cmd/compile: fix x=x assignments
No point in doing anything for x=x assignments.
In addition, skipping these assignments prevents generating:
VARDEF x
COPY x -> x
which is bad because x is incorrectly considered
dead before the vardef.
Fixes #14904
Change-Id: I6817055ec20bcc34a9648617e0439505ee355f82
Reviewed-on: https://go-review.googlesource.com/21470
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index 359f4b2..1c2e528 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -661,6 +661,17 @@
return
}
+ if n.Left == n.Right && n.Left.Op == ONAME {
+ // An x=x assignment. No point in doing anything
+ // here. In addition, skipping this assignment
+ // prevents generating:
+ // VARDEF x
+ // COPY x -> x
+ // which is bad because x is incorrectly considered
+ // dead before the vardef. See issue #14904.
+ return
+ }
+
var t *Type
if n.Right != nil {
t = n.Right.Type
diff --git a/src/cmd/compile/internal/gc/ssa_test.go b/src/cmd/compile/internal/gc/ssa_test.go
index 59a2402..0fb0f17 100644
--- a/src/cmd/compile/internal/gc/ssa_test.go
+++ b/src/cmd/compile/internal/gc/ssa_test.go
@@ -99,3 +99,5 @@
func TestPhi(t *testing.T) { runTest(t, "phi_ssa.go") }
func TestSlice(t *testing.T) { runTest(t, "slice.go") }
+
+func TestNamedReturn(t *testing.T) { runTest(t, "namedReturn.go") }
diff --git a/src/cmd/compile/internal/gc/testdata/namedReturn.go b/src/cmd/compile/internal/gc/testdata/namedReturn.go
new file mode 100644
index 0000000..dafb5d7
--- /dev/null
+++ b/src/cmd/compile/internal/gc/testdata/namedReturn.go
@@ -0,0 +1,101 @@
+// run
+
+// This test makes sure that naming named
+// return variables in a return statement works.
+// See issue #14904.
+
+package main
+
+import (
+ "fmt"
+ "runtime"
+)
+
+// Our heap-allocated object that will be GC'd incorrectly.
+// Note that we always check the second word because that's
+// where 0xdeaddeaddeaddead is written.
+type B [4]int
+
+// small (SSAable) array
+type T1 [3]*B
+
+//go:noinline
+func f1() (t T1) {
+ t[0] = &B{91, 92, 93, 94}
+ runtime.GC()
+ return t
+}
+
+// large (non-SSAable) array
+type T2 [8]*B
+
+//go:noinline
+func f2() (t T2) {
+ t[0] = &B{91, 92, 93, 94}
+ runtime.GC()
+ return t
+}
+
+// small (SSAable) struct
+type T3 struct {
+ a, b, c *B
+}
+
+//go:noinline
+func f3() (t T3) {
+ t.a = &B{91, 92, 93, 94}
+ runtime.GC()
+ return t
+}
+
+// large (non-SSAable) struct
+type T4 struct {
+ a, b, c, d, e, f *B
+}
+
+//go:noinline
+func f4() (t T4) {
+ t.a = &B{91, 92, 93, 94}
+ runtime.GC()
+ return t
+}
+
+var sink *B
+
+func f5() int {
+ b := &B{91, 92, 93, 94}
+ t := T4{b, nil, nil, nil, nil, nil}
+ sink = b // make sure b is heap allocated ...
+ sink = nil // ... but not live
+ runtime.GC()
+ t = t
+ return t.a[1]
+}
+
+func main() {
+ failed := false
+
+ if v := f1()[0][1]; v != 92 {
+ fmt.Printf("f1()[0][1]=%d, want 92\n", v)
+ failed = true
+ }
+ if v := f2()[0][1]; v != 92 {
+ fmt.Printf("f2()[0][1]=%d, want 92\n", v)
+ failed = true
+ }
+ if v := f3().a[1]; v != 92 {
+ fmt.Printf("f3().a[1]=%d, want 92\n", v)
+ failed = true
+ }
+ if v := f4().a[1]; v != 92 {
+ fmt.Printf("f4().a[1]=%d, want 92\n", v)
+ failed = true
+ }
+ if v := f5(); v != 92 {
+ fmt.Printf("f5()=%d, want 92\n", v)
+ failed = true
+ }
+ if failed {
+ panic("bad")
+ }
+}
