cmd/compile: don't statically copy string-typed variables

During package initialization, the compiler tries to optimize:

    var A = "foo"
    var B = A

into

    var A = "foo"
    var B = "foo"

so that we can statically initialize both A and B and skip emitting
dynamic initialization code to assign "B = A".

However, this isn't safe in the presence of cmd/link's -X flag, which
might overwrite an initialized string-typed variable at link time. In
particular, if cmd/link changes A's static initialization, it won't
know it also needs to change B's static initialization.

To address this, this CL disables this optimization for string-typed
variables.

Fixes #34675.

Change-Id: I1c18f3b855f6d7114aeb39f96aaaf1b452b88236
Reviewed-on: https://go-review.googlesource.com/c/go/+/198657
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/cmd/compile/internal/gc/sinit.go b/src/cmd/compile/internal/gc/sinit.go
index ae16d41b1ccd24eb2f9f30698a1c31e606dafc57..a6d13d1ac5f1d10813194d935e303e6c24a24a27 100644 (file)
--- a/src/cmd/compile/internal/gc/sinit.go
+++ b/src/cmd/compile/internal/gc/sinit.go
@@ -83,6 +83,9 @@ func (s *InitSchedule) staticcopy(l *Node, r *Node) bool {
        if r.Name.Defn.Op != OAS {
                return false
        }
+       if r.Type.IsString() { // perhaps overwritten by cmd/link -X (#34675)
+               return false
+       }
        orig := r
        r = r.Name.Defn.Right
 

diff --git a/test/linkx.go b/test/linkx.go
index 20b8c773d8bfa90c3c33c0562cefef3b1c508e1c..520a0651827cfb9aa6a8246c1af771dda0da0299 100644 (file)
--- a/test/linkx.go
+++ b/test/linkx.go
@@ -14,10 +14,19 @@ import "fmt"
 var tbd string
 var overwrite string = "dibs"
 
+var tbdcopy = tbd
+var overwritecopy = overwrite
+var arraycopy = [2]string{tbd, overwrite}
+
 var b bool
 var x int
 
 func main() {
        fmt.Println(tbd)
+       fmt.Println(tbdcopy)
+       fmt.Println(arraycopy[0])
+
        fmt.Println(overwrite)
+       fmt.Println(overwritecopy)
+       fmt.Println(arraycopy[1])
 }

diff --git a/test/linkx_run.go b/test/linkx_run.go
index ca9d31612ac3e482c2698848014c2eb79e3921f9..f25053bf28e9c1b7502ba42c6a8f5356a86d80da 100644 (file)
--- a/test/linkx_run.go
+++ b/test/linkx_run.go
@@ -36,7 +36,7 @@ func test(sep string) {
                os.Exit(1)
        }
 
-       want := "hello\ntrumped\n"
+       want := "hello\nhello\nhello\ntrumped\ntrumped\ntrumped\n"
        got := out.String()
        if got != want {
                fmt.Printf("got %q want %q\n", got, want)