72 lines
3.8 KiB
Text
72 lines
3.8 KiB
Text
This file contains a summary of the bugs/features/inconsistencies
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Robbert and Ed found while making the linker usable for the 68000 and amoeba.
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There is something wrong with the way the combination of
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assembler and linker handle bss.
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In the original (Duk's) the assembler translated .space and, worse, .align
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to a sequence of zero's. If this sequence was at the end of a segment
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within a module the assembler didn't put zero in the segment but sets
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os_flen to the amount of space initialized before the zero space
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and os_size to the segements size. (os_size - os_flen) is then the
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size of the space filled by zeroes.
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For the sake of clarity, let us call 0...os_flen-1 initialized space
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and os_flen..os_size-1 uninitialized space.
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Now the linker, it does a nasty trick. It gathers the uninitialized space
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of all modules within a segment and puts it consequtively at the end
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of the segment. I think that I understand the reason: This way you can keep
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your resultant a.out smaller and you don't need a special bss segment
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(a la unix). But it is incorrect, the net effect is that the .align's
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at the end of segments within a module do have the desired effect,
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the space thus allocated is removed to 'higher spheres' thereby
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leaving the first items of that segment in the inmediatly following
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modules at unaligned boundaries.
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What should be done is that the linker leaves the initialized and
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the unitialized code alone and regards the whole a a chunk that can be
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relocated. Only producing a difference of os_size and os_flen for
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the zeroes at the very end of the segment. Thereby collapsing all
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.space (and .align) commands into zero space only if they
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are in consequtive modules at the end of the segment, with modules
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NOT containing any initialized data.
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I already, ad-hoc, changed the code of the assembler to producing 'hard'
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zeroes when aligning. The trick could also be done for .space
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but is a bit harder here.
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The reason: .space is also used to allocate space in the BSS segment
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if that produced zeroes in the a.out file (0..bss_size) we would
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have a.out files that are far too large.
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This feature of the linker also caused weird effects for names that
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are defined as the very last in a section within a module, without
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any data (initialized or uninitialezed) after it.
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The names a regarded as pointing into the uninitialized space and
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thus relocated to the end of the section.
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The sequence
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.sect .data
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begdata:
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.sect ....
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in an head_em.s resulted in the relocation of begdata to the END
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of the .data segment.
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Another problem form the commons:
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1 - Local commons are not handled by led and not produced by as.
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using .comm for unitialized data is not correct because (from C)
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two uninitialized static declarations for the same name in
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different modules will be handled as external commons and thus
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be overlayed.
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2 - The commons are allocated at the very end of the first pass, after the
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initialezed data has been allocated in the segments. The order on which
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the commons are allocated seems to be random. That way it is impossible
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to produce a label that is guaranteed to point to the last byte (+1)
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of a segment containing commons.
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The currently used trick is to declare an extra segment after the
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segment containing the commons. The first bytre in this segment
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inmediatly follows the commons and can be used as _end or endbss.
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The archiver (aal) with the automatic ranlib is buggy.
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The only thing that seems to work at the moment is creation of a fresh
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archive.
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replacing/adding/deleting modules is likely to produce libraries
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with incorrect ranlib entries.
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The major troublemaker seems to be the extra padding byte at the end
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of odd sized modules.
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Led should return a non-zero value when it has found Undefined symbols
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or has another reason for not being able to produce a correct output file.
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