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

[Philip Reames via llvm-dev]

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'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

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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