[parisc-linux] -fPIC position independent code on parisc

[parisc-linux] -fPIC position independent code on parisc

[Alexander Gabert]

hi,

can someone explain the difference between "normal" and "-fPIC"?

i have compiled a test program:

#include <stdio.h>

int main(void) {
        printf("hello\n");
        return(0);
}

first one without -fPIC
gcc -S hello.c

     1          .LEVEL 1.1
     2          .section        .rodata
     3          .align 4
     4  .LC0:
     5  .stringz"hello\n"
     6          .text
     7          .align 4
     8  .globl main
     9                  .type            main,@function
    10  main:
    11          .PROC
    12          .CALLINFO FRAME=64,CALLS,SAVE_RP,SAVE_SP,ENTRY_GR=3
    13          .ENTRY
    14          stw %r2,-20(%r30)
    15          copy %r3,%r1
    16          copy %r30,%r3
    17          stwm %r1,64(%r30)
    18          ldil LR'.LC0,%r19
    19          ldo RR'.LC0(%r19),%r26
    20          bl printf,%r2
    21          nop
    22          ldi 0,%r28
    23          ldw -20(%r3),%r2
    24          ldo 64(%r3),%r30
    25          ldwm -64(%r30),%r3
    26          bv,n %r0(%r2)
    27          .EXIT
    28          .PROCEND
    29  .Lfe1:
    30          .size   main,.Lfe1-main
    31          .ident  "GCC: (GNU) 3.0.4"

second one with -fPIC
gcc -S -fPIC hello_fPIC.c

     1          .LEVEL 1.1
     2          .section        .rodata
     3          .align 4
     4  .LC0:
     5  .stringz"hello\n"
     6          .text
     7          .align 4
     8  .globl main
     9                  .type            main,@function
    10  main:
    11          .PROC
    12          .CALLINFO FRAME=64,CALLS,SAVE_RP,SAVE_SP,ENTRY_GR=4
    13          .ENTRY
    14          stw %r2,-20(%r30)
    15          copy %r3,%r1
    16          copy %r30,%r3
    17          stwm %r1,64(%r30)
    18          stw %r4,8(%r3)
    19          stw %r19,-32(%r30)
    20          copy %r19,%r4
    21          addil LT'.LC0,%r19
    22          ldw RT'.LC0(%r1),%r1
    23          copy %r1,%r26
    24          bl printf,%r2
    25          nop
    26          copy %r4,%r19
    27          ldi 0,%r28
    28          ldw -20(%r3),%r2
    29          ldw 8(%r3),%r4
    30          ldo 64(%r3),%r30
    31          ldwm -64(%r30),%r3
    32          bv,n %r0(%r2)
    33          .EXIT
    34          .PROCEND
    35  .Lfe1:
    36          .size   main,.Lfe1-main
    37          .ident  "GCC: (GNU) 3.0.4"

i know that it is needed for shared libraries...
but what exactly is position independent code?

gcc is version 3.0.4
glibc is version 2.2.5


thanks in advance,

Alex
--
pub  1024/05E1A80C 2001/12/16 Alexander Gabert (http://nikita.ath.cx) <pappy@nikita.ath.cx>
          Key fingerprint =  2D 84 B0 CB F5 67 8A 22  8D 37 6E 6B 8A 3B 7F D6  05 E1 A8 0C

Re: [parisc-linux] -fPIC position independent code on parisc

[Randolph Chung]

> but what exactly is position independent code?

it's exactly what it says, it will work no matter at what position
the code is loaded.

e.g.:

non-PIC
>     18          ldil LR'.LC0,%r19
>     19          ldo RR'.LC0(%r19),%r26
>     20          bl printf,%r2

This loads the address of .LC0 (your string) relative to a fixed address
into r26 (arg0). The address is resolved statically at link time, so
if the code is moved to a different starting address, the address will 
be wrong.

PIC:
>     21          addil LT'.LC0,%r19
>     22          ldw RT'.LC0(%r1),%r1
>     23          copy %r1,%r26
>     24          bl printf,%r2

This calculates the offset of .LC0 relative to the linkage table pointer
(r19) by creating a GOT entry. The linkage table pointer is assigned 
value at runtime based on where the dynamic loader loads the code into 
memory, so the load will always point to the the right place wherever 
it is loaded. 

When you have a shared library, different programs that are linked
against the library may load it at different offsets, so the code must 
be position independent for it to work.

HTH
randolph
-- 
Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports
http://www.tausq.org/

Re: [parisc-linux] -fPIC position independent code on parisc

[Randolph Chung]

(Please reply on list and not via private mail)

> i do not understand the "LR'" syntax
[etc]

Try
http://docs.hp.com/cgi-bin/onlinedocs.py?mpn=92432-90012&service=hpux&path=00/00/19&title=HP%20Assembler%20Reference%20Manual

> it has been linked already, how do i find out, where it is resolved?

After you link an application, you can disassmble the executable.

> but what with those LR' RR' LT' RT'

They are all part of the assembler operators for field selection.

> linkage table pointer?
> what is this? (i am new)

There are several documents that describe the PA runtime linked on the
parisc-linux website. You might want to check them out. The ELF
specification, together with the related architecture supplements, 
describe how relocations work.

randolph
-- 
Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports
http://www.tausq.org/