[llvm-dev] [cfe-dev] [RFC] Introducing a byte type to LLVM

[Philip Reames via llvm-dev]

On 6/5/21 9:26 PM, Chris Lattner via llvm-dev wrote:
> On Jun 4, 2021, at 11:25 AM, John McCall via cfe-dev 
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:On 4 
> Jun 2021, at 11:24, George Mitenkov wrote:
>>
>>     Hi all,
>>
>>     Together with Nuno Lopes and Juneyoung Lee we propose to add a
>>     new byte
>>     type to LLVM to fix miscompilations due to load type punning.
>>     Please see
>>     the proposal below. It would be great to hear the
>>     feedback/comments/suggestions!
>>
>>
>>     Motivation
>>     ==========
>>
>>     char and unsigned char are considered to be universal holders in
>>     C. They
>>     can access raw memory and are used to implement memcpy. i8 is the
>>     LLVM’s
>>     counterpart but it does not have such semantics, which is also not
>>     desirable as it would disable many optimizations.
>>
>> I don’t believe this is correct. LLVM does not have an innate
>> concept of typed memory. The type of a global or local allocation
>> is just a roundabout way of giving it a size and default alignment,
>> and similarly the type of a load or store just determines the width
>> and default alignment of the access. There are no restrictions on
>> what types can be used to load or store from certain objects.
>>
>> C-style type aliasing restrictions are imposed using |tbaa|
>> metadata, which are unrelated to the IR type of the access.
>>
> I completely agree with John.  “i8” in LLVM doesn’t carry any 
> implications about aliasing (in fact, LLVM pointers are going towards 
> being typeless).  Any such thing occurs at the accesses, and are part 
> of TBAA.
>
> I’m opposed to adding a byte type to LLVM, as such semantic carrying 
> types are entirely unprecedented, and would add tremendous complexity 
> to the entire system.

I agree with both John and Chris here.

I've read through the discussion in this thread, and have yet to be 
convinced there is a problem, much less than this is a good solution.  
I'm open to being convinced of those two things, but the writeup in this 
thread doesn't do it.  There's snippet of examples downthread which 
might be convincing, but there's objections raised around language 
semantics which I find very hard to follow.  The fragmentation of the 
thread really doesn't help.

I would suggest the OP take some of the motivating examples, write up a 
web-page with the examples and their interpretation, then revisit the 
topic.  In particular, I strongly suggest anticipating incorrect 
interpretation/objections and explicitly addressing them.

I'll also note that the use of the term capture w.r.t a *load* 
downthread makes absolutely no sense to me.  Stores capture, not loads.

Philip




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