DWARF Standard

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.

(2.5.1.5 "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".