[llvm-dev] RFC: On non 8-bit bytes and the target for it

David Chisnall via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 24 16:02:04 PDT 2019

On 24/10/2019 14:21, JF Bastien via llvm-dev wrote:
> I’d like to understand what programming model you see programmers using. 
> You don’t need 257 bits per byte if you only offer 257 bit integers. 
> Rather, bytes aren’t really a thing at that point. LLVM kinda handles iN 
> already, and your backend would legalize everything to exactly this type 
> and nothing else, right? Would it be sufficient to expose something like 
> int<unsigned Size> with Size=257 for your programming environment?

To add to what JF says:

Typically, a byte means some combination of:

1. The smallest unit that can be indexed in memory (irrelevant for you, 
you have no memory).
2. The smallest unit that can be stored in a register in such a way that 
its representation is opaque to software (i.e. you can't tell the bit 
order of a byte in a multi-byte word).  For you, it's not clear if this 
is 257 bits or something smaller.
3. The smallest unit that is used to build complex types in software. 
Since you have no memory, it's not clear that you can build structs or 
arrays, and therefore this doesn't seem to apply.

 From your description of your VM, it doesn't sound as if you can 
translate from any language with a vaguely C-like abstract machine, so 
I'm not certain why the size of a byte actually matters to you.  LLVM IR 
has a quite C-like abstract machine, and several of these features seem 
like they will be problematic for you.  There is quite a limited subset 
of LLVM IR that can be expressed for your VM and it would be helpful if 
you could enumerate what you expect to be able to support (and why going 
via LLVM is useful, given that you are unlikely to be able to take 
advantage of any existing front ends, many optimisations, or most of the 
target-agnostic code generator.


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