cmd/compile: mark the first word of an interface as a uintptr

The first word of an interface is a pointer, but for the purposes
of GC we don't need to treat it as such.
 1. If it is a non-empty interface, the pointer points to an itab
    which is always in persistentalloc space.
 2. If it is an empty interface, the pointer points to a _type.
   a. If it is a compile-time-allocated type, it points into
      the read-only data section.
   b. If it is a reflect-allocated type, it points into the Go heap.
      Reflect is responsible for keeping a reference to
      the underlying type so it won't be GCd.

If we ever have a moving GC, we need to change this for 2b (as
well as scan itabs to update their itab._type fields).

Write barriers on the first word of interfaces have already been removed.

Change-Id: I643e91d7ac4de980ac2717436eff94097c65d959
Reviewed-on: https://go-review.googlesource.com/97518
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/test/live.go b/test/live.go
index e54336ead7..ecab83e276 100644
--- a/test/live.go
+++ b/test/live.go
@@ -141,7 +141,7 @@ var i9 interface{}
 func f9() bool {
 	g8()
 	x := i9
-	y := interface{}(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+ x.data x.type$" "live at call to str: x.data x.type$"
+	y := interface{}(str()) // ERROR "live at call to convT2Estring: .autotmp_[0-9]+ x.data$" "live at call to str: x.data$"
 	i9 = y                  // make y escape so the line above has to call convT2E
 	return x != y
 }