|constant|lifetime|in .constexpr. expansion of" }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime1.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime1.C
> new file mode 100644
> index 00000000000..43aa7c974c1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime1.C
> @@ -0,0 +1,13 @@
> +// PR c++/96630
> +// { dg-do compile { target c++14 } }
> +
> +struct S {
> + int x = 0;
> + constexpr const int& get() const { return x; }
> +};
> +
> +constexpr const int& test() {
> + auto local = S{}; // { dg-message "note: declared here" }
> + return local.get();
> +}
> +constexpr int x = test(); // { dg-error "accessing object outside its
> lifetime" }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime2.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime2.C
> new file mode 100644
> index 00000000000..22cd919fcda
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime2.C
> @@ -0,0 +1,20 @@
> +// PR c++/98675
> +// { dg-do compile { target c++14 } }
> +
> +struct S {
> + int x = 0;
> + constexpr const int& get() const { return x; }
> +};
> +
> +constexpr int error() {
> + const auto& local = S{}.get(); // { dg-message "note: declared here" }
> + return local;
> +}
> +constexpr int x = error(); // { dg-error "accessing object outside its
> lifetime" }
> +
> +constexpr int ok() {
> + // temporary should only be destroyed after end of full-expression
> + auto local = S{}.get();
> + return local;
> +}
> +constexpr int y = ok();
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime3.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime3.C
> new file mode 100644
> index 00000000000..6329f8cf6c6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime3.C
> @@ -0,0 +1,13 @@
> +// PR c++/70331
> +// { dg-do compile { target c++14 } }
> +
> +constexpr int f(int i) {
> + int *p = &i;
> + if (i == 0) {
> + int j = 123; // { dg-message "note: declared here" }
> + p = &j;
> + }
> + return *p;
> +}
> +
> +constexpr int i = f(0); // { dg-error "accessing object outside its
> lifetime" }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime4.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime4.C
> new file mode 100644
> index 00000000000..181a1201663
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime4.C
> @@ -0,0 +1,11 @@
> +// { dg-do compile { target c++14 } }
> +
> +constexpr const double& test() {
> + const double& local = 3.0; // { dg-message "note: declared here" }
> + return local;
> +}
> +
> +static_assert(test() == 3.0, ""); // { dg-error "constant|accessing object
> outside its lifetime" }
> +
> +// no deference, shouldn't error
> +static_assert((test(), true), "");
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> new file mode 100644
> index 00000000000..a4bc71d890a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> @@ -0,0 +1,11 @@
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-Wno-return-local-addr" }
> +
> +constexpr const int& id(int x) { return x; }
> +
> +constexpr bool test() {
> + const int& y = id(3);
> + return y == 3;
> +}
> +
> +constexpr bool x = test(); // { dg-error "" }
> --
> 2.34.1
>
>
