cmd/link: Add DW_AT_decl_line for DW_TAG_subprogram in DWARF table

[hoeppi-google]

Currently, the DWARF table does not contain DW_AT_decl_line in the DW_TAG_subprogram entries, i.e. the source code line number of the function definition is not listed in the DWARF entry of said function. I propose to add this information to the DWARF table.

DW_AT_decl_line of functions is needed to convert absolute line numbers into relative line numbers w.r.t. to the first line of the function from DWARF information. Such information is useful in the context of PGO.

CC @aclements

[prattmic]

cc @cherrymui @thanm

[prattmic]

I am fairly certain that adding new DWARF data does not require the proposal process. Removing.

[thanm]

@hoeppi-google is this attribute needed for subprogram corresponding to abstract functions? Or only regular DW_TAG_subprogram entries corresponding to vanilla functions?

[thanm]

discussed this offline with @prattmic , sounds like we need abstract function support as well.

[gopherbot]

Change https://go.dev/cl/457696 mentions this issue: cmd/link: set DW_AT_decl_line for function declarations