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From: "wingo at igalia dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug symtab/18148] New: Definitions with DW_AT_const_value never
 make it into the psymtabs
Date: Thu, 19 Mar 2015 19:19:00 -0000
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X-Bugzilla-Product: gdb
X-Bugzilla-Component: symtab
X-Bugzilla-Version: HEAD
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X-Bugzilla-Who: wingo at igalia dot com
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https://sourceware.org/bugzilla/show_bug.cgi?id=18148

            Bug ID: 18148
           Summary: Definitions with DW_AT_const_value never make it into
                    the psymtabs
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: wingo at igalia dot com

LLVM will not allocate storage for the "foo" in "static const int foo = 1". 
Instead it inlines the value at the use sites, and emits a DW_TAG_variable with
DW_AT_const_value for the definition of "foo".

GCC will allocate storage for both at -O0 but at -O2 the same problem exists.

Unfortunately, partial debug info (PDI) instances without a location don't get
added to the psymtabs table.  That means that if you do the following:

   cat <<EOF > one.c
static const int one = 1;

extern int fetch_two (void);

int main (int argc, char *argv[]) 
{
  return one + fetch_two ();
}
EOF
   cat <<EOF > two.c
static const int two = 2;

int fetch_two (void)
{
  return two;
}
EOF
   gcc -g -O2 -c -o one.o one.c
   gcc -g -O2 -c -o two.o two.c
   gcc -g -o test one.o two.o
   gdb ./test

Then at the GDB prompt:

   (gdb) p one
   $1 = 1
   (gdb) p two
   No symbol "two" in current context.

The reason we can get "one" and not "two" is because the symtab for one.c has
been expanded, when we went to look for main.  If we expand the compilation
unit for two.c and flush the symbol cache, if on GDB trunk:

   (gdb) mt expand foo.c
   # only needed on trunk
   (gdb) mt flush-symbol-cache
   (gdb) print two
   $2 = 2

The ultimate cause is that dwarf2read.c:add_partial_symbol punts if the symbol
has no location, thinking it's a declaration and not a definition.  Well if it
has a DT_AT_const_value, it might be a definition too.  Patch coming tomorrow.

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