c++: Fix up handling of vector CONSTRUCTORs with vectors in it in constexpr.cc [PR104226]

Hi!

The middle-end uses sometimes VECTOR_TYPE CONSTRUCTORs that contain
some other VECTOR_TYPE elements in it (should be with compatible element
size and smaller number of elements, e.g. a V8SImode vector can be
constructed as { V4SImode_var_1, V4SImode_var_2 }), and expansion of
__builtin_shufflevector emits these early, so constexpr.cc can see those
too.
constexpr.cc already has special cases for NULL index which is typical
for VECTOR_TYPE CONSTRUCTORs, and for VECTOR_TYPE CONSTRUCTORs that
contain just scalar elts that works just fine - init_subob_ctx just
returns on non-aggregate elts and get_or_insert_ctor_field has
  if (TREE_CODE (type) == VECTOR_TYPE && index == NULL_TREE)
    {
      CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (ctor), index, NULL_TREE);
      return &CONSTRUCTOR_ELTS (ctor)->last();
    } 
handling for it.  But for the vector in vector case init_subob_ctx would
try to create a sub-CONSTRUCTOR and even didn't handle the NULL index
case well, so instead of creating the sub-CONSTRUCTOR after the elts already
in it overwrote the first one.  So
(V8SImode) { { 0, 0, 0, 0 }, { 0, 0, 0, 0 } }
became
(V8SImode) { 0, 0, 0, 0 }
The following patch fixes it by not forcing a sub-CONSTRUCTOR for this
vector in vector case.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2022-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104226
	* constexpr.cc (init_subob_ctx): For vector ctors containing
	vector elements, ensure appending to the same ctor instead of
	creating another one.

	* g++.dg/cpp0x/constexpr-104226.C: New test.


	Jakub

On 1/26/22 04:33, Jakub Jelinek wrote:
> Hi!
> 
> The middle-end uses sometimes VECTOR_TYPE CONSTRUCTORs that contain
> some other VECTOR_TYPE elements in it (should be with compatible element
> size and smaller number of elements, e.g. a V8SImode vector can be
> constructed as { V4SImode_var_1, V4SImode_var_2 }), and expansion of
> __builtin_shufflevector emits these early, so constexpr.cc can see those
> too.
> constexpr.cc already has special cases for NULL index which is typical
> for VECTOR_TYPE CONSTRUCTORs, and for VECTOR_TYPE CONSTRUCTORs that
> contain just scalar elts that works just fine - init_subob_ctx just
> returns on non-aggregate elts and get_or_insert_ctor_field has
>    if (TREE_CODE (type) == VECTOR_TYPE && index == NULL_TREE)
>      {
>        CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (ctor), index, NULL_TREE);
>        return &CONSTRUCTOR_ELTS (ctor)->last();
>      }
> handling for it.  But for the vector in vector case init_subob_ctx would
> try to create a sub-CONSTRUCTOR and even didn't handle the NULL index
> case well, so instead of creating the sub-CONSTRUCTOR after the elts already
> in it overwrote the first one.  So
> (V8SImode) { { 0, 0, 0, 0 }, { 0, 0, 0, 0 } }
> became
> (V8SImode) { 0, 0, 0, 0 }
> The following patch fixes it by not forcing a sub-CONSTRUCTOR for this
> vector in vector case.

OK.

> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2022-01-26  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/104226
> 	* constexpr.cc (init_subob_ctx): For vector ctors containing
> 	vector elements, ensure appending to the same ctor instead of
> 	creating another one.
> 
> 	* g++.dg/cpp0x/constexpr-104226.C: New test.
> 
> --- gcc/cp/constexpr.cc.jj	2022-01-19 00:42:11.000000000 +0100
> +++ gcc/cp/constexpr.cc	2022-01-25 21:44:28.459208756 +0100
> @@ -4658,6 +4658,13 @@ init_subob_ctx (const constexpr_ctx *ctx
>     if (!AGGREGATE_TYPE_P (type) && !VECTOR_TYPE_P (type))
>       /* A non-aggregate member doesn't get its own CONSTRUCTOR.  */
>       return;
> +  if (VECTOR_TYPE_P (type)
> +      && VECTOR_TYPE_P (TREE_TYPE (ctx->ctor))
> +      && index == NULL_TREE)
> +    /* A vector inside of a vector CONSTRUCTOR, e.g. when a larger
> +       vector is constructed from smaller vectors, doesn't get its own
> +       CONSTRUCTOR either.  */
> +    return;
>   
>     /* The sub-aggregate initializer might contain a placeholder;
>        update object to refer to the subobject and ctor to refer to
> --- gcc/testsuite/g++.dg/cpp0x/constexpr-104226.C.jj	2022-01-25 21:50:34.977031244 +0100
> +++ gcc/testsuite/g++.dg/cpp0x/constexpr-104226.C	2022-01-25 21:51:41.851086559 +0100
> @@ -0,0 +1,15 @@
> +// PR c++/104226
> +// { dg-do compile }
> +// { dg-options "-Wno-psabi" }
> +
> +typedef unsigned short __attribute__((__vector_size__(16))) U;
> +typedef unsigned int __attribute__((__vector_size__(16))) V;
> +typedef unsigned int __attribute__((__vector_size__(32))) W;
> +
> +U
> +foo (void)
> +{
> +  return __builtin_convertvector (__builtin_shufflevector ((V){}, (W){},
> +							   0, 0, 1, 0,
> +							   5, 5, 0, 2), U);
> +}
> 
> 	Jakub
>