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is casting of function descriptor assignments for ia64 ONLY a good idea?
- From: Piet/Pete Delaney <piet at sgi dot com>
- To: Keith Owens <kaos at ocs dot com dot au>
- Cc: linux-ia64 at linuxia64 dot org, srn at engr dot sgi dot com, davidm at hpl dot hp dot com, bug-gdb at gnu dot org, bug-gcc at gnu dot org
- Date: Wed, 2 Jan 2002 21:46:35 -0800
- Subject: is casting of function descriptor assignments for ia64 ONLY a good idea?
- References: <20020102162714.A1790@sgi.com> <14034.1010028604@kao2.melbourne.sgi.com>
SUMMARY:
Keith Owens just pointed out that ia64 function descriptor assignments MUST be cast:
pointer = ((unsigned long *)(&my_printf))[0])
howerver it appears that other platforms MUST NOT be cast.
I was wondering if that a good idea. It seems it might require hacking
a lot of existing code.
Why is this necessary for just ia64?
I've used pointers to functions a lot in the past and I don't recall
ever haveing a problem like this.
See details below...
-piet
On Thu, Jan 03, 2002 at 02:30:04PM +1100, Keith Owens wrote:
> On Wed, 2 Jan 2002 16:27:14 -0800,
> Piet/Pete Delaney <piet@sgi.com> wrote:
> >#include <stdio.h>
> >
> >
> >typedef struct disassemble_info {
> > void (*fprintf_func)();
> >} disassemble_info_t;
> >
> >disassemble_info_t disinfo;
> >
> >void my_printf(void)
> >{
> > (void) printf("Hello World\n");
> >}
> >
> >main(void)
> >{
> > disinfo.fprintf_func = my_printf;
> > printf("my_printf is at 0x%llx.\n", my_printf);
> > printf("disinfo is at 0x%llx.\n", &disinfo);
> > disinfo.fprintf_func();
> >}
>
> You are printing the address of the ia64 function descriptor, not the
> function itself.
>
> printf("&my_printf is at 0x%llx.\n", &my_printf);
> printf("my_printf is at 0x%llx.\n", ((unsigned long *)(&my_printf))[0]);
>
> &my_printf is at 0x40000000000009f0.
> my_printf is at 0x4000000000000690.
> disinfo is at 0x6000000000000dd8.
> Hello World
>
> (gdb) disassemble my_printf
> Dump of assembler code for function my_printf:
> 0x4000000000000690 <my_printf>: [MII] alloc r34=ar.pfs,5,4,0
> 0x4000000000000691 <my_printf+1>: mov r35=r12
> 0x4000000000000692 <my_printf+2>: mov r33=b0 >
> (gdb) disassemble 0x4000000000000690
> Dump of assembler code for function my_printf:
> 0x4000000000000690 <my_printf>: [MII] alloc r34=ar.pfs,5,4,0
> 0x4000000000000691 <my_printf+1>: mov r35=r12
> 0x4000000000000692 <my_printf+2>: mov r33=b0 > >
Looks like you appear to be right!
I wonder if this is a problem with src backward compatabity for the C compiler
to make the cast:
((unsigned long *)(&my_printf))[0])
need for ia64 but not allowed for irix and ia32.
It would/will require #ifdef's or some architecture specific handeling.
That doesn't seem like a fantastic idea if we can avoid it.
In the test below you can see that the casting does in fact 'fix'
the problem on ia64 but it breaks irix and ia32. ia32 seems a
bit brooken allready, the upper bits of the address were wrong.
Perhaps that's 'just' a printf problem due to it being a 32bit arch.
I wonder what the 64 bit SPARC linux port does. I only have a 32 bit
SPARC linux at home.
Why does ia64 require this casting?
-piet
------------------------------------------------------------------
#include <stdio.h>
typedef struct disassemble_info {
void (*fprintf_func)();
} disassemble_info_t;
disassemble_info_t disinfo;
void my_printf(void)
{
(void) printf("Hello World\n");
}
main(void)
{
disinfo.fprintf_func = my_printf;
printf("my_printf is at 0x%llx.\n", my_printf);
printf("&my_printf is at 0x%llx.\n", &my_printf);
printf("&my_printf[0] is at 0x%llx.\n", ((unsigned long *)(&my_printf))[0]);
printf("disinfo is at 0x%llx.\n", &disinfo);
disinfo.fprintf_func();
}
------------------------------------------------------------------
------------------------------------------------------------------
irix
------------------------------------------------------------------
268 piet1 20:43 /tmp> gcc test.c
269 piet1 20:43 /tmp> gdb a.out
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "mips-sgi-irix6.2"...
(gdb) run
Starting program: /tmp/a.out
my_printf is at 0x10000fd0.
&my_printf is at 0x10000fd0.
&my_printf[0] is at 0x27bdffd0. <<-- casting breaks irix ports
disinfo is at 0x100143e0.
Hello World
Program exited with code 014.
