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cmd/compile/internal/staticinit: make staticopy safe

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Currently, cmd/compile optimizes `var a = true; var b = a` into `var a
= true; var b = true`. But this may not be safe if we need to
initialize any other global variables between `a` and `b`, and the
initialization involves calling a user-defined function that reassigns
`a`.

This CL changes staticinit to keep track of the initialization
expressions that we've seen so far, and to stop applying the
staticcopy optimization once we've seen an initialization expression
that might have modified another global variable within this package.

To help identify affected initializers, this CL adds a -d=staticcopy
flag to warn when a staticcopy is suppressed and turned into a dynamic
copy.

Currently, `go build -gcflags=all=-d=staticcopy std` reports only four
instances:

```
encoding/xml/xml.go:1600:5: skipping static copy of HTMLEntity+0 with map[string]string{...}
encoding/xml/xml.go:1869:5: skipping static copy of HTMLAutoClose+0 with []string{...}
net/net.go:661:5: skipping static copy of .stmp_31+0 with poll.ErrNetClosing
net/http/transport.go:2566:5: skipping static copy of errRequestCanceled+0 with ~R0
```

Fixes #51913.

Change-Id: Iab41cf6f84c44f7f960e4e62c28a8aeaade4fbcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/395541
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
This commit is contained in:
Matthew Dempsky 2022-03-24 12:27:39 -07:00 • committed by Gopher Robot
parent 1cdabf0c8b
commit afa3f8e104
6 changed files with 130 additions and 26 deletions
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1
src/cmd/compile/internal/base/debug.go
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@ -47,6 +47,7 @@ type DebugFlags struct {
Shapify int `help:"print information about shaping recursive types"`
Slice int `help:"print information about slice compilation"`
SoftFloat int `help:"force compiler to emit soft-float code" concurrent:"ok"`
StaticCopy int `help:"print information about missed static copies" concurrent:"ok"`
SyncFrames int `help:"how many writer stack frames to include at sync points in unified export data"`
TypeAssert int `help:"print information about type assertion inlining"`
WB int `help:"print information about write barriers"`