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Re: Tree CONSTRUCTORs and Ada array indices


> I'm having fun with arrays in Ada ;) and wondering if anyone can tell me
> what's right here.

No surprise, arrays are probably the base types for which there is the largest 
gap between Ada and the C family of languages, for which GCC was written.
You can see in the E_Array_Subtype case of gnat_to_gnu_entity the hoops we 
have to jump through to make them work.

> In GCC this gets represented as an ARRAY_TYPE, whose TYPE_DOMAIN is a 32-bit
> signed integer type; the TYPE_MAX_VALUE of this is 1 (as a size_int), the
> TYPE_MIN_VALUE of this is -1 as a size_int, i.e. 4294967295. I believe
> using (unsigned 32-bit) size_int for min/max is correct (?) despite the
> domain being a signed type.

Historically it was required that TYPE_DOMAIN be a subtype of 'sizetype', see 
build_index_type.  The Ada compiler was originally using a signed 'sizetype' 
because of this (all the other compilers were using an unsigned one) but a 
couple of changes (POINTER_PLUS_EXPR and LTO) essentially forced a uniform 
'sizetype' across languages and the Ada compiler wasn't the winner. ;-)

> An array of this type is then initialized with a CONSTRUCTOR. The
> CONSTRUCTOR has three elements, with INTEGER_CST indices, being in order
> 2^32-1, 0, 1 (all of sizetype).
> 
> I substitute the CONSTRUCTOR into an ARRAY_REF <blah>[4294967295]{lb:
> 4294967295 sz: 2}, i.e. that picks the element with index (sizetype)2^32-1.
> 
> fold() in fold-const.c then fails to fold, because it does binary search for
> 2^32-1 through the constructor elements, and first checks against the
> middle CONSTRUCTOR_ELT, which has index 0.

IIRC I wrote this code (before the signedness change).

> So, where's the bug here? Should all these indices have sizetype? Should
> fold-const.c's binary search respect the TYPE_DOMAIN of the type of the
> CONSTRUCTOR being searched? (Lots of build_int_cst to reinterpret the
> CONSTRUCTOR_ELT indices, and/or the index being searched for, in said
> TYPE_DOMAIN ?)

According to Richard, we should now be able to set TYPE_DOMAIN to something 
else than a subtype of 'sizetype' but this is a mid-term goal and may require 
some substantial work.  The short-term solution is probably to do nothing, 
arrays with negative indices are not very common in real life even in Ada.

-- 
Eric Botcazou


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