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

If the struct is a bunch of 0-sized fields and one pointer field.

Fixes #74092

Change-Id: I87c5d162c8c9fdba812420d7f9d21de97295b62c
Reviewed-on: https://go-review.googlesource.com/c/go/+/681937
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[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)) => (IMake _typ val)
-(StructSelect [0] (IData x)) => (IData x)
+(IMake _typ (StructMake val)) => imakeOfStructMake(v)
+(StructSelect [_] (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

@@ -423,7 +423,14 @@ 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 {
-				data = data.Args[0]
+				// 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
+					}
+				}
 			}
 			return x.decomposeAsNecessary(pos, b, data, mem, rc.next(data.Type))
 		}

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

@@ -2689,3 +2689,17 @@ 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

@@ -1829,26 +1829,7 @@ 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 {
-	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
+	return t.Size() == int64(PtrSize) && PtrDataSize(t) == int64(PtrSize)
 }
 
 // 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. Normally
-	// this means the type is pointer-ish.
+	// in the data field of an interface, instead of indirectly.
+	// This flag is just a cached computation of Size_ == PtrBytes == goarch.PtrSize.
 	TFlagDirectIface TFlag = 1 << 5
 
 	// Leaving this breadcrumb behind for dlv. It should not be used, and no

src/internal/abi/type.go

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