Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

[Peter Zijlstra]

On Sat, Jun 12, 2021 at 01:20:15PM -0700, Fangrui Song wrote:

> For applications, I think instrumentation based PGO can be 1%~4% faster
> than sample-based PGO (e.g. AutoFDO) on x86.

Why? What specifically is missed by sample-based? I thought that LBR
augmented samples were very useful for exactly this.

> Sample-based PGO has CPU requirement (e.g. Performance Monitoring Unit).
> (my gut feeling is that there may be larger gap between instrumentation
> based PGO and sample-based PGO for aarch64/ppc64, even though they can
> use sample-based PGO.)
> Instrumentation based PGO can be ported to more architectures.

Every architecture that cares about performance had better have a
hardware PMU. Both argh64 and ppc64 have one.

> In addition, having an infrastructure for instrumentation based PGO
> makes it easy to deploy newer techniques like context-sensitive PGO
> (just changed compile options; it doesn't need new source level
> annotation).

What's this context sensitive stuff you speak of? The link provided
earlier is devoid of useful information.