Issue 170427.1: Location View Numbering

Author:	Alexandre Oliva
Champion:	Mark Wielaard
Date submitted:	2017-04-27
Date revised:
Date closed:
Type:	Enhancement
Status:	Open
DWARF Version:	6

Section 6.2, pg. 149-160.

This proposal introduces a new implicit column to the line number table, namely "view numbers", so that multiple program states can be identified at the same program counter, and extends loclists with means to add view numbers to address ranges, enabling locations to start or end at specific views.

This may improve debug information, enabling generators to indicate inlined entry points and preferred breakpoints for statements even if instructions associated with the corresponding source locations were not emitted at the given PC, and to emit variable locations that indicate the initial values of inlined arguments, and side effects of operations as they would be expected to take effect from the source code, even when multiple statements have their side effects all encoded at the same PC: with view numbers, debug information consumers may be able to logically advance the perceived program state, so as to reflect user-expected changes specified in the source code, even if the operations were reordered or optimized out in the executable code.

View numbers

In Section 6.2, page 149, line 4: change "with one row" to "with at least one row". Before line 11, add a bullet "a view number, identifying a logical program state".

In Section 6.2.2, page 150, add to Table 6.3, between 'address' and 'op_index', a new register named "view", defined as "An unsigned integer that identifies each distinct entry associated with the same 'address' in the line number matrix."

In Section 6.2.2, page 152, add before line 1 the following sentence: "The address and view registers, taken together, identify a logical state within the source program."

In Section 6.3, page 153, add to Table 6.4 a line "view | 0" after address.

In Section 6.2.5, page 160, add a Subsection "6.2.5.0 View Numbers":

View numbers identify multiple logical states of the source program that are all associated with the same address in the program.

View numbers are computed as a side effect of line number opcodes, observing the following rules:

opcodes that modify the address register (with the exception of DW_LNS_fixed_advance_pc) simultaneously reset the view register to zero
opcodes that append a row to the matrix subsequently increment the view register

Rationale: location lists can refer to address and view, not op_index, so views are reset at address changes, not at op_index changes. Opcodes that advance op_index only will only reset the view when they happen to advance the address, e.g. by exceeding maximum_operations_per_instruction in op_index.

DW_LNS_fixed_advance_pc is the only opcode that may change the address without resetting the view. It is available for compilers to use when an address change is uncertain, e.g., when advancing past opcodes that may expand to an empty sequence, e.g. possibly-empty alignment sequences, optional no-operation opcodes or the like.

In Section 6.2.5.1, page 160, insert before item 3: "Reset the view register if the value of the address register changed in 2.", and after item 3: "Increment the view register"

In Section 6.2.5.2, page 162, 1. DW_LNS_copy, line 10, add after "Then it": "increments the view register, ".

In Section 6.2.5.2, page 162, append to the description of 2. DW_LNS_advance_pc: "Then, if the value of the address register changed, it resets the view register to zero."

In Section 6.2.5.2, page 163, append to the first paragraph of 8. DW_LNS_const_add_pc: ", and it resets the view register to zero if the value of the address register changed."

In Section 6.2.5.2, page 163, add a third paragraph to 9. DW_LNS_fixed_advance_pc: "This is the only address-changing opcode that does NOT reset the view register. It helps compilers that emit line number programs to determine view numbers even when advancing past opcodes that might expand to nothing."

In Section 6.2.5.3, page 164, in 2. DW_LNE_set_address, change "op_index register" to "view and op_index registers".

Views in loclists

In Section 2.6.2, page 43, add a bullet before "Default location description":

View description. This kind of entry must precede a Bounded location description, and it augments the starting and ending address of the Bounded location description with [view numbers|link to 6.2.5.0]. The addresses of a bounded location description that is not preceded by a view description are argumented with view number zero.

In Section 2.6.2, page 45, add before 6++. DW_LLE_default_location:

6. DW_LLE_view_pair

    This is the only form of view description, and it has two unsigned
    LEB128 operands.  The first and second values specify respectively
    the view numbers that augment the starting and ending addresses of
    the bounded location description the view description precedes.

In Section 7.7.3, add to Table 7.10 the following entry:

DW_LLE_view_pair | 0x09