On Fri, 27 Jan 2023, 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're sure we're done with manifestly constant evaluation" rather
> 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.
> <case CALL_EXPR>: 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<tree> 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<tree, tree>::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<constructor_elt, va_gc> *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" } }
> --
> 2.39.1.348.g5dec958dcf
>
>
