Revert "cmd/compile: allow multi-field structs to be stored directly in interfaces"

This reverts commit cd55f86 (CL 681937)

Reason for revert: still causing compiler failures on Google test code

Change-Id: I5cd482fd607fd060a523257082d48821b5f965d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/695016
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/_gen/generic.rules

@@ -921,8 +921,8 @@
   @x.Block (Load <v.Type> (OffPtr <v.Type.PtrTo()> [t.FieldOff(int(i))] ptr) mem)
 
 // Putting struct{*byte} and similar into direct interfaces.
-(IMake _typ (StructMake val)) => imakeOfStructMake(v)
-(StructSelect [_] (IData x)) => (IData x)
+(IMake _typ (StructMake val)) => (IMake _typ val)
+(StructSelect [0] (IData x)) => (IData x)
 
 // un-SSAable values use mem->mem copies
 (Store {t} dst (Load src mem) mem) && !CanSSA(t) =>

src/cmd/compile/internal/ssa/expand_calls.go

@@ -423,14 +423,7 @@ func (x *expandState) decomposeAsNecessary(pos src.XPos, b *Block, a, m0 *Value,
 		if a.Op == OpIMake {
 			data := a.Args[1]
 			for data.Op == OpStructMake || data.Op == OpArrayMake1 {
-				// A struct make might have a few zero-sized fields.
-				// Use the pointer-y one we know is there.
-				for _, a := range data.Args {
-					if a.Type.Size() > 0 {
-						data = a
-						break
-					}
-				}
+				data = data.Args[0]
 			}
 			return x.decomposeAsNecessary(pos, b, data, mem, rc.next(data.Type))
 		}

src/cmd/compile/internal/ssa/rewrite.go

@@ -2621,17 +2621,3 @@ func panicBoundsCToAux(p PanicBoundsC) Aux {
 func panicBoundsCCToAux(p PanicBoundsCC) Aux {
 	return p
 }
-
-// When v is (IMake typ (StructMake ...)), convert to
-// (IMake typ arg) where arg is the pointer-y argument to
-// the StructMake (there must be exactly one).
-func imakeOfStructMake(v *Value) *Value {
-	var arg *Value
-	for _, a := range v.Args[1].Args {
-		if a.Type.Size() > 0 {
-			arg = a
-			break
-		}
-	}
-	return v.Block.NewValue2(v.Pos, OpIMake, v.Type, v.Args[0], arg)
-}

src/cmd/compile/internal/ssa/rewritegeneric.go

@@ -1822,7 +1822,26 @@ func IsReflexive(t *Type) bool {
 // Can this type be stored directly in an interface word?
 // Yes, if the representation is a single pointer.
 func IsDirectIface(t *Type) bool {
-	return t.Size() == int64(PtrSize) && PtrDataSize(t) == int64(PtrSize)
+	switch t.Kind() {
+	case TPTR:
+		// Pointers to notinheap types must be stored indirectly. See issue 42076.
+		return !t.Elem().NotInHeap()
+	case TCHAN,
+		TMAP,
+		TFUNC,
+		TUNSAFEPTR:
+		return true
+
+	case TARRAY:
+		// Array of 1 direct iface type can be direct.
+		return t.NumElem() == 1 && IsDirectIface(t.Elem())
+
+	case TSTRUCT:
+		// Struct with 1 field of direct iface type can be direct.
+		return t.NumFields() == 1 && IsDirectIface(t.Field(0).Type)
+	}
+
+	return false
 }
 
 // IsInterfaceMethod reports whether (field) m is

src/cmd/compile/internal/types/type.go

@@ -121,8 +121,8 @@ const (
 	TFlagGCMaskOnDemand TFlag = 1 << 4
 
 	// TFlagDirectIface means that a value of this type is stored directly
-	// in the data field of an interface, instead of indirectly.
-	// This flag is just a cached computation of Size_ == PtrBytes == goarch.PtrSize.
+	// in the data field of an interface, instead of indirectly. Normally
+	// this means the type is pointer-ish.
 	TFlagDirectIface TFlag = 1 << 5
 
 	// Leaving this breadcrumb behind for dlv. It should not be used, and no

src/internal/abi/type.go

@@ -2524,7 +2524,8 @@ func StructOf(fields []StructField) Type {
 	}
 
 	switch {
-	case typ.Size_ == goarch.PtrSize && typ.PtrBytes == goarch.PtrSize:
+	case len(fs) == 1 && fs[0].Typ.IsDirectIface():
+		// structs of 1 direct iface type can be direct
 		typ.TFlag |= abi.TFlagDirectIface
 	default:
 		typ.TFlag &^= abi.TFlagDirectIface
@@ -2693,7 +2694,8 @@ func ArrayOf(length int, elem Type) Type {
 	}
 
 	switch {
-	case array.Size_ == goarch.PtrSize && array.PtrBytes == goarch.PtrSize:
+	case length == 1 && typ.IsDirectIface():
+		// array of 1 direct iface type can be direct
 		array.TFlag |= abi.TFlagDirectIface
 	default:
 		array.TFlag &^= abi.TFlagDirectIface