On Tue, Dec 7, 2010 at 3:58 PM, H.J. Lu <[email protected]> wrote:
> On Tue, Dec 7, 2010 at 3:57 PM, Andrew Pinski <[email protected]> wrote:
>> On Tue, Dec 7, 2010 at 3:53 PM, H.J. Lu <[email protected]> wrote:
>>> On Tue, Dec 7, 2010 at 3:15 PM, Cary Coutant <[email protected]> wrote:
>>>>> Here is my proposal. Any 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. The problem is LTO performs the best when
>>> generating the final executable/DSO. That is we want the full IR in the
>>> output of "ld -r".
>>
>> What happens when ld -r is the final link? Think 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.
---
• 2 kinds of object files
○ non-IR object file has
§ non-IR sections
○ IR object file has
§ IR sections
§ non-IR sections
• The output of "ld -r" with mixed IR/non-IR objects should work with:
○ Compilers/linkers with IR support.
○ Compilers/linkers without IR support.
• Add the mixed object file which has
○ IR sections
○ non-IR sections:
§ Object codes from IR sections.
§ Object codes from non-IR object files.
○ 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.
• Linker action:
○ Classify each input object file:
□ If there is a ".objectonly\004" section, it is a mixed object
file.
□ If there is a IR section, it is an IR object file.
□ Otherwise, it is a non-IR object file.
○ Relocatable non-IR link:
§ Prepare for an object-only output.
§ Prepare for a regular output.
§ For each mixed object file:
□ Add IR and non-IR sections to the regular output.
□ For object-only section:
® Extract object only file.
® Add it to the object-only output.
® Discard object-only section.
§ For each IR object file:
□ Add IR and non-IR sections to the regular output.
§ For each non-IR object file:
□ Add non-IR sections to the regular output.
□ Add non-IR sections to the object-only output.
§ Final output:
□ 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.
□ Remove the object-only output.
○ Normal link and relocatable IR link:
§ Prepare for output.
§ IR link:
□ For each mixed object file:
® Compile and add IR sections to the output.
® Discard non-IR sections.
® Object-only section:
◊ Extract object only file.
◊ Add it to the output.
◊ Discard object-only section.
□ For each IR object file:
® Compile and add IR sections to the output.
® Discard non-IR sections.
□ For each non-IR object file,
® Add non-IR sections to the output.
§ Non-IR link:
□ For each mixed object file:
® Add non-IR sections to the output.
® Discard IR sections and object-only section.
□ For each IR object file:
® Add non-IR sections to the output.
® Discard IR sections .
□ For each non-IR object file:
® Add non-IR sections to the output.
