[LLVMdev] Memory Altering/Accessing Instructions

[Prakash Prabhu]

Hi all,

Would it be correct to say that the only instructions in LLVM IR that
modify/access memory potentially are the following:

(1) LoadInst : Ref
(2) StoreInst : Mod
(3) VAArgInst : Ref (?)
(4) AllocaInst : Mod
(5) MallocInst : Mod
(6) FreeInst : Mod
(7) CallInst : Mod/Ref ?

Also, my earlier impression was that the GEP instruction only computes
the effective address and does not modify/access any memory location.
Is that true ?
If I have missed other instructions that could alter/modify memory,
please let me know.

Thanks for your time.

- Prakash
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[John Criswell]

Prakash Prabhu wrote:
> Hi all,
>
> Would it be correct to say that the only instructions in LLVM IR that
> modify/access memory potentially are the following:
>

I believe that every instruction has a
mayWriteToMemory()/mayReadToMemory() method that you can use to
determine this information. See
http://llvm.org/doxygen/classllvm_1_1Instruction.html (the LLVM doxygen
info on llvm::Instruction) for more details. I believe these methods
describe whether memory is read/written in a way visible from the LLVM
IR; I don't believe they'll take into account things like read/writes
due to register spills created by the code generator.

> (1) LoadInst : Ref
> (2) StoreInst : Mod
> (3) VAArgInst : Ref (?)
> (4) AllocaInst : Mod
> (5) MallocInst : Mod
> (6) FreeInst : Mod
> (7) CallInst : Mod/Ref ?
>
> Also, my earlier impression was that the GEP instruction only computes
> the effective address and does not modify/access any memory location.
> Is that true ?
>

This is correct.

> If I have missed other instructions that could alter/modify memory,
> please let me know.
>
> Thanks for your time.
>

-- John T.

[Prakash Prabhu]

Hi John,

Thanks for the reply. I am planning to use this information as a part
of Analysis, so I guess it should be ok since it is before code
generation.

- Prakash