llvm dev - Nov 2017 - Is it ok to allocate > half of address space?

I'm not a compiler writer, but what about embedded devices with a very small
address space?

This just seems like an arbitrary limit for no real reason, especially when
it's "anchored" by the address space which could be absolutely
anything.

On 27 Nov 2017, at 17:22, Marcus Johnson via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I'm not a compiler writer, but what about embedded devices with a very
small address space?
> 
> This just seems like an arbitrary limit for no real reason, especially when
it's "anchored" by the address space which could be absolutely
anything.
It’s pretty uncommon to find embedded devices where this is a problem.  It’s
basically only an issue on 16-bit microcontrollers with more than 32KB of
memory.  32-bit (e.g. ARM M-profile) microcontrollers are unlikely to have more
than the 2GB of RAM required for this to be an issue.

Even if LLVM allows it, it’s only useful if the source language is not C, where
the size of an object is size_t and the difference between two pointers is
ptrdiff_t.  If size_t is unsigned and ptrdiff_t is the signed version of the
same size type[1] then pointer subtraction of the end and the start of any
object larger than half of the range of size_t will give very surprising
results.

David

[1] C requires that size_t be unsigned and ptrdiff_t be signed, but does not
require them to be the same size.  It would be possible to have a C
implementation with a larger ptrdiff_t than size_t, but I’ve never seen one.