> 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?
This will work if the local variable is in the same segment as
the code. It may work generally because I believe there is a fixed
relationship between the code and data segments on linux (this isn't
true on hpux). The issue is whether the data is readonly or needs
to be writeable.
> Is there a more optimal sequence?
In PA 2.0, the b,l/depi instructions can be replaced by the mfia
instruction. This saves one instruction. If you just need to load
a byte, etc, the ldo can be replaced by a ldb, ldh, ldw or ldd
instruction. If the data is close, the addil isn't needed. Otherwise,
there isn't much else that can be done to make the sequence more
optimal.
Dave
--
J. David Anglin dave.anglin@xxxxxxxxxxxxxx
National Research Council of Canada (613) 990-0752 (FAX: 952-6602)
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