[LLVMdev] LLD improvement plan

[Reid Kleckner]

On Thu, May 7, 2015 at 8:36 AM, Shankar Easwaran <shankare at codeaurora.org>
wrote:

> The atom model is optimized when you compile the code with
> -ffunction-sections and -fdata-sections.
>

Not really, -ffunction-sections -fdata-sections gives a section-based
linker the same flexibility as an atom-based linker.

In the atom model, these extra sections simply serve as a crutch to ensure
that object files can be safely atomized. If you commit to the atom model,
you shouldn't even need these sections, you should only need a flag that
says "this section is safe for atomization". On MachO, this is what
.subsections_via_symbols is for.


> Once targets start having -fno-unique-section-names as the default the
> atom model looks more promising.
>

Why does LLD care about -fno-unique-section-names? I thought this was just
an object file size optimization.


> Everyone likes to have the image size small, and making
> -ffunction-sections/-fdata-sections (or) -fno-unique-section-names the
> default make sense and the atom model design directly has a relation to it.
> In fact it simplifies the linker to not have extra data structures IMO.


Again, the same is achievable in the section model by turning on these
flags and enabling --gc-sections or equivalent functionality.

The only advantage I see to using the atom model for ELF and COFF is that
we could dead strip object files built without -ffunction-sections
-fdata-sections, which would allow us to turn these flags off in the
compiler and save on object file size. I suspect the complexity of the atom
model isn't worth the file size savings it offers.
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