compiler: implement unsafe.Alignof and unsafe.Sizeof for generic code

For some reason, these aren't lowered when a generic function is
instantiated by the SSA package.

I've left unsafe.Offsetof to be implemented later, it's a bit difficult
to do correctly the way the code is currently structured.

compiler/compiler.go

@@ -1585,6 +1585,20 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 		ptr := argValues[0]
 		len := argValues[1]
 		return b.CreateGEP(ptr, []llvm.Value{len}, ""), nil
+	case "Alignof": // unsafe.Alignof
+		align := b.targetData.ABITypeAlignment(argValues[0].Type())
+		return llvm.ConstInt(b.uintptrType, uint64(align), false), nil
+	case "Offsetof": // unsafe.Offsetof
+		// This builtin is a bit harder to implement and may need a bit of
+		// refactoring to work (it may be easier to implement if we have access
+		// to the underlying Go SSA instruction). It is also rarely used: it
+		// only applies in generic code and unsafe.Offsetof isn't very commonly
+		// used anyway.
+		// In other words, postpone it to some other day.
+		return llvm.Value{}, b.makeError(pos, "todo: unsafe.Offsetof")
+	case "Sizeof": // unsafe.Sizeof
+		size := b.targetData.TypeAllocSize(argValues[0].Type())
+		return llvm.ConstInt(b.uintptrType, size, false), nil
 	case "Slice": // unsafe.Slice
 		// This creates a slice from a pointer and a length.
 		// Note that the exception mentioned in the documentation (if the

compiler/testdata/generics.go

@@ -1,5 +1,7 @@
 package main
 
+import "unsafe"
+
 type Coord interface {
 	int | float32
 }
@@ -9,6 +11,8 @@ type Point[T Coord] struct {
 }
 
 func Add[T Coord](a, b Point[T]) Point[T] {
+	checkSize(unsafe.Alignof(a))
+	checkSize(unsafe.Sizeof(a))
 	return Point[T]{
 		X: a.X + b.X,
 		Y: a.Y + b.Y,
@@ -22,3 +26,5 @@ func main() {
 	var ai, bi Point[int]
 	Add(ai, bi)
 }
+
+func checkSize(uintptr)

compiler/testdata/generics.ll

@@ -94,6 +94,8 @@ entry:
   store float %b.X, float* %b.repack14, align 8
   %b.repack15 = getelementptr inbounds %"main.Point[float32]", %"main.Point[float32]"* %b, i32 0, i32 1
   store float %b.Y, float* %b.repack15, align 4
+  call void @main.checkSize(i32 4, i8* undef) #2
+  call void @main.checkSize(i32 8, i8* undef) #2
   %complit.repack = getelementptr inbounds %"main.Point[float32]", %"main.Point[float32]"* %complit, i32 0, i32 0
   store float 0.000000e+00, float* %complit.repack, align 8
   %complit.repack17 = getelementptr inbounds %"main.Point[float32]", %"main.Point[float32]"* %complit, i32 0, i32 1
@@ -157,6 +159,8 @@ store.throw7:                                     ; preds = %store.next
   unreachable
 }
 
+declare void @main.checkSize(i32, i8*) #0
+
 declare void @runtime.nilPanic(i8*) #0
 
 ; Function Attrs: nounwind
@@ -185,6 +189,8 @@ entry:
   store i32 %b.X, i32* %b.repack14, align 8
   %b.repack15 = getelementptr inbounds %"main.Point[int]", %"main.Point[int]"* %b, i32 0, i32 1
   store i32 %b.Y, i32* %b.repack15, align 4
+  call void @main.checkSize(i32 4, i8* undef) #2
+  call void @main.checkSize(i32 8, i8* undef) #2
   %complit.repack = getelementptr inbounds %"main.Point[int]", %"main.Point[int]"* %complit, i32 0, i32 0
   store i32 0, i32* %complit.repack, align 8
   %complit.repack17 = getelementptr inbounds %"main.Point[int]", %"main.Point[int]"* %complit, i32 0, i32 1