[commit] Enable hardware watchpoints for inaccessible constand addresses

[Daniel Jacobowitz <dan at codesourcery dot com>]

I originally implemented support for watchpoints on inaccessible
memory here:

http://sourceware.org/ml/gdb-patches/2007-08/msg00417.html

That patch was eventually revised to this version:

http://sourceware.org/ml/gdb-patches/2008-02/msg00472.html

In between those two, I lost a check in can_use_hardware_watchpoints
that has turned out to be necessary after all.  With today's GDB,
we use a hardware watchpoint for "*global_ptr" because we detect that
it has one used (non-lazy) memory value; but "*0x0" has none, and we
weren't considering the lazy (because it was unreadable) value.

This time, I added a test specifically for this scenario.  Tested on
x86_64-linux, checked in.

-- 
Daniel Jacobowitz
CodeSourcery

2010-09-04  Daniel Jacobowitz  <dan@codesourcery.com>

	* breakpoint.c (can_use_hardware_watchpoint): Handle the first
	value specially.

2010-09-04  Daniel Jacobowitz  <dan@codesourcery.com>

	* gdb.base/watchpoint.exp: Call test_inaccessible_watchpoint
	before disabling hardware watchpoints.
	(test_inaccessible_watchpoint): Check that hardware watchpoints
	are used.  Test for watchpoints on a constant address.

Index: breakpoint.c
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.c,v
retrieving revision 1.512
diff -u -p -r1.512 breakpoint.c
--- breakpoint.c	3 Sep 2010 15:42:02 -0000	1.512
+++ breakpoint.c	4 Sep 2010 15:02:57 -0000
@@ -8327,10 +8327,12 @@ can_use_hardware_watchpoint (struct valu
     {
       if (VALUE_LVAL (v) == lval_memory)
 	{
-	  if (value_lazy (v))
-	    /* A lazy memory lvalue is one that GDB never needed to fetch;
-	       we either just used its address (e.g., `a' in `a.b') or
-	       we never needed it at all (e.g., `a' in `a,b').  */
+	  if (v != head && value_lazy (v))
+	    /* A lazy memory lvalue in the chain is one that GDB never
+	       needed to fetch; we either just used its address (e.g.,
+	       `a' in `a.b') or we never needed it at all (e.g., `a'
+	       in `a,b').  This doesn't apply to HEAD; if that is
+	       lazy then it was not readable, but watch it anyway.  */
 	    ;
 	  else
 	    {
Index: testsuite/gdb.base/watchpoint.exp
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.base/watchpoint.exp,v
retrieving revision 1.30
diff -u -p -r1.30 watchpoint.exp
--- testsuite/gdb.base/watchpoint.exp	16 Aug 2010 19:19:22 -0000	1.30
+++ testsuite/gdb.base/watchpoint.exp	4 Sep 2010 15:02:58 -0000
@@ -662,7 +662,28 @@ proc test_inaccessible_watchpoint {} {
 	    }
 	}
 
-	gdb_test "watch *global_ptr" ".*atchpoint \[0-9\]+: \\*global_ptr"
+	# See whether a watchpoint on a normal variable is a hardware
+	# watchpoint or not.  The watchpoints on NULL should be hardware
+	# iff this one is.
+	set watchpoint_msg "Watchpoint"
+	gdb_test_multiple "watch global_ptr" "watch global_ptr" {
+	    -re "Watchpoint \[0-9\]+: global_ptr\r\n.*$gdb_prompt $" {
+		pass "watch global_ptr"
+	    }
+	    -re "Hardware watchpoint \[0-9\]+: global_ptr\r\n.*$gdb_prompt $" {
+		set watchpoint_msg "Hardware watchpoint"
+		pass "watch global_ptr"
+	    }
+	}
+	delete_breakpoints
+
+	# Make sure that we can watch a constant address, and correctly
+	# use a HW watchpoint if supported.
+	gdb_test "watch *(int *) 0" \
+	    "$watchpoint_msg \[0-9\]+: \\*\\(int \\*\\) 0"
+	delete_breakpoints
+
+	gdb_test "watch *global_ptr" "$watchpoint_msg \[0-9\]+: \\*global_ptr"
 	gdb_test "set \$global_ptr_breakpoint_number = \$bpnum" ""
 	gdb_test "next" ".*global_ptr = buf.*" "global_ptr next"
 	gdb_test_multiple "next" "next over ptr init" {
@@ -771,6 +792,8 @@ if [initialize] then {
 	test_complex_watchpoint
     }
 
+    test_inaccessible_watchpoint
+
     # Verify that a user can force GDB to use "slow" watchpoints.
     # (This proves rather little on kernels that don't support
     # fast watchpoints, but still...)
@@ -849,8 +872,6 @@ if [initialize] then {
       }
     }
 
-    test_inaccessible_watchpoint
-
     # See above.
     if [istarget "mips-idt-*"] then {
 	gdb_exit