[PATCH] D21405: [PGO] IRPGO pre-cleanup pass changes

[Sean Silva via llvm-commits]

silvas added a comment.

In http://reviews.llvm.org/D21405#459242, @mehdi_amini wrote:

> I really don't like the fact that the PGO pipeline will be different from the non-PGO pipeline (other than what is required for instrumentations). I wonder what other people will think of that.


This was one of my initial concerns too: http://lists.llvm.org/pipermail/llvm-dev/2015-August/089058.html
Jake and I (actually mostly Jake) did some basic measurements related to this and did not find any significant difference.

Rong also did some measurements related to this in the initial RFC thread: http://lists.llvm.org/pipermail/llvm-dev/2015-August/089425.html

I agree though: generally speaking, we should avoid running too many passes before instrumentation precisely because they cause divergence between the PGO and non-PGO pipelines. Thankfully Jake and I found that on our games only pre-inlining was needed (running other optimizations before instrumentation did not help significantly for our test cases beyond the benefit of just pre-inlining).

(note: due to PR27299, a run of simplifycfg after inlining is needed. But this run of simplifycfg doesn't really affect the performance of the instrumented build at all)


http://reviews.llvm.org/D21405