[dev.typeparams] cmd/compile: simplify interface conversions

Simplify the implementation of interface conversions in the compiler.
Don't pass fields that aren't needed (the data word, usually) to the runtime.

For generics, we need to put a dynamic type in an interface. The new
dataWord function is exactly what we need (the type word will come
from a dictionary).

Change-Id: Iade5de5c174854b65ad248f35c7893c603f7be3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/340029
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

1 files changed, 27 insertions, 64 deletions

diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index 79a49c0dff..3d1d9d6ba1 100644
--- a/src/runtime/iface.go
+++ b/src/runtime/iface.go
@@ -316,20 +316,30 @@ var (
 // The convXXX functions succeed on a nil input, whereas the assertXXX
 // functions fail on a nil input.
 
-func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
+// convT converts a value of type t, which is pointed to by v, to a pointer that can
+// be used as the second word of an interface value.
+func convT(t *_type, v unsafe.Pointer) unsafe.Pointer {
 	if raceenabled {
-		raceReadObjectPC(t, elem, getcallerpc(), abi.FuncPCABIInternal(convT2E))
+		raceReadObjectPC(t, v, getcallerpc(), abi.FuncPCABIInternal(convT))
 	}
 	if msanenabled {
-		msanread(elem, t.size)
+		msanread(v, t.size)
 	}
 	x := mallocgc(t.size, t, true)
-	// TODO: We allocate a zeroed object only to overwrite it with actual data.
-	// Figure out how to avoid zeroing. Also below in convT2Eslice, convT2I, convT2Islice.
-	typedmemmove(t, x, elem)
-	e._type = t
-	e.data = x
-	return
+	typedmemmove(t, x, v)
+	return x
+}
+func convTnoptr(t *_type, v unsafe.Pointer) unsafe.Pointer {
+	// TODO: maybe take size instead of type?
+	if raceenabled {
+		raceReadObjectPC(t, v, getcallerpc(), abi.FuncPCABIInternal(convTnoptr))
+	}
+	if msanenabled {
+		msanread(v, t.size)
+	}
+	x := mallocgc(t.size, t, false)
+	memmove(x, v, t.size)
+	return x
 }
 
 func convT16(val uint16) (x unsafe.Pointer) {
@@ -389,63 +399,16 @@ func convTslice(val []byte) (x unsafe.Pointer) {
 	return
 }
 
-func convT2Enoptr(t *_type, elem unsafe.Pointer) (e eface) {
-	if raceenabled {
-		raceReadObjectPC(t, elem, getcallerpc(), abi.FuncPCABIInternal(convT2Enoptr))
-	}
-	if msanenabled {
-		msanread(elem, t.size)
-	}
-	x := mallocgc(t.size, t, false)
-	memmove(x, elem, t.size)
-	e._type = t
-	e.data = x
-	return
-}
-
-func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
-	t := tab._type
-	if raceenabled {
-		raceReadObjectPC(t, elem, getcallerpc(), abi.FuncPCABIInternal(convT2I))
-	}
-	if msanenabled {
-		msanread(elem, t.size)
-	}
-	x := mallocgc(t.size, t, true)
-	typedmemmove(t, x, elem)
-	i.tab = tab
-	i.data = x
-	return
-}
-
-func convT2Inoptr(tab *itab, elem unsafe.Pointer) (i iface) {
-	t := tab._type
-	if raceenabled {
-		raceReadObjectPC(t, elem, getcallerpc(), abi.FuncPCABIInternal(convT2Inoptr))
-	}
-	if msanenabled {
-		msanread(elem, t.size)
-	}
-	x := mallocgc(t.size, t, false)
-	memmove(x, elem, t.size)
-	i.tab = tab
-	i.data = x
-	return
-}
-
-func convI2I(inter *interfacetype, i iface) (r iface) {
-	tab := i.tab
-	if tab == nil {
-		return
+// convI2I returns the new itab to be used for the destination value
+// when converting a value with itab src to the dst interface.
+func convI2I(dst *interfacetype, src *itab) *itab {
+	if src == nil {
+		return nil
 	}
-	if tab.inter == inter {
-		r.tab = tab
-		r.data = i.data
-		return
+	if src.inter == dst {
+		return src
 	}
-	r.tab = getitab(inter, tab._type, false)
-	r.data = i.data
-	return
+	return getitab(dst, src._type, false)
 }
 
 func assertI2I(inter *interfacetype, tab *itab) *itab {