[Bug target/63764] [5 Regression] ICE: in verify_ssa, at tree-ssa.c:939

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63764

--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Tue, 18 Nov 2014, jakub at gcc dot gnu.org wrote:

> > Maybe build_array_ref needs to ensure that references to elements of 
> > non-lvalue vectors don't become lvalues?  (This would be different to 
> > non-lvalue arrays arising from non-lvalue structs and unions, where the 
> > result of an array reference *is* an lvalue but modifying it, or accessing 
> > it after the next sequence point, has undefined behavior.)
> 
> Apparently I misremembered it, trying it again, I see the VCE still there in
> *vecp upon entry to convert_vector_to_pointer_for_subscript, the difference
> between VIEW_CONVERT_EXPR and NON_LVALUE_EXPR is just that
> c_common_mark_addressable_vec looks through VCE and marks the inner part as
> addressable (because VCE is handled_component_p), while for non_lvalue_expr it
> does not.
> 
> So, shall we convert_vector_to_pointer_for_subscript add something like
>   if (!lvalue_p (*vecp))
>     error_at (...); ?
> What wording for it?

I'd think it's reasonable to subscript a non-lvalue vector (including e.g. 
the return value of a function with vector return type) - this might of 
course need to create a temporary object, as with non-lvalue arrays.