[PATCH] Fix PR12526: -location watchpoints for bitfield arguments

Patrick Palka
2014-09-07T18:37:32Z

{ v2: Here is my crude attempt at adding a testcase for this changeset. I
also fixed the bug that I mentioned earlier. }

PR 12526 reports that -location watchpoints against bitfield arguments
trigger false positives when bits around the bitfield, but not the
bitfield itself, are modified.

This happens because -location watchpoints naturally operate at the byte
level, not at the bit level. When the address of a bitfield lvalue is
taken, information about the bitfield (i.e. its offset and size) is lost
in the process.

This information must first be retained throughout the lifetime of the
-location watchpoint. This patch achieves this by adding two new fields
to the watchpoint struct: val_bitpos and val_bitsize. These fields are
set when a watchpoint is first defined in watch_command_1(). They are
both equal to zero if the watchpoint is not a -location watchpoint or if
the argument is not a bitfield.

Then these bitfield parameters are used inside update_watchpoint() and
watchpoint_check() to extract the actual value of the bitfield from the
watchpoint address, with the help of a local helper function
extract_bitfield_from_watchpoint_value().

Finally when creating a HW breakpoint pointing to a bitfield, we
optimize the address and length of the breakpoint. By skipping over the
bytes that don't cover the bitfield, this step reduces the frequency at
which a read watchpoint for the bitfield is triggered. It also reduces
the number of times a false-positive call to check_watchpoint() is
triggered for a write watchpoint.

gdb/
PR breakpoints/12526
* breakpoint.h (struct watchpoint): New fields val_bitpos and
val_bitsize.
* breakpoint.c (watch_command_1): Use these fields to retain
bitfield information.
(extract_bitfield_from_watchpoint_value): New function.
(watchpoint_check): Use it.
(update_watchpoint): Use it. Optimize the address and length
of a HW watchpoint pointing to a bitfield.
* value.h (unpack_value_bitfield): New prototype.
* value.c (unpack_value_bitfield): Make extern.

gdb/testsuite/
PR breakpoints/12526
* gdb.base/watch-bitfields.exp: New file.
* gdb.base/watch-bitfields.c: New file.
---
gdb/breakpoint.c | 74 +++++++++++++++++++++++++++++-
gdb/breakpoint.h | 5 ++
gdb/testsuite/gdb.base/watch-bitfields.c | 54 ++++++++++++++++++++++
gdb/testsuite/gdb.base/watch-bitfields.exp | 56 ++++++++++++++++++++++
gdb/value.c | 2 +-
gdb/value.h | 5 ++
6 files changed, 194 insertions(+), 2 deletions(-)
create mode 100644 gdb/testsuite/gdb.base/watch-bitfields.c
create mode 100644 gdb/testsuite/gdb.base/watch-bitfields.exp

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 683ed2b..2d33ff7 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -1703,6 +1703,29 @@ watchpoint_del_at_next_stop (struct watchpoint *w)
b->disposition = disp_del_at_next_stop;
}

