[LLVMdev] [PROPOSAL] ELF safe/unsafe sections

Shankar Easwaran shankare at codeaurora.org
Tue Jul 30 17:33:22 PDT 2013

On 7/30/2013 5:43 PM, Nick Kledzik wrote:
> On Jul 29, 2013, at 10:09 AM, Shankar Easwaran wrote:
>> On 7/29/2013 11:24 AM, Nick Kledzik wrote:
>>> On Jul 25, 2013, at 2:10 PM, Rui Ueyama wrote:
>>>> Is there any reason -ffunction-sections and -fdata-sections wouldn't work? If it'll work, it may be be better to say "if you want to get a better linker output use these options", rather than defining new ELF section.
>>> >From my understanding, -ffunction-sections is a good semantic match.  But it introduces a lot of bloat in the .o file which the linker must process.
>>> For reference, with mach-o we just added a flag to the overall .o file that says all sections are "safe".  The compiler always generates safe object files (unless there is inline code with non-local labels) and always sets the flag.   Hand written assembly files did not have the flag by default, but savvy assembly programmers can set it.
>> We could set this flag for ELF too in the ELF header, but it wouldnot not confirm to the ELF ABI.
>> To account safe sections, we should just create an additional section in the ELF (gcc creates a lot many sections to handle executable stack and for LTO). This would just be another section to dictate what sections are safe.
> Or just create a new empty section with a magic name whose existence tells the linker that all sections are safe.
>> Isnt it better to have this flag set for every section in Darwin too, makes it flexible. I am not sure about the ABI concerns on Darwin though.
> I don't see the benefit of that level of detail.  Either the compiler produced the object file, so all sections are safe. Or it was hand written.  If hand written, it is much easier to either say all sections are safe or none are.  Listing which are safe and which are not would be a pain.
I can think of two usecases when the compiler needs to emit safe 
sections on a section by section basis.

* code having inline assembly (it only affects the text section)
* the compiler trying to do some optimizations that deals with data 
placed outside function boundaries.

Does this make sense ?


Shankar Easwaran

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