write_pieced_value: Fix size capping logic

A field f in a structure composed of DWARF pieces may be located in
multiple pieces, where the first and last of those may contain bits from
other fields as well.  So when writing to f, the beginning of the first
and the end of the last of those pieces may have to be skipped.  But the
logic in write_pieced_value for handling one of those pieces is flawed
when the first and last piece are the same, i.e., f is contained in a
single piece:

  < - - - - - - - - - piece_size - - - - - - - - - ->
  +-------------------------------------------------+
  | skipped_bits |   f_bits   | / / / / / / / / / / |
  +-------------------------------------------------+

The current logic determines the size of the sub-piece to operate on by
limiting the piece size to the bit size of f and then subtracting the
skipped bits:

  min (piece_size, f_bits) - skipped_bits

Instead of:

  min (piece_size - skipped_bits, f_bits)

So the resulting sub-piece size is corrupted, leading to wrong handling of
this piece in write_pieced_value.

Note that the same bug was already found in read_pieced_value and fixed
there (but not in write_pieced_value), see PR 15391.

This patch swaps the calculations, bringing them into the same (correct)
order as in read_pieced_value.

gdb/ChangeLog:

	* dwarf2loc.c (write_pieced_value): Fix order of calculations for
	size capping.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/var-pieces.exp: Add test case for modifying a
	variable at nonzero offset.

4 files changed, 17 insertions, 2 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c2bceeb..880cd60 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* dwarf2loc.c (write_pieced_value): Fix order of calculations for
+	size capping.
+
 2017-06-13  Yao Qi  <yao.qi@linaro.org>
 
 	* mips-linux-nat.c: Move include features/mips*-linux.c to
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 127167d..2a45a79 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1964,8 +1964,6 @@ write_pieced_value (struct value *to, struct value *from)
 	  bits_to_skip -= this_size_bits;
 	  continue;
 	}
-      if (this_size_bits > type_len - offset)
-	this_size_bits = type_len - offset;
       if (bits_to_skip > 0)
 	{
 	  dest_offset_bits = bits_to_skip;
@@ -1978,6 +1976,8 @@ write_pieced_value (struct value *to, struct value *from)
 	  dest_offset_bits = 0;
 	  source_offset_bits = offset;
 	}
+      if (this_size_bits > type_len - offset)
+	this_size_bits = type_len - offset;
 
       this_size = (this_size_bits + source_offset_bits % 8 + 7) / 8;
       source_offset = source_offset_bits / 8;
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 4993a2d..688efdf 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 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.
+
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
 	* gdb.dwarf2/var-access.c: New file.
 	* gdb.dwarf2/var-access.exp: New test.
 	* lib/gdb-utils.exp (string_to_regexp): Quote braces as well.
diff --git a/gdb/testsuite/gdb.dwarf2/var-access.exp b/gdb/testsuite/gdb.dwarf2/var-access.exp
index a52327d..bd92a44 100644
--- a/gdb/testsuite/gdb.dwarf2/var-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/var-access.exp
@@ -178,6 +178,11 @@ gdb_test "print/d s1" " = \\{a = 63, b = 3, c = 0, d = 1\\}" \
     "verify s1.a"
 gdb_test "print/d a" " = \\{0, 1, 63, 3, 4, 5, 6, 7\\}" \
     "verify s1.a through a"
+gdb_test_no_output "set var s1.b = 42"
+gdb_test "print/d s1" " = \\{a = 63, b = 42, c = 0, d = 1\\}" \
+    "verify s1.b"
+gdb_test "print/d a" " = \\{0, 1, 63, 42, 4, 5, 6, 7\\}" \
+    "verify s1.b through a"
 
 # Byte-aligned register- and memory pieces.
 gdb_test_no_output "set var \$[lindex $regname 0] = 81" \