commit e93523245b704bc126705620969b4736b00350c5
Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Date:   Tue Jun 13 15:20:26 2017 +0200

    PR gdb/21226: Take DWARF stack value pieces from LSB end
    
    When taking a DW_OP_piece or DW_OP_bit_piece from a DW_OP_stack_value, the
    existing logic always takes the piece from the lowest-addressed end, which
    is wrong on big-endian targets.  The DWARF standard states that the
    "DW_OP_bit_piece operation describes a sequence of bits using the least
    significant bits of that value", and this also matches the current logic
    in GCC.  For instance, the GCC guality test case pr54970.c fails on s390x
    because of this.
    
    This fix adjusts the piece accordingly on big-endian targets.  It is
    assumed that:
    
    * DW_OP_piece shall take the piece from the LSB end as well;
    
    * pieces reaching outside the stack value bits are considered undefined,
      and a zero value can be used instead.
    
    gdb/ChangeLog:
    
            PR gdb/21226
            * dwarf2loc.c (read_pieced_value): Anchor stack value pieces at
            the LSB end, independent of endianness.
    
    gdb/testsuite/ChangeLog:
    
            PR gdb/21226
            * gdb.dwarf2/nonvar-access.exp: Add checks for verifying that
            stack value pieces are taken from the LSB end.

### a/gdb/ChangeLog
### b/gdb/ChangeLog
## -1,5 +1,11 @@
 2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
+	PR gdb/21226
+	* dwarf2loc.c (read_pieced_value): Anchor stack value pieces at
+	the LSB end, independent of endianness.
+
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
 	* dwarf2loc.c (write_pieced_value): Fix order of calculations for
 	size capping.
 
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1858,6 +1858,10 @@ read_pieced_value (struct value *v)
 		if (unavail)
 		  mark_value_bits_unavailable (v, offset, this_size_bits);
 	      }
+
+	    copy_bitwise (contents, dest_offset_bits,
+			  intermediate_buffer, source_offset_bits % 8,
+			  this_size_bits, bits_big_endian);
 	  }
 	  break;
 
@@ -1866,26 +1870,30 @@ read_pieced_value (struct value *v)
 			     p->v.mem.in_stack_memory,
 			     p->v.mem.addr + source_offset,
 			     buffer.data (), this_size);
+	  copy_bitwise (contents, dest_offset_bits,
+			intermediate_buffer, source_offset_bits % 8,
+			this_size_bits, bits_big_endian);
 	  break;
 
 	case DWARF_VALUE_STACK:
 	  {
-	    size_t n = this_size;
+	    struct objfile *objfile = dwarf2_per_cu_objfile (c->per_cu);
+	    struct gdbarch *objfile_gdbarch = get_objfile_arch (objfile);
+	    ULONGEST stack_value_size_bits
+	      = 8 * TYPE_LENGTH (value_type (p->v.value));
 
-	    if (n > c->addr_size - source_offset)
-	      n = (c->addr_size >= source_offset
-		   ? c->addr_size - source_offset
-		   : 0);
-	    if (n == 0)
-	      {
-		/* Nothing.  */
-	      }
-	    else
-	      {
-		const gdb_byte *val_bytes = value_contents_all (p->v.value);
+	    /* Use zeroes if piece reaches beyond stack value.  */
+	    if (p->size > stack_value_size_bits)
+	      break;
 
-		intermediate_buffer = val_bytes + source_offset;
-	      }
+	    /* Piece is anchored at least significant bit end.  */
+	    if (gdbarch_byte_order (objfile_gdbarch) == BFD_ENDIAN_BIG)
+	      source_offset_bits += stack_value_size_bits - p->size;
+
+	    copy_bitwise (contents, dest_offset_bits,
+			  value_contents_all (p->v.value),
+			  source_offset_bits,
+			  this_size_bits, bits_big_endian);
 	  }
 	  break;
 
@@ -1899,6 +1907,10 @@ read_pieced_value (struct value *v)
 		   : 0);
 	    if (n != 0)
 	      intermediate_buffer = p->v.literal.data + source_offset;
+
+	    copy_bitwise (contents, dest_offset_bits,
+			  intermediate_buffer, source_offset_bits % 8,
+			  this_size_bits, bits_big_endian);
 	  }
 	  break;
 
@@ -1915,12 +1927,6 @@ read_pieced_value (struct value *v)
 	  internal_error (__FILE__, __LINE__, _("invalid location type"));
 	}
 
-      if (p->location != DWARF_VALUE_OPTIMIZED_OUT
-	  && p->location != DWARF_VALUE_IMPLICIT_POINTER)
-	copy_bitwise (contents, dest_offset_bits,
-		      intermediate_buffer, source_offset_bits % 8,
-		      this_size_bits, bits_big_endian);
-
       offset += this_size_bits;
     }
 }
### a/gdb/testsuite/ChangeLog
### b/gdb/testsuite/ChangeLog
## -1,5 +1,11 @@
 2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
+	PR gdb/21226
+	* gdb.dwarf2/nonvar-access.exp: Add checks for verifying that
+	stack value pieces are taken from the LSB end.
+
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
 	* gdb.dwarf2/var-pieces.exp: Add test case for modifying a
 	variable at nonzero offset.
 
--- a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
@@ -128,14 +128,26 @@ Dwarf::assemble $asm_file {
 		    {name def_t}
 		    {type :$struct_t_label}
 		    {location {
-			const1u 0
+			const2s -184
 			stack_value
 			bit_piece 9 0
-			const1s -1
+			const4u 1752286
 			stack_value
 			bit_piece 23 0
 		    } SPECIAL_expr}
 		}
+		# Composite location with some empty pieces.
+		DW_TAG_variable {
+		    {name part_def_a}
+		    {type :$array_a9_label}
+		    {location {
+			piece 3
+			const4u 0xf1927314
+			stack_value
+			piece 4
+			piece 2
+		    } SPECIAL_expr}
+		}
 		# Implicit location: immediate value.
 		DW_TAG_variable {
 		    {name def_implicit_s}
@@ -221,9 +233,12 @@ gdb_test "print/x *implicit_b_ptr" " = $val"
 
 # Byte-aligned fields, pieced together from DWARF stack values.
 gdb_test "print def_s" " = \\{a = 0, b = -1\\}"
+switch $endian { big {set val 0x92} little {set val 0x73} }
+gdb_test "print/x part_def_a\[4\]" " = $val"
+gdb_test "print/x part_def_a\[8\]" " = <optimized out>"
 
 # Non-byte-aligned fields, pieced together from DWARF stack values.
-gdb_test "print def_t" " = \\{a = 0, b = -1\\}"
+gdb_test "print def_t" " = \\{a = -184, b = 1752286\\}"
 
 # Simple variable without location.
 gdb_test "print undef_int" " = <optimized out>"