DWARF Standard

Section 6.4.2, pg various
Three CFI instructions use DWARF expressions as operands - DW_CFA_def_cfa_expression, 
DW_CFA_expression, DW_CFA_val_expression.

DW_CFA_def_cfa_expression paragraph says that the "operand (is) encoded as a 
DW_FORM_exprloc value representing a DWARF expression". Note tat this wording 
also eliminates a possibility for this 'exprloc' to encode a location description.

DW_CFA(_val)_expression paragraphs say that the operand is "a DW_FORM_block value 
representing a DWARF expression".

Given that all three can use only DWARF expressions (not location descriptions) - 
should DW_CFA_def_cfa_expression be changed so there would be a unified wording 
for all three to use DW_FORM_block? Moreover, page 239 says that 
DW_CFA_def_cfa_expression uses BLOCK (not EXPRLOC), which contradicts the 
instruction definition at the page 178.

Change seems to be backward-compatible as at the representation level both 
'block' and 'exprloc' are identical - ULEB128 length field followed by information bytes.


----------------
PROPOSED CHANGES
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6.4.2.3 Register Rule Instructions

  6.  DW_CFA_def_cfa_expression
      Replace "a DW_FORM_exprloc" with "an exprloc". New text:

      The DW_CFA_def_cfa_expression instruction takes a single operand
      encoded as an exprloc value representing a DWARF expression.


  9.  DW_CFA_expression
      Replace "a DW_FORM_block" with "an exprloc". New text:

      The DW_CFA_expression instruction takes two operands: an unsigned LEB128
      value representing a register number, and an exprloc value
      representing a DWARF expression.


  10. DW_CFA_val_expression
      Replace "a DW_FORM_block" with "an exprloc". New text:

      The DW_CFA_val_expression instruction takes two operands: an unsigned
      LEB128 value representing a register number, and an exprloc value
      representing a DWARF expression.


7.24 Call Frame Information

  Table 7.29: Call frame instruction encodings
  Change "BLOCK" to "exprloc" and add "register" to DW_CFA_val_expression.

  New table entries:
    | DW_CFA_def_cfa_expression | 0 | 0x0f | exprloc          |              |
    | DW_CFA_expression         | 0 | 0x10 | ULEB128 register | exprloc      |
    | DW_CFA_val_expression     | 0 | 0x16 | ULEB128 register | exprloc      |

-- 
2023-01-13 -- Add proposed changes.