runtime: simplify mem profile checking in mallocgc
Checking whether the current allocation needs to be profiled is
currently branch-y and weirdly a lot of code. The branches are
generally predictable, but it's a surprising number of instructions.
Part of the problem is that MemProfileRate is just a global that can be
set at any time, so we need to load it and check certain settings
explicitly. In an ideal world, we would just always subtract from
nextSample and have a single branch to take the slow path if we
subtract below zero.
If MemProfileRate were a function, we could trash all the nextSample
values intentionally in each mcache. This would be slow, but
MemProfileRate changes rarely while the malloc hot path is well, hot.
Unfortunate...
Although this ideal world is, AFAICT, impossible, we can still get
close. If we cache the value of MemProfileRate in each mcache, then we
can force malloc to take the slow path whenever MemProfileRate changes.
This does require two additional loads, but crucially, these loads are
independent of everything else in mallocgc. Furthermore, the branch
dependent on those loads is incredibly predictable in practice.
This CL on its own has little-to-no impact on mallocgc. But this
codepath is going to be duplicated in several places in the next CL, so
it'll pay to simplify it. Also, we're very much trying to remedy a
death-by-a-thousand-cuts situation, and malloc is currently still kind
of a monster -- it will not help if mallocgc isn't really streamlined
itself.
Lastly, there's a nice property now that all nextSample values get
immediately re-sampled when MemProfileRate changes.
Change-Id: I6443d0cf9bd7861595584442b675ac1be8ea3455
Reviewed-on: https://go-review.googlesource.com/c/go/+/615815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>