Re: GDB & static const variables
Re: GDB & static const variables
- Subject: Re: GDB & static const variables
- From: Jim Ingham <email@hidden>
- Date: Thu, 16 Jul 2009 12:04:34 -0700
As other folks have said, it is most likely the case that gcc
optimized this away. But you can tell for sure:
> cat test.m
#include <Foundation/Foundation.h>
Boolean TestFunction( void )
{
static const int doubleByteDegreeMark = 42;
static int anotherVar = 12;
return ( doubleByteDegreeMark == 42 && anotherVar == 12 );
}
> gcc -g -c -O0 test.m
> dwarfdump -n TestFunction -c test.o
----------------------------------------------------------------------
File: test.o (x86_64)
----------------------------------------------------------------------
Searching .debug_info for 'TestFunction'... 1 match:
0x00000575: TAG_subprogram [17] *
AT_external( 0x01 )
AT_name( "TestFunction" )
AT_decl_file( "/private/tmp/test.m" )
AT_decl_line( 3 )
AT_prototyped( 0x01 )
AT_type( {0x00000497} ( Boolean ) )
AT_low_pc( 0x0000000000000000 )
AT_high_pc( 0x0000000000000033 )
AT_frame_base( 0x00000000
0x0000000000000000 - 0x0000000000000001: rsp+8
0x0000000000000001 - 0x0000000000000004: rsp+16
0x0000000000000004 - 0x0000000000000033: rbp+16 )
AT_sibling( {0x000005dd} )
0x0000059a: TAG_variable [18]
AT_name( "doubleByteDegreeMark" )
AT_decl_file( "/private/tmp/test.m" )
AT_decl_line( 4 )
AT_type( {0x000005dd} ( const int ) )
AT_location( [0x0000000000000a1c] )
0x000005c0: TAG_variable [18]
AT_name( "anotherVar" )
AT_decl_file( "/private/tmp/test.m" )
AT_decl_line( 5 )
AT_type( {0x0000009f} ( int ) )
AT_location( [0x0000000000000a18] )
0x000005dc: NULL
So in the case where this is compiled with the C compiler, there is a
variable declaration in the debug info for "doubleByteDegreeMark" and
you should be able to print it. But as Quincy guessed, in the C++
case, this gets optimized to a constant, and there is no debug info
for it:
> gcc -g -x objective-c++ -c -O0 test.m
> dwarfdump -n TestFunction -c test.o
----------------------------------------------------------------------
File: test.o (x86_64)
----------------------------------------------------------------------
Searching .debug_info for 'TestFunction'... 1 match:
0x0000057f: TAG_subprogram [16] *
AT_external( 0x01 )
AT_name( "TestFunction" )
AT_decl_file( "/private/tmp/test.m" )
AT_decl_line( 2 )
AT_MIPS_linkage_name( "_Z12TestFunctionv" )
AT_type( {0x000004a8} ( Boolean ) )
AT_low_pc( 0x0000000000000000 )
AT_high_pc( 0x0000000000000015 )
AT_frame_base( 0x00000000
0x0000000000000000 - 0x0000000000000001: rsp+8
0x0000000000000001 - 0x0000000000000004: rsp+16
0x0000000000000004 - 0x0000000000000015: rbp+16 )
AT_sibling( {0x000005c4} )
0x000005a7: TAG_variable [17]
AT_name( "anotherVar" )
AT_decl_file( "/private/tmp/test.m" )
AT_decl_line( 5 )
AT_type( {0x000000a1} ( int ) )
AT_location( [0x00000000000009e0] )
0x000005c3: NULL
That's arguably a bug in the compiler. Even if the compiler chooses
to optimize the name to a constant, it can still record that fact in
the debug information. At -O0 particularly, the compiler is supposed
to preserve as much information as it can to make the debugging
experience faithful to your source code.
Of course, it may be that the compiler handles const's the way it
handles #defines, and the compiler doesn't by default emit debug
information for #defines. So the compiler folks may say they aren't
going to fix this bug...
But there's nothing the debugger can do about this, the info is not
present.
Hope this helps,
Jim
On Jul 16, 2009, at 9:39 AM, Eric Gorr wrote:
On Jul 16, 2009, at 12:29 PM, Fritz Anderson wrote:
On 16 Jul 2009, at 10:25 AM, Eric Gorr wrote:
Boolean TestFunction( void )
{
static const int doubleByteDegreeMark = 42;
static int anotherVar = 12;
return ( doubleByteDegreeMark == 42 && anotherVar == 12 );
}
from GDB, if I do 'info locals' it prints out:
(gdb) info locals
anotherVar = 12
if I try to print out the value of doubleByteDegreeMark, it gives
me the error message:
(gdb) print doubleByteDegreeMark
No symbol "doubleByteDegreeMark" in current context.
So, my (probably really simple) question is how can I print out
the value of doubleByteDegreeMark from GDB?
Given that doubleByteDegreeMark is a constant, and referred to only
within the function, gcc optimized by doing away with the storage,
and substituted the literal value 42; and then it saw "42 == 42,"
and optimized that to true; and then saw "true && anotherVar ==
12," and optimized that to "anotherVar == 12." I don't know why it
didn't optimize that to simply "true." Maybe at the optimization
level you chose, it didn't do the analysis to get that far.
There being no storage for doubleByteDegreeMark, gdb didn't have
any stored value to display. I don't know the details of why it
didn't know about it just as a constant; I've seen it display enum
constants.
Thank you for your comments.
I still see the same problem if change the test function to:
Boolean TestFunction( int val )
{
static const int doubleByteDegreeMark = 42;
static int anotherVar = 12;
return ( val == doubleByteDegreeMark || val == anotherVar );
}
Now, I suppose it is still possible that the compiler will have
simply optimized this to:
Boolean TestFunction( int val )
{
static int anotherVar = 12;
return ( val == 42 || val == anotherVar );
}
or something.
This is the debug build with all optimizations turned off.
Under other circumstances, should 'info locals' display
doubleByteDegreeMark? Should I be able to print doubleByteDegreeMark?
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Xcode-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Xcode-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden