cmd/compile: constant-fold loads from constant dictionaries and types

Retrying the original CL with a small modification. The original CL
did not handle the case of reading an itab out of a dictionary
correctly.  When we read an itab out of a dictionary, we must treat
the type inside that itab as maybe being put in an interface.

Original CL: 486895
Revert CL: 490156

Change-Id: Id2dc1699d184cd8c63dac83986a70b60b4e6cbd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/491495
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/reflectdata/reflect.go b/src/cmd/compile/internal/reflectdata/reflect.go
index eeda3cb4e2..ff2eb1aba9 100644
--- a/src/cmd/compile/internal/reflectdata/reflect.go
+++ b/src/cmd/compile/internal/reflectdata/reflect.go
@@ -835,7 +835,14 @@ func TypeLinksymLookup(name string) *obj.LSym {
 }
 
 func TypeLinksym(t *types.Type) *obj.LSym {
-	return TypeSym(t).Linksym()
+	lsym := TypeSym(t).Linksym()
+	signatmu.Lock()
+	if lsym.Extra == nil {
+		ti := lsym.NewTypeInfo()
+		ti.Type = t
+	}
+	signatmu.Unlock()
+	return lsym
 }
 
 // Deprecated: Use TypePtrAt instead.
@@ -1869,7 +1876,9 @@ func MarkTypeUsedInInterface(t *types.Type, from *obj.LSym) {
 		// Shape types shouldn't be put in interfaces, so we shouldn't ever get here.
 		base.Fatalf("shape types have no methods %+v", t)
 	}
-	tsym := TypeLinksym(t)
+	MarkTypeSymUsedInInterface(TypeLinksym(t), from)
+}
+func MarkTypeSymUsedInInterface(tsym *obj.LSym, from *obj.LSym) {
 	// Emit a marker relocation. The linker will know the type is converted
 	// to an interface if "from" is reachable.
 	r := obj.Addrel(from)