Issue 211005.1: Interaction of DW_OP_piece (and similar) and DW_OP_call2 (and similar)

Author: Allen Webster
Champion: Adrian Prantl
Date submitted: 2021-10-05
Date revised:
Date closed:
Type: Clarification
Status: Open
DWARF Version: 6

Section 2.6, pg various

The specification does not say much about how the concept of pieces interact with the ability of a DWARF expression to encode "calls".

The only thing it does say that seems possibly definitive is in 2.6.1: "Each simple location description that is a DWARF expression is evaluated independently of any others."

Which would suggest that the intended meaning of a "piece" op is to mark the boundaries between completely separate expressions that do not share a stack at all. Under this interpretation implementations would be able to decode the opcode stream once to mark the ranges of each simple location within the composite location and then handle them independently. Further under this interpretation a "piece" op in a "call" would be illegal, as that would imply that a call from a DWARF expression calls into a compound location, and the spec claims that a "call" op is transferring control at the level of DWARF expressions.

( "These operations transfer control of DWARF expression evaluation ...")

This would be great, but the problem is that in practice this doesn't seem to be how it's being used.

Looking just at GDB:

case DW_OP_piece:
    ULONGEST size;
    /* Record the piece.  */
    op_ptr = read_uleb128 (op_ptr, op_end, &size);
    add_piece (ctx, 8 * size, 0);
    /* Pop off the address/regnum, and reset the location type.  */
    if (ctx->location != DWARF_VALUE_LITERAL
        && ctx->location != DWARF_VALUE_OPTIMIZED_OUT)
      dwarf_expr_pop (ctx);
    ctx->location = DWARF_VALUE_MEMORY;
  goto no_push;

Here the interpretation of the "piece" op is just to pop one value from the stack, not to fully reset the stack. Under this interpretation a composite location has been treated as the same thing as a DWARF expression. Callees can emit pieces into the same composite location as callers, and the "simple location" after the first "piece" op is not necessarily isolated from the work that was done in first "simple location".