[LLVMdev] [Proposal] Parallelize post-IPO stage.

[Andrew Trick]

On Jul 14, 2013, at 6:38 PM, Eric Christopher <echristo at gmail.com> wrote:

> On Sun, Jul 14, 2013 at 5:57 PM, Andrew Trick <atrick at apple.com> wrote:
>> 
>> On Jul 12, 2013, at 3:49 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>> 
>> 6) Miscellaneous
>> ===========
>>  Will partitioning degrade performance in theory.  I think it depends on
>> the definition of
>> performance.  If performance means execution-time, I guess it dose not.
>> However, if performance includes code-size, I think it may have some
>> negative impact.
>> Following is few scenario:
>> 
>>  - constants generated by the post-IPO passes are not shared across
>> partitions
>>  - dead func may be detected during the post-IPO stage, and they may not be
>> deleted.
>> 
>> 
>> In don't know if it's feasible, but stable linker output, independent of the
>> partioning, is highly desirable. One of the most irritating performance
>> regressions to track down involves different versions of the host linker. If
>> partitioning decisions are thrown into the mix, this could be annoying. Is
>> it possible for the final link to do a better job cleaning up?
> 
> While I haven't yet read the rest of the proposal I'm going to comment
> on this in particular. In my view this is an absolute requirement as
> the compiler should produce the same output given the same input every
> time with no deviation.

The partitioning should be deterministic. It’s just that the linker output now depends on the partitioning heuristics. As long that decision is based on the input (not the host system), then it still meets Eric’s requirements. I just think it’s unfortunate that post-IPO partitioning (or more generally, parallel codegen) affects the output, but may be hard to avoid. It would be nice to be able to tune the partitioning for compile time without worrying about code quality.

Sorry for the tangential thought here... it seems that most of Shuxin’s proposal is actually independent of LTO, even though the prototype and primary goal is enabling LTO.

-Andy
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