interface to partial support for DW_OP_piece in dwarf2expr.[ch]

Jim Blandy
2004-08-03T07:15:54Z

Full support for the Dwarf DW_OP_piece operator in location
expressions would require major work on 'struct value' and its users.
But in many cases, compilers use DW_OP_piece in a restricted way, such
that all the multi-piece locations actually generated could be
expressed using the 'struct symbol' and 'struct value' we have today.

For example, on the PowerPC E500, the 32-bit upper and lower halves of
the 64-bit gprs have separate register numbers in Dwarf information,
but GDB has (pseudo-)registers that refer to the entire 64 bits. So a
Dwarf 3 location expression that places one piece in an upper-half
register and another piece in the corresponding lower-half register
could be represented by a 'struct value' V where VALUE_LVAL (V) ==
lval_register and VALUE_REGNO (V) is the pseudo-register number.

Recognizing these cases requires calling out to architecture-specific
code. When an patch implementing this was first posted, there was
general agreement that dwarf2expr.[ch] ought not call architecture
methods directly, as it did in the patch, but should instead return
the list of pieces; and that dwarf2expr.[ch]'s callers (specifically,
the code in dwarf2loc.c) should take care of whatever wrangling was
needed to turn that list of pieces into the right 'struct value',
architecture-specific or otherwise. That thread starts here:

http://sources.redhat.com/ml/gdb-patches/2003-05/msg00425.html

Here's a sketch of how we might extend the Dwarf expression evaluation
interface to return piece lists, meant to implement the suggestions in
that thread. If this looks reasonable, then I'll do some more work
and post it for further review.

*** dwarf2expr.h.~1.5.~ 2003-06-08 13:27:13.000000000 -0500
--- dwarf2expr.h 2004-08-03 02:07:08.000000000 -0500
*************** struct dwarf_expr_context
*** 74,79 ****
--- 74,122 ----
/* Non-zero if the result is in a register. The register number
will be on the expression stack. */
int in_reg;
+
+ /* An array of pieces. PIECES points to its first element;
+ NUM_PIECES is its length.
+
+ Each time DW_OP_piece is executed, we add a new element to the
+ end of this array, recording the current top of the stack, the
+ current in_reg flag, and the size given as the operand to
+ DW_OP_piece. We then pop the top value from the stack, clear the
+ in_reg flag, and resume evaluation.
+
+ The Dwarf spec doesn't say whether DW_OP_piece pops the top value
+ from the stack. We do, ensuring that clients of this interface
+ expecting to see a value left on the top of the stack (say, code
+ evaluating frame base expressions or CFA's specified with
+ DW_CFA_def_cfa_expression) will get an error if the expression
+ actually marks all the values it computes as pieces.
+
+ If an expression never uses DW_OP_piece, num_pieces will be zero.
+ (It would be nice to present these cases as expressions yeilding
+ a single piece, with in_reg clear, so that callers need not
+ distinguish between the no-DW_OP_piece and one-DW_OP_piece cases.
+ But expressions with no DW_OP_piece operations have no value to
+ place in a piece's 'size' field; the size comes from the
+ surrounding data. So the two cases need to be handled
+ separately.) */
+ int num_pieces;
+ struct dwarf_expr_piece *pieces;
+ };
+
+
+ /* A piece of an object, as recorded by DW_OP_piece. */
+ struct dwarf_expr_piece
+ {
+ /* If IN_REG is zero, then the piece is in memory, and VALUE is its address.
+ If IN_REG is non-zero, then the piece is in a register, and VALUE
+ is the register number. */
+ int in_reg;
+
+ /* This piece's address or register number. */
+ CORE_ADDR value;
+
+ /* The length of the piece, in bytes. */
+ ULONGEST size;
};

struct dwarf_expr_context *new_dwarf_expr_context (void);