Implement struct padding

Fixes #6

codegen.go

@@ -428,6 +428,7 @@ func (f FuncType) GenZero(c *Compiler, loc Operand) {
 func (s StructType) GenZero(c *Compiler, loc Operand) {
 	off := 0
 	for _, field := range s.compositeType {
+		off = -(-off & -field.Ty.Metrics().Align) // Align upwards
 		floc := loc
 		if off > 0 {
 			ftmp := c.Temporary()

compiler_test.go

@@ -336,17 +336,22 @@ func TestStruct(t *testing.T) {
 	testCompile(t, `
 		type Foo struct { a, b I32; c I64 }
 		type Bar struct { a, b, c I8 }
+		type Baz struct { a I8; b I64; c I8 }
 		fn fooFn(_ Foo)
 		fn barFn(_ Bar)
+		fn bazFn(_ Baz)
 		pub fn main() I32 {
 			var foo Foo
 			fooFn(foo)
 			var bar Bar
 			barFn(bar)
+			var baz Baz
+			bazFn(baz)
 			return 0
 		}
 	`, `
 		type :b3 = { b 3 }
+		type :blb = { b, l, b }
 		type :w2l = { w 2, l }
 		export function w $main() {
 		@start
@@ -368,6 +373,15 @@ func TestStruct(t *testing.T) {
 
 			call $barFn(:b3 %t4)
 
+			%t7 =l alloc8 24
+			storeb 0, %t7
+			%t8 =l add %t7, 8
+			storel 0, %t8
+			%t9 =l add %t7, 16
+			storeb 0, %t9
+
+			call $bazFn(:blb %t7)
+
 			ret 0
 		}
 	`)

types.go

@@ -365,13 +365,7 @@ func (s StructType) Concrete() ConcreteType {
 	return s
 }
 func (s StructType) Metrics() (m TypeMetrics) {
-	for _, field := range s.compositeType {
-		fm := field.Ty.Metrics()
-		if m.Align < fm.Align {
-			m.Align = fm.Align
-		}
-		m.Size += fm.Size
-	}
+	m.Size, m.Align = s.metrics("")
 	return
 }
 func (s StructType) Format(indent int) string {
@@ -398,14 +392,37 @@ func (s StructType) layout(c *Compiler) CompositeLayout {
 	return layout
 }
 func (s StructType) Offset(name string) int {
-	n := 0
+	if name == "" {
+		return -1
+	} else {
+		off, _ := s.metrics(name)
+		return off
+	}
+}
+func (s StructType) metrics(name string) (off int, align int) {
+	// This is the internal function behind both Metrics and Offset
+	// name == "" -> Metrics
+	// name != "" -> Offset
+
 	for _, field := range s.compositeType {
+		m := field.Ty.Metrics()
+		off = -(-off & -m.Align) // Align upwards
 		if field.Name == name {
-			return n
+			return
+		}
+		off += m.Size
+
+		if m.Align > align {
+			align = m.Align
 		}
-		n += field.Ty.Metrics().Size
 	}
-	return -1
+
+	if name == "" {
+		off = -(-off & -align) // Align struct size to max alignment for arrays
+		return
+	} else {
+		return -1, -1
+	}
 }
 
 func (a UnionType) Equals(other Type) bool {