[REPOST] Invalid Code when reading from unaligned zero-sized array

[Eric Botcazou]

> What we support is out of bounds accesses for heap vars if the var's type
> has flexible array member or something we treat similarly and there is the
> possibility that there could be payload after the heap var that could be 
> accessed from the flexible array members or similar arrays.

My question was about the above similar arrays, i.e. whether we consider all 
trailing arrays in structures as flexible-like or not.  No strong opinion.

> So, I don't see what is the big deal with BLKmode, because all the cases
> which actually could have flexible array member extra payloads (or similar)
> must necessarily live in memory, if it is the compiler that decides whether
> to put it into memory or keep in registers etc., then it can't be heap
> allocated.

The invariant is that types for which objects can effectively have variable 
size must have BLKmode, otherwise you need to add very ugly code in the RTL 
expander to mask the lie.

-- 
Eric Botcazou