John,
I need your help crafting some assembly for the early startup of the
dynamic linker.
During early startup we need to do some work *before* the GOT is relocated.
This means that static local variable accesses are handled via a
special wrapper e.g.
~~~
struct local *l;
ELF_MACHINE_LOAD_ADDRESS (l, local);
retry:
root = l->root;
~~~
Where local is a static local variable defined at the top of the C file.
Previously I had implemented ELF_MACHINE_LOAD_ADDRESS like this:
~~~
#define ELF_MACHINE_LOAD_ADDRESS(var, symbol) \
asm (" addil LT'" #symbol ", %%r19\n" \
" ldw RT'" #symbol "(%%sr0,%%r1), %0\n" \
: "=&r" (var) : : "r1" );
~~~
As you will notice the LT'/RT' pair will generate an access through
the GOT for this variable e.g.
~~~
ELF_MACHINE_LOAD_ADDRESS (l, local);
18b30: 2a 60 10 00 addil L%800,r19,r1
18b34: 48 26 00 00 ldw 0(r1),r6
~~~
This appears to have worked because in previous builds the GOT
relocations required for this access were processed *before* the
access.
How do I load the address of a static local variable without using the GOT?
I use the following for computing the location of _DYNAMIC:
~~~
Elf32_Addr dynamic;
asm (
" b,l 1f,%0\n"
" depi 0,31,2,%0\n"
"1: addil L'_DYNAMIC - ($PIC_pcrel$0 - 8),%0\n"
" ldo R'_DYNAMIC - ($PIC_pcrel$0 - 12)(%%r1),%0\n"
: "=r" (dynamic) : : "r1");
~~~
Can I use this sequence to find the address of a static local variable?
Is there a more optimal sequence?
Cheers,
Carlos.
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