+/* Extract a bitfield value from value VAL using the bit parameters contained in
+ watchpoint W. */
+
+static struct value *
+extract_bitfield_from_watchpoint_value (struct watchpoint *w, struct value *val)
+{
+ struct value *bit_val;
+
+ if (val == NULL)
+ return NULL;
+
+ bit_val = allocate_value (value_type (val));
+
+ unpack_value_bitfield (bit_val,
+ w->val_bitpos,
+ w->val_bitsize,
+ value_contents_for_printing (val),
+ value_offset (val),
+ val);
+
+ return bit_val;
+}
+
/* Assuming that B is a watchpoint:
- Reparse watchpoint expression, if REPARSE is non-zero
- Evaluate expression and store the result in B->val
@@ -1877,6 +1900,12 @@ update_watchpoint (struct watchpoint *b, int reparse)
watchpoints. */
if (!b->val_valid && !is_masked_watchpoint (&b->base))
{
+ if (b->val_bitsize != 0)
+ {
+ v = extract_bitfield_from_watchpoint_value (b, v);
+ if (v != NULL)
+ release_value (v);
+ }
b->val = v;
b->val_valid = 1;
}
@@ -1906,8 +1935,31 @@ update_watchpoint (struct watchpoint *b, int reparse)
CORE_ADDR addr;
int type;
struct bp_location *loc, **tmp;
+ int bitpos = 0, bitsize = 0;
+
+ if (value_bitsize (v) != 0)
+ {
+ /* Extract the bit parameters out from the bitfield
+ sub-expression. */
+ bitpos = value_bitpos (v);
+ bitsize = value_bitsize (v);
+ }
+ else if (v == result && b->val_bitsize != 0)
+ {
+ /* If VAL_BITSIZE != 0 then RESULT is actually a bitfield
+ lvalue whose bit parameters are saved in the fields
+ VAL_BITPOS and VAL_BITSIZE. */
+ bitpos = b->val_bitpos;
+ bitsize = b->val_bitsize;
+ }

addr = value_address (v);
+ if (bitsize != 0)
+ {
+ /* Skip the bytes that don't contain the bitfield. */
+ addr += bitpos / 8;
+ }
+
type = hw_write;
if (b->base.type == bp_read_watchpoint)
type = hw_read;
@@ -1922,7 +1974,15 @@ update_watchpoint (struct watchpoint *b, int reparse)

loc->pspace = frame_pspace;
loc->address = addr;
- loc->length = TYPE_LENGTH (value_type (v));
+
+ if (bitsize != 0)
+ {
+ /* Just cover the bytes that make up the bitfield. */
+ loc->length = ((bitpos % 8) + bitsize + 7) / 8;
+ }
+ else
+ loc->length = TYPE_LENGTH (value_type (v));
+
loc->watchpoint_type = type;
}
}
@@ -5039,6 +5099,9 @@ watchpoint_check (void *p)
mark = value_mark ();
fetch_subexp_value (b->exp, &pc, &new_val, NULL, NULL, 0);

+ if (b->val_bitsize != 0)
+ new_val = extract_bitfield_from_watchpoint_value (b, new_val);
+
/* We use value_equal_contents instead of value_equal because
the latter coerces an array to a pointer, thus comparing just
the address of the array instead of its contents. This is
@@ -11172,6 +11235,7 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
struct expression *exp;
const struct block *exp_valid_block = NULL, *cond_exp_valid_block = NULL;
struct value *val, *mark, *result;
+ int saved_bitpos = 0, saved_bitsize = 0;
struct frame_info *frame;
const char *exp_start = NULL;
const char *exp_end = NULL;
@@ -11305,6 +11369,12 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
mark = value_mark ();
fetch_subexp_value (exp, &pc, &val, &result, NULL, just_location);

+ if (val && just_location)
+ {
+ saved_bitpos = value_bitpos (val);
+ saved_bitsize = value_bitsize (val);
+ }
+
if (just_location)
{
int ret;
@@ -11440,6 +11510,8 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
else
{
w->val = val;
+ w->val_bitpos = saved_bitpos;
+ w->val_bitsize = saved_bitsize;
w->val_valid = 1;
}

diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
index f6d06ce..2b80af1 100644
--- a/gdb/breakpoint.h
+++ b/gdb/breakpoint.h
@@ -779,6 +779,11 @@ struct watchpoint
then an error occurred reading the value. */
int val_valid;

