[LLVMdev] Is lld the linker we need for our project ?

[Shankar Easwaran]

On 3/14/2014 5:13 AM, Michael Spencer wrote:
> On Fri, Mar 14, 2014 at 1:59 AM, Rodolphe Suescun <rsu.medal at dolphin.fr> wrote:
>> Hi,
>>
>> Thanks a lot for your answer. It seems lld is still the best
>> solution, even if it does not work "right out of the box" for
>> us today.
>>
>> We already have a solution for the "objcopy" part (added the
>> required output format to llvm-objdump).
> Interesting solution. Would it be much work to implement an
> llvm-objcopy that only supports -O binary? I plan to fully implement
> objcopy one of these days, but having just that as a start would be
> very useful.
>
>> The ScriptLayout class seems to be empty for now (on the master
>> branch at least), but we do not need linker scripts today.
>> All that is required for now is to be able to assign a fixed
>> address to a few atoms (the ones that will hold the reset &
>> interrupt vectors) and place code/data sections in code/data
>> memory (so we can simulate generated code and fix and optimize
>> our LLVM target).
>> I guess that can be done by adapting the DefaultLayout code in
>> our own Layout class, but any hint or documentation about how
There are two ways for getting this to work :-

(a)
There is a requirement that an output section could have a fixed address 
by using --section-start.

The way I thought to go about this was to have a structure, called

class AtomSectionAttributes {
virtual StringRef getOutputSectionName()  = 0;
};

class MergedSectionAttributes {
virtual uint64_t getAddress() = 0;
virtual StringRef name() = 0;
virtual bool needsNewProgramHeader() = 0;
};

When the user says -section-start .text = 0xf000000, what lld could do 
is create a mergeSection upfront, and set the MergedSectionAttributes 
information.

The assignVirtualAddress would change by looking at the 
mergedSectionAttribute to set the appropriate virtualaddress.

(b)

The hierarchy that we have is as below :-

DefaultLayout
  |
ScriptLayout
  |
TargetLayout

All the ELF writers have a TargetLayout object contained in them.

Use a linker script, you need to parse the SECTION command, and override 
the following functions in the ScriptLayout class to take care of the 
users intention specified in the linker script.

getSectionOrder
getSectionName
hasOutputSecgment
addAtom
assignVirtualAddress
assignFileOffsets


Does this help ?

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