[PATCH] c++: __builtin_bit_cast To C array target type [PR103140]

Jason Merrill jason@redhat.com
Thu Nov 25 15:57:36 GMT 2021


On 11/8/21 15:03, Will Wray via Gcc-patches wrote:
> This patch allows __builtin_bit_cast to materialize a C array as its To type.
> 
> It was developed as part of an implementation of P1997, array copy-semantics,
> but is independent, so makes sense to submit, review and merge ahead of it.
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.c (check_bit_cast_type): handle ARRAY_TYPE check,
> 	(cxx_eval_bit_cast): handle ARRAY_TYPE copy.
> 	* semantics.c (cp_build_bit_cast): warn only on unbounded/VLA.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/bit-cast2.C: update XFAIL tests.
> 	* g++.dg/cpp2a/bit-cast-to-array1.C: New test.
> ---
>   gcc/cp/constexpr.c                              |  8 ++++-
>   gcc/cp/semantics.c                              |  7 ++---
>   gcc/testsuite/g++.dg/cpp2a/bit-cast-to-array1.C | 40 +++++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/bit-cast2.C          |  8 ++---
>   4 files changed, 53 insertions(+), 10 deletions(-)
> 
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index 453007c686b..be1cdada6f8 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -4124,6 +4124,11 @@ static bool
>   check_bit_cast_type (const constexpr_ctx *ctx, location_t loc, tree type,
>   		     tree orig_type)
>   {
> +  if (TREE_CODE (type) == ARRAY_TYPE)
> +      return check_bit_cast_type (ctx, loc,
> +				  TYPE_MAIN_VARIANT (TREE_TYPE (type)),
> +				  orig_type);
> +
>     if (TREE_CODE (type) == UNION_TYPE)
>       {
>         if (!ctx->quiet)
> @@ -4280,7 +4285,8 @@ cxx_eval_bit_cast (const constexpr_ctx *ctx, tree t, bool *non_constant_p,
>     tree r = NULL_TREE;
>     if (can_native_interpret_type_p (TREE_TYPE (t)))
>       r = native_interpret_expr (TREE_TYPE (t), ptr, len);
> -  else if (TREE_CODE (TREE_TYPE (t)) == RECORD_TYPE)
> +  else if (TREE_CODE (TREE_TYPE (t)) == RECORD_TYPE
> +	   || TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
>       {
>         r = native_interpret_aggregate (TREE_TYPE (t), ptr, 0, len);
>         if (r != NULL_TREE)
> diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
> index 2443d032749..b3126b12abc 100644
> --- a/gcc/cp/semantics.c
> +++ b/gcc/cp/semantics.c
> @@ -11562,13 +11562,10 @@ cp_build_bit_cast (location_t loc, tree type, tree arg,
>       {
>         if (!complete_type_or_maybe_complain (type, NULL_TREE, complain))
>   	return error_mark_node;
> -      if (TREE_CODE (type) == ARRAY_TYPE)
> +      if (TREE_CODE (type) == ARRAY_TYPE && !TYPE_DOMAIN (type))
>   	{
> -	  /* std::bit_cast for destination ARRAY_TYPE is not possible,
> -	     as functions may not return an array, so don't bother trying
> -	     to support this (and then deal with VLAs etc.).  */
>   	  error_at (loc, "%<__builtin_bit_cast%> destination type %qT "
> -			 "is an array type", type);
> +			 "is a VLA variable-length array type", type);

Null TYPE_DOMAIN doesn't mean VLA, it means unknown length.

Probably better to check for null or non-constant TYPE_SIZE rather than 
specifically for VLA.