+ /* When watching the location of a bitfield, contains the offset and size of
+ the bitfield. Otherwise contains 0. */
+ int val_bitpos;
+ int val_bitsize;
+
/* Holds the frame address which identifies the frame this
watchpoint should be evaluated in, or `null' if the watchpoint
should be evaluated on the outermost frame. */
diff --git a/gdb/testsuite/gdb.base/watch-bitfields.c b/gdb/testsuite/gdb.base/watch-bitfields.c
new file mode 100644
index 0000000..fb57885
--- /dev/null
+++ b/gdb/testsuite/gdb.base/watch-bitfields.c
@@ -0,0 +1,54 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2014 Free Software Foundation, Inc.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+struct foo
+{
+ unsigned long a:1;
+ unsigned char b:2;
+ unsigned long c:3;
+ char d:4;
+ int e:5;
+ char f:6;
+ int g:7;
+ long h:8;
+} q = { 0 };
+
+int
+main (void)
+{
+ q.a = 1;
+ q.b = 2;
+ q.c = 3;
+ q.d = 4;
+ q.e = 5;
+ q.f = 6;
+ q.g = -7;
+ q.h = -8;
+ q.a--;
+ q.h--;
+ q.c--;
+ q.b--;
+ q.e--;
+ q.d--;
+ q.c--;
+ q.f--;
+ q.g--;
+ q.h--;
+
+
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.base/watch-bitfields.exp b/gdb/testsuite/gdb.base/watch-bitfields.exp
new file mode 100644
index 0000000..3f25384
--- /dev/null
+++ b/gdb/testsuite/gdb.base/watch-bitfields.exp
@@ -0,0 +1,56 @@
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite
+
+standard_testfile
+
+if {[prepare_for_testing $testfile.exp $testfile $srcfile debug]} {
+ return -1
+}
+
+if {![runto_main]} {
+ return -1
+}
+
+# Continue inferior execution, expecting the watchpoint EXPR to be triggered
+# having old value OLD and new value NEW.
+proc expect_watchpoint { expr old new } {
+ set expr_re [string_to_regexp $expr]
+ gdb_test "print $expr" "\\$\\d+ = $old\\s"
+ gdb_test "cont" "$expr_re\\s.*Old value = $old\\s+New value = $new\\s.*"
+ gdb_test "print $expr" "\\$\\d+ = $new\\s"
+}
+
+# Check that -location watchpoints against bitfields trigger properly.
+gdb_test "watch -l q.a"
+gdb_test "watch -l q.e"
+expect_watchpoint "q.a" 0 1
+expect_watchpoint "q.e" 0 5
+expect_watchpoint "q.a" 1 0
+expect_watchpoint "q.e" 5 4
+gdb_test "cont" ".*exited normally.*"
+
+# Check that regular watchpoints against expressions involving bitfields
+# trigger properly.
+runto_main
+gdb_test "watch q.d + q.f + q.g"
+expect_watchpoint "q.d + q.f + q.g" 0 4
+expect_watchpoint "q.d + q.f + q.g" 4 10
+expect_watchpoint "q.d + q.f + q.g" 10 3
+expect_watchpoint "q.d + q.f + q.g" 3 2
+expect_watchpoint "q.d + q.f + q.g" 2 1
+expect_watchpoint "q.d + q.f + q.g" 1 0
+gdb_test "cont" ".*exited normally.*"
diff --git a/gdb/value.c b/gdb/value.c
index 6620f96..fdc8858d 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -3231,7 +3231,7 @@ unpack_field_as_long (struct type *type, const gdb_byte *valaddr, int fieldno)
are unavailable/optimized out, DEST_VAL is correspondingly
marked unavailable/optimized out. */

-static void
+void
unpack_value_bitfield (struct value *dest_val,
int bitpos, int bitsize,
const gdb_byte *valaddr, int embedded_offset,
diff --git a/gdb/value.h b/gdb/value.h
index 4cdbf21..e3603c3 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -613,6 +613,11 @@ extern int unpack_value_field_as_long (struct type *type, const gdb_byte *valadd
int embedded_offset, int fieldno,
const struct value *val, LONGEST *result);

+extern void unpack_value_bitfield (struct value *dest_val,
+ int bitpos, int bitsize,
+ const gdb_byte *valaddr, int embedded_offset,
+ const struct value *val);
+
extern struct value *value_field_bitfield (struct type *type, int fieldno,
const gdb_byte *valaddr,
int embedded_offset,
--
2.1.0.60.g85f0837