This got "broken" by my ENTRY_VALUE patches, and apparently even Alex'
"Keep static VTA locs in cselib tables only" patch doesn't cure it.

We have:
(debug_insn 6 3 7 2 (var_location:HI D#2 (mem:HI (reg/v/f:DI 5 di [orig:62 p ] [62]) [0 *p_1(D)+0 S2 A16])) pr45003-3.c:8 -1
     (nil))

(debug_insn 7 6 8 2 (var_location:HI D#1 (debug_expr:HI D#2)) pr45003-3.c:8 -1
     (nil))

(debug_insn 8 7 9 2 (var_location:SI a (sign_extend:SI (debug_expr:HI D#1))) pr45003-3.c:8 -1
     (nil))

(insn:TI 9 8 10 2 (set (reg:SI 5 di [orig:60 D.1725 ] [60])
        (zero_extend:SI (mem:HI (reg/v/f:DI 5 di [orig:62 p ] [62]) [2 *p_1(D)+0 S2 A16]))) pr45003-3.c:9 117 {*zero_extendhisi2_movzwl}
     (nil))

The PR45003 improvement noted during var-tracking the reverse operation of
(zero_extend:SI (mem:HI (di))))
i.e. that the value of D#2 (and D#1) can be computed as (subreg:HI (reg:SI di))
after insn 9.  Thus before ENTRY_VALUE changes, we'd see that the p pointer value isn't live after that insn anywhere and the only way to compute it would be to go back, see that (mem:HI (value of p on entry)) is live in (subreg:HI (reg:SI di)) and just emit the sign extension of it.
Now, with ENTRY_VALUE, even when we keep it as lowest priority alternative,
we have a place where the value of p on entry is live - (entry_value:DI rdi)
and thus we just use it and don't look further.
This testcase will work just fine with GDB 7.4 which finally supports DW_OP_GNU_entry_value (just verified that), but that wasn't the actual goal of the testcase, it is trivial to modify the testcase so that it will fail even with GDB 7.4:

--- gcc/testsuite/gcc.dg/guality/pr45003-3.c.jj	2010-11-19 20:56:33.000000000 +0100
+++ gcc/testsuite/gcc.dg/guality/pr45003-3.c	2011-12-16 10:01:36.170231171 +0100
@@ -24,8 +24,12 @@ int
 main ()
 {
   unsigned short us = 0x8078;
-  foo (&us);
+  unsigned short *pus = &us;
+  asm volatile ("" : "+r" (pus));
+  foo (pus);
   short s = -32648;
-  bar (&s);
+  short *ps = &s;
+  asm volatile ("" : "+r" (ps));
+  bar (ps);
   return 0;
 }

Then on the call site we don't know what is the value passed to the functions, and as those values aren't needed afterwards, they aren't saved anywhere.
So, yes, entry_value should have the lowest priority of everything.

To fix this we'd need to cache during vt_emit_notes not just a single expression, but at most two, one that doesn't use any entry_value expressions and one that does.  To build the complete expressions for the notes we'd first try if we can build up an expression entirely without any entry_values, and only if it fails, supply one with entry_values.  If we did no caching, we could of course just try to expand twice and ignore entry_value in the first round, but we really can't avoid the caching, the memory consumption and compile time complexity was huge on some testcases.

Any updates here?  Should we simply XFAIL the tests?

Then it would XPASS with latest gdb.
With the gdb 7.4.50.20120103-8.fc17 I'm only seeing lots of guality XPASSes on x86_64 and no FAILs and on i686 again lots of XPASSes and just a single
FAIL: gcc.dg/guality/vla-1.c  -Os  line 17 sizeof (a) == 6
fail.  We definitely need to do something about this, but perhaps it can wait for 4.8.  Alex, do you think you could look at this?  Thanks.

Thus, P2.  We still should consider XFAILing the tests to make the testsuite
clean.

On it (at last!)

GCC 4.7.0 is being released, adjusting target milestone.

Author: aoliva
Date: Fri Apr 13 15:55:52 2012
New Revision: 186420

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=186420
Log:
PR debug/51570
* var-tracking.c (expand_depth): New type.
(onepart_aux, expand_loc_callback_data): Change depth type to it.
(loc_exp_dep_alloc): Adjust initializer.
(update_depth): Use new type.  Add entryvals.
(vt_expand_var_loc_chain): Take note of expansions with
ENTRY_VALUEs, but don't accept them right away.  Run an optional
second pass accepting the minimum ENTRY_VALUE count found in the
first pass.
(vt_expand_loc_callback, INIT_ELCD): Adjust.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/var-tracking.c

Fixed in the trunk.  Letting the dust settle before backporting.

GCC 4.7.1 is being released, adjusting target milestone.

GCC 4.7.2 has been released.

Not backporting.  Fixed.