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

[John McCall via cfe-dev]

On 13 Jun 2021, at 11:26, Ralf Jung wrote:

> Hi Johannes,
>
>>> I think Joshua gave a very nice motivation already.
>>
>> I don't dispute that but I am still not understanding the need for 
>> bytes. None of the examples I have seen so far
>> clearly made the point that it is the byte types that provide a 
>> substantial benefit. The AA example below does neither.
>
> I hope 
> <https://lists.llvm.org/pipermail/llvm-dev/2021-June/151110.html> 
> makes a convincing case that under the current semantics, when one 
> does an "i64" load of a value that was stored at pointer type, we have 
> to say that this load returns poison. In particular, saying that this 
> implicitly performs a "ptrtoint" is inconsistent with optimizations 
> that are probably too important to be changed to accommodate this 
> implicit "ptrtoint".

I think it is fact rather obvious that, if this optimization as 
currently written is indeed in conflict with the current semantics, it 
is the optimization that will have to give.  If the optimization is too 
important for performance to give up entirely, we will simply have to 
find some more restricted pattern that wee can still soundly optimize.

Perhaps the clearest reason is that, if we did declare that integer 
types cannot carry pointers and so introduced byte types that could, C 
frontends would have to switch to byte types for their integer types, 
and so we would immediately lose this supposedly important optimization 
for C-like languages, and so, since optimizing C is very important, we 
would immediately need to find some restricted pattern under which we 
could soundly apply this optimization to byte types.  That’s assuming 
that this optimization is actually significant, of course.

John.