[PATCH] D29701: ThinLTOBitcodeWriter: Write available_externally copies of VCP eligible functions to merged module.

[Peter Collingbourne via Phabricator via llvm-commits]

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In https://reviews.llvm.org/D29701#674285, @mehdi_amini wrote:

> In https://reviews.llvm.org/D29701#670366, @pcc wrote:
>
> > Note: I patched in https://github.com/pcc/llvm-project/commit/5a5904d6721f895eafdd2fc476872b98806c36e6 to measure perf impact. Total wall time spent in addRegularLTO when linking chrome was 8.2743s, as compared to about 6 seconds before (see https://reviews.llvm.org/D27324).
>
>
> So >30% overhead on addRegularLTO, but what is the overhead for `runRegularLTO`?


Just did a fresh set of runs on chrome with https://github.com/pcc/llvm-project/commits/lto-timers patched in, and took median of 5 for each measurement:

                 before after
  addRegularLTO 8.0407s 9.7432s (+21%)
  runRegularLTO 9.6064s 10.4748s (+9%)

Note that we haven't done everything we can -- a couple more things I can think of are to exclude functions that take non-integer arguments (other than "this"), and to set optnone on functions in the merged module. Taking a look at chrome's merged module, there are 6727 available_externally functions, of which 1340 take a pointer argument other than "this".


https://reviews.llvm.org/D29701