>   	  return error_mark_node;
>   	}
>         if (!trivially_copyable_p (type))
> diff --git a/gcc/testsuite/g++.dg/cpp2a/bit-cast-to-array1.C b/gcc/testsuite/g++.dg/cpp2a/bit-cast-to-array1.C
> new file mode 100644
> index 00000000000..e6e50c06389
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/bit-cast-to-array1.C
> @@ -0,0 +1,40 @@
> +// { dg-do compile }
> +
> +class S { int s; };
> +S s();
> +class U { int a, b; };
> +U u();
> +
> +void
> +foo (int *q)
> +{
> +  __builtin_bit_cast (int [1], 0);
> +  __builtin_bit_cast (S [1], 0);
> +  __builtin_bit_cast (U [1], u);
> +}
> +
> +template <int N>
> +void
> +bar (int *q)
> +{
> +  int intN[N] = {};
> +  int int2N[2*N] = {};
> +  __builtin_bit_cast (int [N], intN);
> +  __builtin_bit_cast (S [N], intN);
> +  __builtin_bit_cast (U [N], int2N);
> +}
> +
> +template <typename T1, typename T2, typename T3>
> +void
> +baz (T1 ia, T2 sa, T3 ua)
> +{
> +  __builtin_bit_cast (T1, *ia);
> +  __builtin_bit_cast (T2, *sa);
> +  __builtin_bit_cast (T3, *ua);
> +}
> +
> +void
> +qux (S* sp, int *ip, U* up)
> +{
> +  baz <int[1], S[1], U[1]> (ip, sp, up);
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/bit-cast2.C b/gcc/testsuite/g++.dg/cpp2a/bit-cast2.C
> index 6bb1760e621..7f1836ee4e9 100644
> --- a/gcc/testsuite/g++.dg/cpp2a/bit-cast2.C
> +++ b/gcc/testsuite/g++.dg/cpp2a/bit-cast2.C
> @@ -14,7 +14,7 @@ foo (int *q)
>     __builtin_bit_cast (int, s);		// { dg-error "'__builtin_bit_cast' source type 'S' is not trivially copyable" }
>     __builtin_bit_cast (S, 0);		// { dg-error "'__builtin_bit_cast' destination type 'S' is not trivially copyable" }
>     __builtin_bit_cast (int &, q);	// { dg-error "'__builtin_bit_cast' destination type 'int&' is not trivially copyable" }
> -  __builtin_bit_cast (int [1], 0);	// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is an array type" }
> +  __builtin_bit_cast (S [1], 0);	// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is not trivially copyable" }
>     __builtin_bit_cast (V, 0);		// { dg-error "invalid use of incomplete type 'struct V'" }
>     __builtin_bit_cast (int, v);
>     __builtin_bit_cast (int, *p);		// { dg-error "invalid use of incomplete type 'struct V'" }
> @@ -29,7 +29,7 @@ bar (int *q)
>     __builtin_bit_cast (int, s);		// { dg-error "'__builtin_bit_cast' source type 'S' is not trivially copyable" }
>     __builtin_bit_cast (S, 0);		// { dg-error "'__builtin_bit_cast' destination type 'S' is not trivially copyable" }
>     __builtin_bit_cast (int &, q);	// { dg-error "'__builtin_bit_cast' destination type 'int&' is not trivially copyable" }
> -  __builtin_bit_cast (int [1], 0);	// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is an array type" }
> +  __builtin_bit_cast (S [1], 0);	// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is not trivially copyable" }
>     __builtin_bit_cast (V, 0);		// { dg-error "invalid use of incomplete type 'struct V'" }
>     __builtin_bit_cast (int, *p);		// { dg-error "invalid use of incomplete type 'struct V'" }
>     __builtin_bit_cast (U, 0);		// { dg-error "'__builtin_bit_cast' source size '\[0-9]*' not equal to destination type size '\[0-9]*'" }
> @@ -43,7 +43,7 @@ baz (T3 s, T4 *p, T1 *q)
>     __builtin_bit_cast (int, s);		// { dg-error "'__builtin_bit_cast' source type 'S' is not trivially copyable" }
>     __builtin_bit_cast (T3, 0);		// { dg-error "'__builtin_bit_cast' destination type 'S' is not trivially copyable" }
>     __builtin_bit_cast (T1 &, q);		// { dg-error "'__builtin_bit_cast' destination type 'int&' is not trivially copyable" }
> -  __builtin_bit_cast (T2, 0);		// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is an array type" }
> +  __builtin_bit_cast (T2, 0);		// { dg-error "'__builtin_bit_cast' destination type \[^\n\r]* is not trivially copyable" }
>     __builtin_bit_cast (T4, 0);		// { dg-error "invalid use of incomplete type 'struct V'" }
>     __builtin_bit_cast (int, *p);		// { dg-error "invalid use of incomplete type 'struct V'" }
>     __builtin_bit_cast (U, (T1) 0);	// { dg-error "'__builtin_bit_cast' source size '\[0-9]*' not equal to destination type size '\[0-9]*'" }
> @@ -53,5 +53,5 @@ baz (T3 s, T4 *p, T1 *q)
>   void
>   qux (int *q)
>   {
> -  baz <int, int [1], S, V> (s, p, q);
> +  baz <int, S [1], S, V> (s, p, q);
>   }
> 



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