711f6a8603
We started attaching `DW_AT_object_pointer`s on method declarations in https://github.com/llvm/llvm-project/pull/122742. However, that caused the `.debug_info` section size to increase significantly (by around ~10% on some projects). This was mainly due to the large number of new `DW_FORM_ref4` values. This patch tries to address that regression by changing the `DW_FORM_ref4` to a `DW_FORM_implicit_const` for declarations. The value of `DW_FORM_implicit_const` will be the *index* of the object parameter in the list of formal parameters of the subprogram (i.e., if the first `DW_TAG_formal_parameter` is the object pointer, the `DW_FORM_implicit_const` would be `0`). The DWARFv5 spec only mentions the use of the `reference` attribute class to for `DW_AT_object_pointer`. So using a `DW_FORM_impilicit_const` would be an extension to (and not something mandated/specified by) the standard. Though it'd make sense to extend the wording in the spec to allow for this optimization. That way we don't pay for the 4 byte references on every attribute occurrence. In a local build of clang this barely affected the `.debug_info` section size (but did increase `.debug_abbrev` by up to 10%, which doesn't impact the total debug-info size much however). We guarded this on LLDB tuning (since using `DW_FORM_implicit_const` for this purpose may surprise consumers) and DWARFv5 (since that's where `DW_FORM_implicit_const` was first standardized).
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