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[BPF] annotate DIType metadata for builtin preseve_array_access_index()

Authored by yonghong-song on Aug 2 2019, 10:40 AM.



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.

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Event Timeline

yonghong-song created this revision.Aug 2 2019, 10:40 AM
Herald added projects: Restricted Project, Restricted Project. · View Herald TranscriptAug 2 2019, 10:40 AM
ast accepted this revision.Aug 2 2019, 11:57 AM
This revision is now accepted and ready to land.Aug 2 2019, 11:57 AM

set metadata unless metadata is non-NULL. I have not found a case where metadata is NULL. But nevertheless, let us do this to avoid set a potential NULL metadata.

This revision was automatically updated to reflect the committed changes.