[llvm-dev] What about multiple MachineMemOperands in one MI (BranchFolding/MachineInstr::mayAlias)?

[Amara Emerson via llvm-dev]

> On Sep 27, 2019, at 9:28 AM, Philip Reames via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> On 9/27/19 7:33 AM, Matt Arsenault via llvm-dev wrote:
>> 
>> 
>>> On Sep 27, 2019, at 09:07, Björn Pettersson A via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> Obviously we do not store into two locations (it is still a single two byte store).
>>> So is it (always) correct to interpret the list of MachineMemOperands as the instruction will store to one of the locations?
>> 
>> 
>> I think it’s bug to have multiple memory operands if the instruction only accesses one location. The operands should have been merged in some way unless the instruction can truly access two distinct addresses
I would actually expect gather/scatter loads/stores to have multiple MMOs but according to SelectionDAG we generate just one, which seems technically incorrect.
> I'm a bit less sure of this.  It's on the surface reasonable, but there are some interesting questions.
> 
> We definitely interpret a list of MMOs as indicating a set of locations which are possibly(?) accessed.  The only piece I'm unsure about is that the existence of an MMO requires the access occurs.  If we do, that raises some interesting consistency questions for cases such as:
> 
> Load/Store merging (a superset of the branch folding case)
> Predicate loads and stores (since the access may not happen)
> Load/stores in dead code (i.e. the typical UB contradiction cases)
> Instructions w/multiple accesses to the same MMO combined w/constant memory to imm folding which only handles some cases
Maybe this should be specified properly somewhere, like a MIR langref. In GlobalISel we rely on MMOs being present and correct for legalization, which bakes in a 1-1 mapping assumption, at least for simple loads & stores.
> I'm tempted to suggest we treat the list of MMOs as a potential superset of the implied access, not a direct one-to-one mapping.
> 
> (None of this should imply branch folding shouldn't merge the MMOs.  That would just become an optimization quality issue, not a correctness one.)
> 
> Philip
> 
> 
> 
>> 
>> -Matt
>> 
>> 
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