[llvm-dev] Intrinsics InstrReadMem memory properties

[Son Tuan VU via llvm-dev]

Hi Johannes,

Thanks for your reply. I now see more clearly how things work with these
properties. However, what would be an object whose address is potentially
known by a callee? I suppose the intrinsic arguments and global variable?

So IIUC, if not restricted by *only properties, an intrinsic could access
to:
- only its arguments if IntrArgMemOnly specified,
- its arguments and the global variable as well if Intr*Mem (other than
IntrNoMem) specified.

Please tell me if I'm correct or not!

Thanks again,



On Wed, Jul 24, 2019, 17:27 Doerfert, Johannes <jdoerfert at anl.gov> wrote:

> Hi Son Tuan Vu,
>
> if not restricted by *writeonly*, *readonly*, or *readnone* (basically), a
> call can access any object for which the
> callee could potentially know the address. That means, if the address of
> an object cannot be known to the callee,
> it cannot access that object. An example is given below. Thus, a dead
> store can be eliminated if the memory cannot
> be read by any subsequent operation. If you think there is a bug, could
> you provide a reproducer?
>
> Example:
>
> void unknown();
> void foo() {
>    int *A = malloc(100 * sizeof(A[0]));
>    int B[100];
>   for (int i = 0; i < 100; i++)
>     A[i] = B[i] = i;
>
>   // The addresses/objects A and B are not known to the unknown function
> and the stores above can be removed.
>   unknown();
>
>   free(A);
> }
>
> I hope this helps,
>   Johannes
>
>
> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Son Tuan VU
> via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: Wednesday, July 24, 2019 08:20
> To: llvm-devmemory
> Subject: [llvm-dev] Intrinsics InstrReadMem memory properties
>
> Hello,
>
> According to include/llvm/IR/Intrinsics.td, InstrReadMem property
> indicates that the intrinsic only reads from and does not write to memory.
>
> Does this mean that it can read anywhere in the memory? Because we already
> have 'InstrArgMemOnly' for intrinsics which only access memory that its
> argument(s) point(s) to.
>
> If 'InstrReadMem' really means read from anywhere in the memory, this
> should imply that,  if there's an intrinsic having this property *after* a
> dead store, the latter should not be eliminated by optimizations?
>
> This is not the current behavior of LLVM though, so it seems that my
> guesses are wrong... But at least, can someone show me the mistake here?
>
> Thanks for your time,
>
> Son Tuan Vu
>
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