Issue 070426.1: Constant expressions in location lists
Background
A problem we've come up with in improving our support for optimized
code on HP-UX is in trying to describe a variable that sometimes has
a constant value (not available in memory or in a register) during
portions of its live range. A variable that is constant everywhere
can be described by DW_AT_const_value, but there is no way (that I
can see) to represent a constant value in a location list.
Proposal
I propose to add a new "DW_OP_implicit_value" operator that would
provide the constant value for ranges where a variable has no actual
location, but is known to have a constant value. Like the DW_OP_reg*
operators, it must occur either alone or as the only operator in a
piece expression.
Modify Section 2.5.1, "Register Name Operators", as follows:
2.5.1 Register Name and Implicit Value Operators
The following DWARF operations can be used to name a register
or to provide an implicit value of arbitrary size and type.
They can be used only in location expressions, and must be
used alone (either as a DWARF expression consisting of just
that one operation or as the sole operator in a piece
expression).
*Note that ...*
1. DW_OP_reg0, ...
...
2. DW_OP_regx
...
3. DW_OP_implicit_value
The DW_OP_implicit_value operation takes two operands:
an unsigned LEB128 length, followed by a block
representing the value in the native representation of the
target machine. The length operand gives the length in
bytes of the block that follows. This operation should be
used only in location lists for ranges where the value of
a variable does not exist in memory or in a register, but
is known to be constant.
In Section 7.7.1, "DWARF Expressions", add the following row to
Figure 22:
No. of
Operation Code Operands Notes
----------------------------------------------------------------
DW_OP_constant_value 0x9b 2 ULEB128 length followed
by length bytes
----------------------------------------------------------------
==================================================================
Proposal accepted without modification.
