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Re: "ld -r" on mixed IR/non-IR objects (
On Tue, Dec 7, 2010 at 3:58 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Dec 7, 2010 at 3:57 PM, Andrew Pinski <pinskia@gmail.com> wrote:
>> On Tue, Dec 7, 2010 at 3:53 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Tue, Dec 7, 2010 at 3:15 PM, Cary Coutant <ccoutant@google.com> wrote:
>>>>> Here is my proposal. ÂAny comments?
>>>>
>>>> We talked about ld -r a while back during the WHOPR project, and the
>>>> two ways that the linker could work: (1) combine all the .o files and
>>>> use the plugin to run LTRANS on the IR files, producing a pure,
>>>> optimized, object file; and (2) combine the non-IR object files as ld
>>>> -r normally would, and combine that result somehow with the IR from
>>>> the other files, for later optimization. If I remember correctly,
>>>> there was support for both modes of operation. The first mode is
>>>> easily handled with the current design (untested as far as I know --
>>>> there are probably bugs, and I'm not sure if we get the symbol
>>>> visibility correct in those cases).
>>>
>>> We considered it. ÂThe problem is LTO performs the best when
>>> generating the final executable/DSO. ÂThat is we want the full IR in the
>>> output of "ld -r".
>>
>> What happens when ld -r is the final link? ÂThink loadable linux
>> kernel modules and some other stuff that abuse elf relocatable
>> objects?
>>
>
> "ld -plugin ... -r" will be treated as final link.
>
Here is the updated proposal.
--
H.J.
---
â 2 kinds of object files
â non-IR object file has
 non-IR sections
â IR object file has
 IR sections
 non-IR sections
â The output of "ld -r" with mixed IR/non-IR objects should work with:
â Compilers/linkers with IR support.
â Compilers/linkers without IR support.
â Add the mixed object file which has
â IR sections
â non-IR sections:
 Object codes from IR sections.
 Object codes from non-IR object files.
â Object-only section:
 Section name won't be generated by any tools, something like
".objectonly\004".
 Contains non-IR object file.
 Input is discarded after link.
â Linker action:
â Classify each input object file:
â If there is a ".objectonly\004" section, it is a mixed object file.
â If there is a IR section, it is an IR object file.
â Otherwise, it is a non-IR object file.
â Relocatable non-IR link:
 Prepare for an object-only output.
 Prepare for a regular output.
 For each mixed object file:
â Add IR and non-IR sections to the regular output.
â For object-only section:
 Extract object only file.
 Add it to the object-only output.
 Discard object-only section.
 For each IR object file:
â Add IR and non-IR sections to the regular output.
 For each non-IR object file:
â Add non-IR sections to the regular output.
â Add non-IR sections to the object-only output.
 Final output:
â If there are IR objects, non-IR objects and the object-only
output isn't empty:
 Put the object-only output into the object-only section.
 Add the object-only section to the regular output.
â Remove the object-only output.
â Normal link and relocatable IR link:
 Prepare for output.
 IR link:
â For each mixed object file:
 Compile and add IR sections to the output.
 Discard non-IR sections.
 Object-only section:
â Extract object only file.
â Add it to the output.
â Discard object-only section.
â For each IR object file:
 Compile and add IR sections to the output.
 Discard non-IR sections.
â For each non-IR object file,
 Add non-IR sections to the output.
 Non-IR link:
â For each mixed object file:
 Add non-IR sections to the output.
 Discard IR sections and object-only section.
â For each IR object file:
 Add non-IR sections to the output.
 Discard IR sections .
â For each non-IR object file:
 Add non-IR sections to the output.