On Wed, Aug 07, 2019 at 03:38:02PM -0400, Jason Merrill wrote:
> On Wed, Aug 7, 2019, 2:35 PM Marek Polacek <pola...@redhat.com> wrote:
>
> > On Wed, Aug 07, 2019 at 12:23:25PM -0400, Jason Merrill wrote:
> > > On Wed, Aug 7, 2019, 9:11 AM Marek Polacek <pola...@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 <pola...@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.
> >
>
> Sure, perhaps we could share the error message between the two functions.
> In general it's better to reject something in potential_... if we can.
Yeah, that makes sense. How about this further tweak then?
2019-08-07 Marek Polacek <pola...@redhat.com>
* constexpr.c (inline_asm_in_constexpr_error): New.
(cxx_eval_constant_expression) <case ASM_EXPR>: Call it.
(potential_constant_expression_1) <case ASM_EXPR>: Likewise.
* g++.dg/cpp2a/inline-asm3.C: New test.
diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index e86b0789b84..63c890560e0 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -4411,6 +4411,16 @@ lookup_placeholder (const constexpr_ctx *ctx, bool lval,
tree type)
return ob;
}
+/* Complain about an attempt to evaluate inline assembly. */
+
+static void
+inline_asm_in_constexpr_error (location_t loc)
+{
+ error_at (loc, "inline assembly is not a constant expression");
+ inform (loc, "only unevaluated inline assembly is allowed in a "
+ "%<constexpr%> function in C++2a");
+}
+
/* Attempt to reduce the expression T to a constant value.
On failure, issue diagnostic and return error_mark_node. */
/* FIXME unify with c_fully_fold */
@@ -5291,13 +5301,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx,
tree t,
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");
- }
+ inline_asm_in_constexpr_error (cp_expr_loc_or_input_loc (t));
*non_constant_p = true;
return t;
@@ -6488,10 +6492,9 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
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;
+ if (flags & tf_error)
+ inline_asm_in_constexpr_error (loc);
+ return false;
case OBJ_TYPE_REF:
if (cxx_dialect >= cxx2a)
diff --git gcc/testsuite/g++.dg/cpp2a/inline-asm3.C
gcc/testsuite/g++.dg/cpp2a/inline-asm3.C
new file mode 100644
index 00000000000..b636e3b2d87
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/inline-asm3.C
@@ -0,0 +1,12 @@
+// P1668R1: Permit unevaluated inline asm in constexpr functions
+// { dg-do compile { target c++2a } }
+// { dg-additional-options "-Wno-pedantic" }
+
+constexpr int
+foo ()
+{
+ constexpr int i = ({ asm(""); 42; }); // { dg-error "inline assembly is not a
constant expression" }
+ return i;
+}
+
+constexpr int i = foo ();
