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

[Hal Finkel via cfe-dev]

On 6/6/21 00:26, Chris Lattner via cfe-dev wrote:
> On Jun 4, 2021, at 11:25 AM, John McCall via cfe-dev 
> <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.
>
> -Chris


I'll take this opportunity to point out that, at least historically, the 
reason why a desire to optimize around ptrtoint keeps resurfacing is 
because:

  1. Common optimizations introduce them into code that did not 
otherwise have them (SROA, for example, see convertValue in SROA.cpp).

  2. They're generated by some of the ABI code for argument passing (see 
clang/lib/CodeGen/TargetInfo.cpp).

  3. They're present in certain performance-sensitive code idioms (see, 
for example, ADT/PointerIntPair.h).

It seems to me that, if there's design work to do in this area, one 
should consider addressing these now-long-standing issues where we 
introduce ptrtoint by replacing this mechanism with some other one.

  -Hal

>
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