cmd/compile: fix and improve alias detection
"aliased" is the function responsible for detecting whether we can
turn "a, b = x, y" into just "a = x; b = y", or we need to pre-compute
y and save it in a temporary variable because it might depend on a.
It currently has two issues:
1. It suboptimally treats assignments to blank as writes to heap
memory. Users generally won't write "_, b = x, y" directly, but it
comes up a lot in generated code within the compiler.
This CL changes it to ignore blank assignments.
2. When deciding whether the assigned variable might be referenced by
pointers, it mistakenly checks Class() and Name.Addrtaken() on "n"
(the *value* expression being assigned) rather than "a" (the
destination expression).
It doesn't appear to result in correctness issues (i.e.,
incorrectly reporting no aliasing when there is potential aliasing),
due to all the (overly conservative) rewrite passes before code
reaches here. But it generates unnecessary code and could have
correctness issues if we improve those other passes to be more
aggressive.
This CL fixes the misuse of "n" for "a" by renaming the variables
to "r" and "l", respectively, to make their meaning clearer.
Improving these two cases shaves 4.6kB of text from cmd/go, and 93kB
from k8s.io/kubernetes/cmd/kubelet:
text data bss dec hex filename
9732136 290072 231552
10253760 9c75c0 go.before
9727542 290072 231552
10249166 9c63ce go.after
97977637 1007051 301344
99286032 5eafc10 kubelet.before
97884549 1007051 301344
99192944 5e99070 kubelet.after
While here, this CL also collapses "memwrite" and "varwrite" into a
single variable. Logically, they're detecting the same thing: are we
assigning to a memory location that a pointer might alias. There's no
need for two variables.
Updates #6853.
Updates #23017.
Change-Id: I5a307b8e20bcd2196e85c55eb025d3f01e303008
Reviewed-on: https://go-review.googlesource.com/c/go/+/261677
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>