Re: [PATCH 2/2] c++: speculative constexpr and is_constant_evaluated [PR108243]

[Patrick Palka <ppalka@redhat.com>]

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.
> > > > 	<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" } }
> > > 
> > > 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.
> > 	<case CALL_EXPR>: 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<tree> 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<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;
> >   @@ -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.

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.

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.

-- >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) <case CALL_EXPR>: 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);
 	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 } } }
-- 
2.39.1.418.g7876265d61