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Re: C++ PATCH for c++/91346 - Implement C++2a P1668R1, allow unevaluated asm in constexpr


On Wed, Aug 07, 2019 at 12:23:25PM -0400, Jason Merrill wrote:
> On Wed, Aug 7, 2019, 9:11 AM Marek Polacek <polacek@redhat.com> wrote:
> 
> > On Tue, Aug 06, 2019 at 09:25:45PM -0400, Jason Merrill wrote:
> > > Let's downgrade the errors in earlier standard modes to pedwarn. Ok with
> > > that change.
> >
> > Works for me, here's what I'll apply once it passes testing.
> >
> > I removed the diagnostic in potential_constant_expression_1/ASM_EXPR so
> > that
> > we don't generate duplicate pedwarns for the same thing.  Hope that's OK.
> >
> > 2019-08-07  Marek Polacek  <polacek@redhat.com>
> >
> >         PR c++/91346 - Implement P1668R1, allow unevaluated asm in
> > constexpr.
> >         * constexpr.c (cxx_eval_constant_expression): Handle ASM_EXPR.
> >         (potential_constant_expression_1) <case ASM_EXPR>: Allow.
> >         * cp-tree.h (finish_asm_stmt): Adjust.
> >         * parser.c (cp_parser_asm_definition): Grab the locaion of "asm"
> > and
> >         use it.  Change an error to a pedwarn.  Allow asm in C++2a, warn
> >         otherwise.
> >         * pt.c (tsubst_expr): Pass a location down to finish_asm_stmt.
> >         * semantics.c (finish_asm_stmt): New location_t parameter.  Use it.
> >
> >         * g++.dg/cpp2a/inline-asm1.C: New test.
> >         * g++.dg/cpp2a/inline-asm2.C: New test.
> >         * g++.dg/cpp1y/constexpr-neg1.C: Adjust dg-error.
> >
> > diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> > index 36a66337433..e86b0789b84 100644
> > --- gcc/cp/constexpr.c
> > +++ gcc/cp/constexpr.c
> > @@ -5289,6 +5289,18 @@ cxx_eval_constant_expression (const constexpr_ctx
> > *ctx, tree t,
> >        r = void_node;
> >        break;
> >
> > +    case ASM_EXPR:
> > +      if (!ctx->quiet)
> > +       {
> > +         error_at (cp_expr_loc_or_input_loc (t),
> > +                   "inline assembly is not a constant expression");
> > +         inform (cp_expr_loc_or_input_loc (t),
> > +                 "only unevaluated inline assembly is allowed in a "
> > +                 "%<constexpr%> function in C++2a");
> > +       }
> > +      *non_constant_p = true;
> > +      return t;
> > +
> >      default:
> >        if (STATEMENT_CODE_P (TREE_CODE (t)))
> >         {
> > @@ -6469,13 +6481,18 @@ potential_constant_expression_1 (tree t, bool
> > want_rval, bool strict, bool now,
> >        /* GCC internal stuff.  */
> >      case VA_ARG_EXPR:
> >      case TRANSACTION_EXPR:
> > -    case ASM_EXPR:
> >      case AT_ENCODE_EXPR:
> >      fail:
> >        if (flags & tf_error)
> >         error_at (loc, "expression %qE is not a constant expression", t);
> >        return false;
> >
> > +    case ASM_EXPR:
> > +      /* In C++2a, unevaluated inline assembly is permitted in constexpr
> > +        functions.  If it's used in earlier standard modes, we pedwarn in
> > +        cp_parser_asm_definition.  */
> > +      return true;
> >
> 
> Actually, do we need this change? If it's (possibly) unevaluated, we
> shouldn't get here.

We can get here when using asm() in ({ }) like this (ugh):

constexpr int
foo (bool b)
{
  if (b)
   {
     constexpr int i = ({ asm(""); 42; });
     return i;
    }
  else
    return 42;
}
static_assert(foo(false) == 42, "");

With the current state of potential_constant_expression_1, we generate

inline-asm3.C: In function ‘constexpr int foo(bool)’:
inline-asm3.C:10:27: error: inline assembly is not a constant expression
   10 |      constexpr int i = ({ asm(""); 42; });
      |                           ^~~
inline-asm3.C:10:27: note: only unevaluated inline assembly is allowed in a ‘constexpr’ function in C++2a

which I thought was better than what we emit with the hunk revered:

inline-asm3.C: In function ‘constexpr int foo(bool)’:
inline-asm3.C:10:27: error: expression ‘<statement>’ is not a constant expression
   10 |      constexpr int i = ({ asm(""); 42; });
      |                           ^~~

But I'm happy to revert that hunk if you want.

Marek


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