cmd/gc: fix x=x crash

The 'nodarg' function is used to obtain a Node*
representing a function argument or result.
It returned a brand new Node*, but that violates
the guarantee in most places in the compiler that
two Node*s refer to the same variable if and only if
they are the same Node* pointer. Reestablish that
invariant by making nodarg return a preexisting
named variable if present.

Having fixed that, avoid any copy during x=x in
componentgen, because the VARDEF we emit
before the copy marks the lhs x as dead incorrectly.

The change in walk.c avoids modifying the result
of nodarg. This was the only place in the compiler
that did so.

Fixes issue 8097.

LGTM=r, khr
R=golang-codereviews, r, khr
CC=golang-codereviews, iant
https://codereview.appspot.com/102820043

1 files changed, 26 insertions, 0 deletions

diff --git a/test/live.go b/test/live.go
index 21d3e6a5f..286fcc306 100644
--- a/test/live.go
+++ b/test/live.go
@@ -564,3 +564,29 @@ func f38(b bool) {
 	}
 	println()
 }
+
+// issue 8097: mishandling of x = x during return.
+
+func f39() (x []int) {
+	x = []int{1}
+	println() // ERROR "live at call to printnl: x"
+	return x
+}
+
+func f39a() (x []int) {
+	x = []int{1}
+	println() // ERROR "live at call to printnl: x"
+	return
+}
+
+func f39b() (x [10]*int) {
+	x = [10]*int{new(int)} // ERROR "live at call to new: x"
+	println() // ERROR "live at call to printnl: x"
+	return x
+}
+
+func f39c() (x [10]*int) {
+	x = [10]*int{new(int)} // ERROR "live at call to new: x"
+	println() // ERROR "live at call to printnl: x"
+	return
+}