[llvm-dev] Intrinsics InstrReadMem memory properties

[Son Tuan VU via llvm-dev]

Ok, now I think I've found a bug:

Consider this C code:
void bar(int b) {
  int a[10];
  memset(a, b, 10);
}

which generates this IR code:
define dso_local void @bar(i32 %b) #0 {
entry:
  %b.addr = alloca i32, align 4
  %a = alloca [10 x i32], align 16
  store i32 %b, i32* %b.addr, align 4
  %arraydecay = getelementptr inbounds [10 x i32], [10 x i32]* %a, i64 0,
i64 0
  %0 = bitcast i32* %arraydecay to i8*
  %1 = load i32, i32* %b.addr, align 4
  %2 = trunc i32 %1 to i8
  call void @llvm.memset.p0i8.i64(i8* align 16 %0, i8 %2, i64 10, i1 false)
  ret void
}

Now I have a pass that inserts an intrinsic with IntrReadMem into the IR:
define dso_local void @bar(i32 %b) #0 {
entry:
  %b.addr = alloca i32, align 4
  %a = alloca [10 x i32], align 16
  store i32 %b, i32* %b.addr, align 4
  %arraydecay = getelementptr inbounds [10 x i32], [10 x i32]* %a, i64 0,
i64 0
  %0 = bitcast i32* %arraydecay to i8*
  %1 = load i32, i32* %b.addr, align 4
  %2 = trunc i32 %1 to i8
  call void @llvm.memset.p0i8.i64(i8* align 16 %0, i8 %2, i64 10, i1 false)
*  tail call void @mem_read_test(i8* %0)*
  ret void
}

; Function Attrs: nounwind readonly
declare void @mem_read_test(i8*) #2

However, the call to memset() still got optimized away by DSE. What am I
missing here? Or this is indeed a bug in DSE?

Son Tuan Vu


On Wed, Jul 24, 2019 at 6:47 PM Doerfert, Johannes <jdoerfert at anl.gov>
wrote:

> You are on the right track. Addresses could get exposed in various ways,
> a probably non-exclusive list is:
>  - passed as arguments
>  - communicated through a global
>  - via I/O, or more general, system calls. This includes all forms of
>    synchronization, e.g., inter-lane communication.
>  - transitively passed by any of the means above, e.g., the address of a
>    pointer to the object could be exposed.
>
> So if we take the example below and add:
>   bar(&A[50]);
> just before the call to unknown, we have to assume A is known to unknown
> now, at least if we do not have information about bar that would suggest
> otherwise.
>
>
> On 07/24, Son Tuan VU wrote:
> > 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
> > >
>
> --
>
> Johannes Doerfert
> Researcher
>
> Argonne National Laboratory
> Lemont, IL 60439, USA
>
> jdoerfert at anl.gov
>
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