# Location View Numbering

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 6.2.3, 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

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2024-12-09: Noted that GCC produces view numbers by default (and has
for a few years), but there is no consumer support or experience.
Voted to defer this proposal until there is consumer experience.