[PATCH] D27324: IPO: Introduce ThinLTOBitcodeWriter pass.

Mon Dec 5 15:38:02 PST 2016

pcc added a comment.

Mehdi asked me to look at the cost of the split module design. To do that, I took my prototype [0] and made a change [1] to shrink the size of the regular LTO module. With that the total time spent in LTO::addRegularLTO and LTO::addThinLTO on my machine during a ThinLTO link of Chromium is:

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   6.1434 ( 58.6%)   0.2158 ( 36.2%)   6.3591 ( 57.4%)   6.1709 ( 55.6%)  LTO::addRegularLTO
   4.3320 ( 41.4%)   0.3805 ( 63.8%)   4.7125 ( 42.6%)   4.9269 ( 44.4%)  LTO::addThinLTO
  10.4754 (100.0%)   0.5963 (100.0%)  11.0717 (100.0%)  11.0978 (100.0%)  Total

So that's about 6 seconds spent loading and linking all of Chromium's vtables, as compared to 5 seconds spent building the combined summary. If the figures were an order of magnitude off I'd be concerned, but these figures seem reasonable enough to me.

[0] https://github.com/pcc/llvm-project/tree/cfi-thinlto
[1] https://github.com/pcc/llvm-project/commit/de5bd8fe4c05c6e9aecf1e5384ef5553bf7332f0

https://reviews.llvm.org/D27324