Re: RFC: fix another regression w/ gcc svn trunk

[Tom Tromey <tromey@redhat.com>]

>>>>> "Joel" == Joel Brobecker <brobecker@adacore.com> writes:

Joel> This looks reasonable to me. For 7.2, I'm also 50/50, so I propose
Joel> the following compromise: Since it is mostly affecting an unreleased
Joel> version of GCC, we can leave this out of 7.2, and reconsider for 7.2.1
Joel> after an observation period on the head?

Sounds reasonable to me.

>> +/* Given an attr with a DW_FORM_dataN value in host byte order,
>> +   zero-extend it as appropriate for the symbol's type.  */

Joel> I think it would be nice to explain in the comment the reason for
Joel> zero-extending as opposed to sign-extending?

Good idea.  I have appended what I am checking in.

Tom

2010-07-27  Tom Tromey  <tromey@redhat.com>

	* dwarf2read.c (dwarf2_const_value_data): Never sign extend.

Index: dwarf2read.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.430
diff -u -r1.430 dwarf2read.c
--- dwarf2read.c	28 Jul 2010 16:23:58 -0000	1.430
+++ dwarf2read.c	28 Jul 2010 19:58:30 -0000
@@ -10446,8 +10446,13 @@
 }
 
 
-/* Given an attr with a DW_FORM_dataN value in host byte order, sign-
-   or zero-extend it as appropriate for the symbol's type.  */
+/* Given an attr with a DW_FORM_dataN value in host byte order,
+   zero-extend it as appropriate for the symbol's type.  The DWARF
+   standard (v4) is not entirely clear about the meaning of using
+   DW_FORM_dataN for a constant with a signed type, where the type is
+   wider than the data.  The conclusion of a discussion on the DWARF
+   list was that this is unspecified.  We choose to always zero-extend
+   because that is the interpretation long in use by GCC.  */
 static void
 dwarf2_const_value_data (struct attribute *attr,
 			 struct symbol *sym,
@@ -10456,12 +10461,7 @@
   LONGEST l = DW_UNSND (attr);
 
   if (bits < sizeof (l) * 8)
-    {
-      if (TYPE_UNSIGNED (SYMBOL_TYPE (sym)))
-	l &= ((LONGEST) 1 << bits) - 1;
-      else
-	l = (l << (sizeof (l) * 8 - bits)) >> (sizeof (l) * 8 - bits);
-    }
+    l &= ((LONGEST) 1 << bits) - 1;
 
   SYMBOL_VALUE (sym) = l;
   SYMBOL_CLASS (sym) = LOC_CONST;