[LLVMdev] [PROPOSAL] ELF safe/unsafe sections

[Reid Kleckner]

Can you guys invent a more specific word than "safe" to describe this
concept?  The best thing I can come up with is something like "atomizable
section".  This is basically describing code and data that the linker
should feel free to strip and reorder, right?  I'm not tracking this
closely enough to be totally sure if that's a reasonable word for this.


On Tue, Jul 30, 2013 at 5:33 PM, Shankar Easwaran
<shankare at codeaurora.org>wrote:

> 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 ?
>
>
> Thanks
>
> Shankar Easwaran
>
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