[llvm-commits] [llvm] r169218 - in /llvm/trunk: include/llvm/ lib/CodeGen/AsmPrinter/ lib/VMCore/ test/CodeGen/ARM/ test/CodeGen/Thumb/ test/CodeGen/X86/ test/DebugInfo/ test/DebugInfo/X86/ test/JitListener/

[Robinson, Paul]

>> When L == 0 we don't emit the lower bound; this is permissible for
>> languages that default to a lower bound of zero, but not for languages
>> that have other defaults (or no default).  LLVM does support non-C-family
>> languages via Dragonegg; FORTRAN and Ada are especially mentioned on the
>> LLVM home page, and their default lower-bound is 1 not 0.
>>
>> I think Ada supports subrange types directly, not just as array bounds,
>> in which case omitting either bound is incorrect.  I don't know whether
>> Dragonegg supports describing subrange types in DWARF though.
>>
>We do support the lower bound being non-zero. :) Here's the code.

There's a crucial difference between "lower bound is zero" and "lower
bound is the default."  The former is a language-independent statement,
the latter is not.

Ada has a default lower-bound of 1.  But it allows specifying a non-default
lower-bound, e.g. 0.  If my Ada program declares an array with bounds 0..1
what does LLVM do?  It looks to me like it would omit DW_AT_lower_bound,
and supply DW_AT_upper_bound = 1 (because the count is 2).  Knowing that
it's an Ada program, my debugger will implicitly provide a default lower
bound of 1, therefore it will think the array bounds are (1..1) not (0..1).

LLVM just lied to my debugger.  Bad compiler!

Omitting DW_AT_lower_bound is a language-dependent operation.  LLVM
currently is not treating it that way; it is assuming the default
lower bound is 0 for all languages.

--paulr