On Wed, Jan 26, 2011 at 3:52 PM, Russell King - ARM Linux
<li...@arm.linux.org.uk> wrote:
> On Wed, Jan 26, 2011 at 10:42:41AM -0500, Nicolas Pitre wrote:
>> On Wed, 26 Jan 2011, Dave Martin wrote:
>> > Alternatively, I wonder .alt.smp.init could make weak references
>> > instead of normal references: then if the referenced section is
>> > discarded, the reference will revert to 0 and the fixup code can
>> > ignore it (at the expense of wasting a bit of space in the fixup table
>> > ... but it's no worse than the current situation). From
>> > experimentation, it seems this doesn't work if the referenced symbol
>> > is defined in the same file, since the linker resolves the reference
>> > before discarding sections, but maybe there's a way around it...
>>
>> That's strange. How can the linker resolve the reference if the
>> referenced symbol is not in the same section as the reference source?
>> What happens when the final link completes?
>
> $ cat t.s
> .section ".exit.text", "ax", %progbits
> mov r0, r0
> a: mov r0, r0
>
> .pushsection ".alt.smp.fixup", "a"
> .weak a
> .long a
> mov r0, r1
> .popsection
> $ arm-linux-as -o t.o t.s
> $ arm-linux-nm t.o
> 00000004 W a
> $ arm-linux-objdump -Dr t.o
>
> Disassembly of section .exit.text:
>
> 00000000 <a-0x4>:
> 0: e1a00000 nop (mov r0,r0)
>
> 00000004 <a>:
> 4: e1a00000 .word 0xe1a00000
>
> Disassembly of section .alt.smp.fixup:
>
> 00000000 <.alt.smp.fixup>:
> 0: 00000000 .word 0x00000000
> 0: R_ARM_ABS32 a
> 4: e1a00001 mov r0, r1
>
> which when linked with:
>
> SECTIONS
> {
> .alt.smp.init : { *(.alt.smp.init) }
>
> /DISCARD/ : { *(.exit.text) }
> }
>
> $ arm-linux-ld -T t.lds -r -o t.oo t.o
> `a' referenced in section `.alt.smp.fixup' of t.o: defined in discarded
> section `.exit.text' of t.o
>
> So I don't think weak symbols work like we want them to.
>
That was the conclusion I came to also ... the linker seems to resolve
references in each object before discarding sections, so the weak
reference has already become concrete and section discard breaks it.
---Dave
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