[LLVMdev] [RFC] Scoped no-alias metadata

[Hal Finkel]

Hello,

I'd like to propose a new form of memory-aliasing metadata: scoped no-alias metadata. This can be used to model local 'restrict' pointers in C99, but should also be useful for other frontends (I think, for example, that Fortran will want this as well). Currently, we only use the restrict qualifier on function arguments in Clang where we translate the restrict qualifier as a 'noalias' function parameter attribute.

In C99, the 'restrict' guarantee holds only for the lifetime of the pointer, and different lifetime regions can exist within a single function. Furthermore, these lifetime regions can be nested. As a result, we'll need to assign a unique identifier to each lifetime region (roughly a block in C99, although this may be only a partial block if a restrict pointer is declared in the middle of the block). Also, during optimization, instructions from these different lifetime regions can be merged, so a particular pointer value may end up associated with multiple lifetime regions.

Similar to TBAA, the scoped lifetime regions are arranged into disjoint tree structures:
!0 = metadata !{ metadata !"scope of foo()" }
!1 = metadata !{ metadata !"scope 1", metadata !0 }
!2 = metadata !{ metadata !"scope 2", metadata !0 }
!3 = metadata !{ metadata !"scope 2.1", metadata !2 }
!4 = metadata !{ metadata !"scope 2.2", metadata !2 }

and these are used to mark the pointer values:
 %arrayidx = getelementptr inbounds %struct.ST* %s, i64 1, i32 2, i32 1, i64 5, i64 13, !sna !3

in C99 this corresponds to something like:
void foo(...) {
  // begin root scope for foo()
  int *restrict a = ...;
  ...
  {
    // things in this block have scope 1
    int *restrict b = ...;
    ...
  }
  ...
  {
    // things here have scope 2
    int *restrict b = ...;
    ...
    {
      // a new scope 2.1
    }
    ...
    *b += 1; // some use of a restrict pointer
    ...
    // more restrict pointers are declared, start a new nested scope 2.2
    int *restrict c = ...;
    ...
  }
}

When BasicAA compares to pointers for aliasing, as it recurses to find each pointer's base pointer, it collects a list of scope ids associated with each pointer. If one of the pointers is associated with a scope id that is identical to one associated with the other pointer, or is a descendant or parent to one associated with the other pointer, then the two pointers are assumed not to alias. When merging two pointer values, if both have scope ids, then the resulting value can carry both scope ids. If one does not have a scope id, then the resulting pointer must carry none (we can preserve optimization ability by moving the metadata from the value to be merged to its uses that are also pointer-manipulation instructions).

I think that the most difficult part of implementing this proposal would be updating the various transformation passes to preserve this metadata.

This is the simplest thing that I've thought of so far; does anyone have a better way? Is there some reason that this won't work?

Thanks again,
Hal

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory