[BPF] annotate DIType metadata for builtin preseve_array_access_index()

Previously, debuginfo types are annotated to
IR builtin preserve_struct_access_index() and
preserve_union_access_index(), but not
preserve_array_access_index(). The debug info
is useful to identify the root type name which
later will be used for type comparison.

For user access without explicit type conversions,
the previous scheme works as we can ignore intermediate
compiler generated type conversions (e.g., from union types to
union members) and still generate correct access index string.

The issue comes with user explicit type conversions, e.g.,
converting an array to a structure like below:
  struct t { int a; char b[40]; };
  struct p { int c; int d; };
  struct t *var = ...;
  ... __builtin_preserve_access_index(&(((struct p *)&(var->b[0]))->d)) ...
Although BPF backend can derive the type of &(var->b[0]),
explicit type annotation make checking more consistent
and less error prone.

Another benefit is for multiple dimension array handling.
For example,
  struct p { int c; int d; } g[8][9][10];
  ... __builtin_preserve_access_index(&g[2][3][4].d) ...
It would be possible to calculate the number of "struct p"'s
before accessing its member "d" if array debug info is
available as it contains each dimension range.

This patch enables to annotate IR builtin preserve_array_access_index()
with proper debuginfo type. The unit test case and language reference
is updated as well.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D65664

llvm-svn: 367724

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 5fa698e7510..a6acdeaf85e 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -17395,6 +17395,10 @@ based on array base ``base``, array dimension ``dim`` and the last access index
 into the array. The return type ``ret_type`` is a pointer type to the array element.
 The array ``dim`` and ``index`` are preserved which is more robust than
 getelementptr instruction which may be subject to compiler transformation.
+The ``llvm.preserve.access.index`` type of metadata is attached to this call instruction
+to provide array or pointer debuginfo type.
+The metadata is a ``DICompositeType`` or ``DIDerivedType`` representing the
+debuginfo version of ``type``.
 
 Arguments:
 """"""""""