[LLVMdev] LLD improvement plan

[Sean Silva]

On Wed, May 6, 2015 at 9:30 AM, Daniel Dilts <diltsman at gmail.com> wrote:

>
>
> On Wed, May 6, 2015 at 12:51 AM, Will Newton <will.newton at gmail.com>
> wrote:
>
>> On Wed, May 6, 2015 at 6:22 AM, Chris Lattner <clattner at apple.com> wrote:
>> > On May 5, 2015, at 6:47 PM, Daniel Dilts <diltsman at gmail.com> wrote:
>> >
>> > Take a look at how debuggers have migrated through the years.  They too
>> >>
>> >> used to have their own script format.  Now most (all?) popular
>> debuggers
>> >> do scripting through embedding an actual programming language.  This
>> >> could be a better way forward for linkers as well -- embed Python in
>> the
>> >> linker, define a Python API for linkable item placement, entry point,
>> >> symbol operations, etc..., and then you also have the rest of Python at
>> >> your fingertips.
>> >
>> >
>> > I mostly care about specifying address where specific symbols will be
>> placed
>> > and specifying the memory layout of the platform.   I normally use
>> > __attribute__((section(""))) to place the symbols in their own sections
>> and
>> > then use the linker script to place the sections at the required
>> addresses.
>> > How would this be accomplished without linker scripts?
>> >
>> >
>> > I’d prefer to use an "__attribute__((address(0x1234)))” myself.  That
>> way
>> > you can control platform specifics with #ifdefs.
>>
>> But that way you have to do layout by hand in C. Generally you won't
>> know the size of the preceding code or data so you won't know what
>> address to put things at at the granularity of a single C level
>> object/function. Better to say "put this in the ROM section" and set
>> the address of the ROM section once in a linker script and let the
>> linker do the layout.
>>
>
> The real use case is on platforms, like ARM, where control registers are
> mapped to a specific address range.  Then it is useful to put an object
> that deals with the control registers at a specific address.
> __attribute__((address(0x1234))) can be replaced with a platform
> specific linker script.  Which is better?  I don't know, I haven't spent
> any time comparing them.
>

There is another important use of linker scripts, namely having a notion of
separate load and virtual addresses. This is important for e.g. an
initialized .data section which is stored in ROM, but needs to be copied to
writable memory.

For more information on linker scripts, see my post "linker script
findings": http://marc.info/?l=llvm-dev&m=135698146015498&w=2

-- Sean Silva


>
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