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.
