Fix handling of DWARF register pieces on big-endian targets

For big-endian targets the logic in read/write_pieced_value tries to take
a register piece from the LSB end.  This requires offsets and sizes to be
adjusted accordingly, and that's where the current implementation has some
issues:

* The formulas for recalculating the bit- and byte-offsets into the
  register are wrong.  They just happen to yield correct results if
  everything is byte-aligned and the piece's last byte belongs to the
  given value.

* After recalculating the bit offset into the register, the number of
  bytes to be copied from the register is not recalculated.  Of course
  this does not matter if everything (particularly the piece size) is
  byte-aligned.

These issues are fixed.  The size calculation is performed with a new
helper function bits_to_bytes().

gdb/ChangeLog:

	* dwarf2loc.c (bits_to_bytes): New function.
	(read_pieced_value): Fix offset calculations for register pieces
	on big-endian targets.
	(write_pieced_value): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/var-access.exp: Add test for non-byte-aligned
	register pieces.

1 files changed, 35 insertions, 21 deletions

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 6140ee6d70a..a7447cba2ad 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1752,6 +1752,15 @@ copy_bitwise_tests (void)
 
 #endif /* GDB_SELF_TEST */
 
+/* Return the number of bytes overlapping a contiguous chunk of N_BITS
+   bits whose first bit is located at bit offset START.  */
+
+static size_t
+bits_to_bytes (ULONGEST start, ULONGEST n_bits)
+{
+  return (start % 8 + n_bits + 7) / 8;
+}
+
 static void
 read_pieced_value (struct value *v)
 {
@@ -1804,7 +1813,7 @@ read_pieced_value (struct value *v)
       if (this_size_bits > max_offset - offset)
 	this_size_bits = max_offset - offset;
 
-      this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
+      this_size = bits_to_bytes (source_offset_bits, this_size_bits);
       buffer.reserve (this_size);
       source_offset = source_offset_bits / 8;
       intermediate_buffer = buffer.data ();
@@ -1817,20 +1826,20 @@ read_pieced_value (struct value *v)
 	    struct frame_info *frame = frame_find_by_id (c->frame_id);
 	    struct gdbarch *arch = get_frame_arch (frame);
 	    int gdb_regnum = dwarf_reg_to_regnum_or_error (arch, p->v.regno);
+	    ULONGEST reg_bits = 8 * register_size (arch, gdb_regnum);
 	    int optim, unavail;
-	    LONGEST reg_offset = source_offset;
 
 	    if (gdbarch_byte_order (arch) == BFD_ENDIAN_BIG
-		&& this_size < register_size (arch, gdb_regnum))
+		&& p->size < reg_bits)
 	      {
 		/* Big-endian, and we want less than full size.  */
-		reg_offset = register_size (arch, gdb_regnum) - this_size;
-		/* We want the lower-order THIS_SIZE_BITS of the bytes
-		   we extract from the register.  */
-		source_offset_bits += 8 * this_size - this_size_bits;
+		source_offset_bits += reg_bits - p->size;
 	      }
+	    this_size = bits_to_bytes (source_offset_bits, this_size_bits);
+	    buffer.reserve (this_size);
 
-	    if (!get_frame_register_bytes (frame, gdb_regnum, reg_offset,
+	    if (!get_frame_register_bytes (frame, gdb_regnum,
+					   source_offset_bits / 8,
 					   this_size, buffer.data (),
 					   &optim, &unavail))
 	      {
@@ -1844,7 +1853,7 @@ read_pieced_value (struct value *v)
 	      }
 
 	    copy_bitwise (contents, dest_offset_bits,
-			  intermediate_buffer, source_offset_bits % 8,
+			  buffer.data (), source_offset_bits % 8,
 			  this_size_bits, bits_big_endian);
 	  }
 	  break;
@@ -1969,7 +1978,7 @@ write_pieced_value (struct value *to, struct value *from)
       if (this_size_bits > max_offset - offset)
 	this_size_bits = max_offset - offset;
 
-      this_size = (this_size_bits + dest_offset_bits % 8 + 7) / 8;
+      this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
       source_offset = source_offset_bits / 8;
       dest_offset = dest_offset_bits / 8;
 
@@ -1994,20 +2003,25 @@ write_pieced_value (struct value *to, struct value *from)
 	    struct frame_info *frame = frame_find_by_id (c->frame_id);
 	    struct gdbarch *arch = get_frame_arch (frame);
 	    int gdb_regnum = dwarf_reg_to_regnum_or_error (arch, p->v.regno);
-	    int reg_offset = dest_offset;
+	    ULONGEST reg_bits = 8 * register_size (arch, gdb_regnum);
 
 	    if (gdbarch_byte_order (arch) == BFD_ENDIAN_BIG
-		&& this_size <= register_size (arch, gdb_regnum))
+		&& p->size <= reg_bits)
 	      {
 		/* Big-endian, and we want less than full size.  */
-		reg_offset = register_size (arch, gdb_regnum) - this_size;
+		dest_offset_bits += reg_bits - p->size;
 	      }
+	    this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
+	    buffer.reserve (this_size);
 
-	    if (need_bitwise)
+	    if (dest_offset_bits % 8 != 0 || this_size_bits % 8 != 0)
 	      {
+		/* Data is copied non-byte-aligned into the register.
+		   Need some bits from original register value.  */
 		int optim, unavail;
 
-		if (!get_frame_register_bytes (frame, gdb_regnum, reg_offset,
+		if (!get_frame_register_bytes (frame, gdb_regnum,
+					       dest_offset_bits / 8,
 					       this_size, buffer.data (),
 					       &optim, &unavail))
 		  {
@@ -2022,14 +2036,14 @@ write_pieced_value (struct value *to, struct value *from)
 				     "bitfield; containing word "
 				     "is unavailable"));
 		  }
-		copy_bitwise (buffer.data (), dest_offset_bits,
-			      contents, source_offset_bits,
-			      this_size_bits,
-			      bits_big_endian);
 	      }
 
-	    put_frame_register_bytes (frame, gdb_regnum, reg_offset, 
-				      this_size, source_buffer);
+	    copy_bitwise (buffer.data (), dest_offset_bits % 8,
+			  contents, source_offset_bits,
+			  this_size_bits, bits_big_endian);
+	    put_frame_register_bytes (frame, gdb_regnum,
+				      dest_offset_bits / 8,
+				      this_size, buffer.data ());
 	  }
 	  break;
 	case DWARF_VALUE_MEMORY: