[Bug symtab/26092] New: Complain about contradictory DW_LNE_end_sequence marker

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26092

            Bug ID: 26092
           Summary: Complain about contradictory DW_LNE_end_sequence
                    marker
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test-case gdb.dwarf2/dw2-ranges-base.exp.  It has a line-table for
dw2-ranges-base.c like this:
...
     Line Number Statements:
      [0x0000014e]  Extended opcode 2: set Address to 0x4004ba
      [0x00000159]  Advance Line by 10 to 11
      [0x0000015b]  Copy
      [0x0000015c]  Advance PC by 12 to 0x4004c6
      [0x0000015e]  Advance Line by 19 to 30
      [0x00000160]  Copy
      [0x00000161]  Extended opcode 1: End of Sequence

      [0x00000164]  Extended opcode 2: set Address to 0x4004ae
      [0x0000016f]  Advance Line by 20 to 21
      [0x00000171]  Copy
      [0x00000172]  Advance PC by 12 to 0x4004ba
      [0x00000174]  Advance Line by 29 to 50
      [0x00000176]  Copy
      [0x00000177]  Extended opcode 1: End of Sequence

      [0x0000017a]  Extended opcode 2: set Address to 0x4004a7
      [0x00000185]  Advance Line by 30 to 31
      [0x00000187]  Copy
      [0x00000188]  Advance PC by 7 to 0x4004ae
      [0x0000018a]  Advance Line by 39 to 70
      [0x0000018c]  Copy
      [0x0000018d]  Extended opcode 1: End of Sequence
...

The Copy followed by End-of-Sequence is as specified in the dwarf assembly, but
incorrect.  F.i., consider:
 ...
      [0x0000015c]  Advance PC by 12 to 0x4004c6
      [0x0000015e]  Advance Line by 19 to 30
      [0x00000160]  Copy
      [0x00000161]  Extended opcode 1: End of Sequence
 ...

Both the Copy and the End-of-Sequence append a row to the matrix using the same
addres: 0x4004c6.

The Copy declares a target instruction at that address.

The End-of-Sequence declares that the sequence ends before that address.

It's a contradiction that the target instruction is both part of the sequence
(according to Copy) and not part of the sequence (according to
End-of-Sequence).

[ Relevant dwarf standard bits:

DW_LNS_copy

The DW_LNS_copy opcode takes no operands. It appends a row to the matrix using
the current values of the state machine registers. Then it sets the
discriminator register to 0, and sets the basic_block, prologue_end and
epilogue_begin registers to “false.”

end_sequence:

A boolean indicating that the current address is that of the first byte
after the end of a sequence of target machine instructions. end_sequence
terminates a sequence of lines; therefore other information in the same
row is not meaningful.

DW_LNE_end_sequence:

The DW_LNE_end_sequence opcode takes no operands. It sets the
end_sequence register of the state machine to “true” and appends a row
to the matrix using the current values of the state-machine registers.
Then it resets the registers to the initial values specified above (see
Section 6.2.2). Every line number program sequence must end with a
DW_LNE_end_sequence instruction which creates a row whose address is
that of the byte after the last target machine instruction of the sequence.

]

The offending Copy is currently silently skipped by
buildsym_compunit::record_line.

It would be good to warn about this invalid use of dwarf, through the
complaints mechanism, or some such.

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