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, 3 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/ssa/decompose.go b/src/cmd/compile/internal/ssa/decompose.go
index 0cabfb61e7..af85090248 100644
--- a/src/cmd/compile/internal/ssa/decompose.go
+++ b/src/cmd/compile/internal/ssa/decompose.go
@@ -200,12 +200,13 @@ func decomposeComplexPhi(v *Value) {
 }
 
 func decomposeInterfacePhi(v *Value) {
+	uintptrType := v.Block.Func.Config.Types.Uintptr
 	ptrType := v.Block.Func.Config.Types.BytePtr
 
-	itab := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
+	itab := v.Block.NewValue0(v.Pos, OpPhi, uintptrType)
 	data := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
 	for _, a := range v.Args {
-		itab.AddArg(a.Block.NewValue1(v.Pos, OpITab, ptrType, a))
+		itab.AddArg(a.Block.NewValue1(v.Pos, OpITab, uintptrType, a))
 		data.AddArg(a.Block.NewValue1(v.Pos, OpIData, ptrType, a))
 	}
 	v.reset(OpIMake)