Current language: auto; currently asm
(gdb) disassemble my_printf
Dump of assembler code for function my_printf:
0x10000fd0 <my_printf>: addiu $sp,$sp,-48
0x10000fd4 <my_printf+4>: sd $ra,32($sp)
0x10000fd8 <my_printf+8>: sd $s8,24($sp)
0x10000fdc <my_printf+12>: sd $gp,16($sp)
0x10000fe0 <my_printf+16>: move $s8,$sp
0x10000fe4 <my_printf+20>: lui $at,0x2
0x10000fe8 <my_printf+24>: addiu $at,$at,-19696
0x10000fec <my_printf+28>: daddu $gp,$at,$t9
0x10000ff0 <my_printf+32>: lw $a0,-32720($gp)
0x10000ff4 <my_printf+36>: daddiu $a0,$a0,16504
0x10000ff8 <my_printf+40>: lw $t9,-32692($gp)
0x10000ffc <my_printf+44>: jalr $t9
0x10001000 <my_printf+48>: nop
0x10001004 <my_printf+52>: move $sp,$s8
0x10001008 <my_printf+56>: ld $ra,32($sp)
0x1000100c <my_printf+60>: ld $s8,24($sp)
0x10001010 <my_printf+64>: ld $gp,16($sp)
0x10001014 <my_printf+68>: addiu $sp,$sp,48
0x10001018 <my_printf+72>: jr $ra
0x1000101c <my_printf+76>: nop
End of assembler dump.
(gdb)
------------------------------------------------------------------
------------------------------------------------------------------
ia32 linux
------------------------------------------------------------------
296 piet2 14:34 ~/src> gcc test.c
297 piet2 14:34 ~/src> gdb a.out
GNU gdb 5.1
Copyright 2001 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
(gdb) run
Starting program: /home/piet/src/a.out
~piet/.cshrc: Entry
~piet/.cshrc: Non Interactive Shell
~piet/.cshrc: Not Interactive shlvl: 5
my_printf is at 0xbffff8e808048460. <<----- ia32 linux high order bits are wrong.
&my_printf is at 0xbffff8e808048460.
&my_printf[0] is at 0xbffff8e883e58955. <<----- casting breaks ia32 linux ports
disinfo is at 0xbffff8e8080496f4.
Hello World
Program exited with code 014.
(gdb) disassemble my_printf
Dump of assembler code for function my_printf:
0x8048460 <my_printf>: push %ebp
0x8048461 <my_printf+1>: mov %esp,%ebp
0x8048463 <my_printf+3>: sub $0x8,%esp
0x8048466 <my_printf+6>: sub $0xc,%esp
0x8048469 <my_printf+9>: push $0x8048558
0x804846e <my_printf+14>: call 0x804833c <printf>
0x8048473 <my_printf+19>: add $0x10,%esp
0x8048476 <my_printf+22>: leave
0x8048477 <my_printf+23>: ret
End of assembler dump.
(gdb)
------------------------------------------------------------------
------------------------------------------------------------------
ia64 linux
------------------------------------------------------------------
260 monica 20:27 ~/src> gcc test.c
261 monica 20:27 ~/src> gdb a.out
GNU gdb Red Hat Linux 7.x (5.0rh-9)
Copyright 2001 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "ia64-redhat-linux"...
(gdb) run
Starting program: /home/piet/src/a.out
~piet/.cshrc: Entry
~piet/.cshrc: Non Interactive Shell
~piet/.cshrc: Not Interactive shlvl: 2
my_printf is at 0x4000000000000880.
&my_printf is at 0x4000000000000880.
&my_printf[0] is at 0x4000000000000560.
disinfo is at 0x6000000000000ba8.
Hello World
Program exited with code 014.
(gdb) disassemble my_printf
Dump of assembler code for function my_printf:
0x4000000000000560 <my_printf>: [MII] alloc r34=ar.pfs,5,4,0
0x4000000000000561 <my_printf+1>: mov r35=r12
0x4000000000000562 <my_printf+2>: mov r33=b0
0x4000000000000570 <my_printf+16>: [MMI] addl r14=80,r1;;
0x4000000000000571 <my_printf+17>: ld8 r36=[r14]
0x4000000000000572 <my_printf+18>: mov r32=r1
0x4000000000000580 <my_printf+32>: [MFB] nop.m 0x0
0x4000000000000581 <my_printf+33>: nop.f 0x0
0x4000000000000582 <my_printf+34>: br.call.sptk.many b0=0x4000000000000400 <_init+272>;;
0x4000000000000590 <my_printf+48>: [MII] mov r1=r32
0x4000000000000591 <my_printf+49>: mov.i ar.pfs=r34
0x4000000000000592 <my_printf+50>: mov b0=r33
0x40000000000005a0 <my_printf+64>: [MFB] mov r12=r35
0x40000000000005a1 <my_printf+65>: nop.f 0x0
0x40000000000005a2 <my_printf+66>: br.ret.sptk.many b0;;
End of assembler dump.
(gdb)
------------------------------------------------------------------