[Bug target/101876] [s390x] vector builtin vec_permi fails to resolve with #pragma GCC target

["mhillen at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101876

--- Comment #3 from Marius Hillenbrand <mhillen at linux dot ibm.com> ---
The issue is caused by inconsistent alignment of vector_types between the types
(a) expected or returned by builtin functions and (b) the typedef in the
example code. In the failing cases, there's a mismatch of 16-Byte and 8-Byte
(compliant with the ABI) alignment. In the successful cases, either both use
16-Byte or 8-Byte alignment.

When gcc starts, s390_init_builtins defines builtin functions with their
corresponding type information. That includes vector builtins. While creating
the builtin types for these builtins, when gcc starts with -march < z13, then
s390_vector_alignment defers to default_vector_alignment, which results in
natural alignment. That results in 16-Byte alignment of the 16-Byte vectors.
Once the pragma switched to an arch level with VX support (TARGET_VX and
TARGET_VX_ABI are true), then s390_vector_alignment selects 8-Byte alignment in
accordance with the vector ABI.

The C++ example (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101877) ICEs
because two structurally identical vector types have different canonical types
(as a result of the different alignment?!).