interp: work around limitation of constfolding in IR builder

The IR builder does not appear to fold comparisons of constant inttoptr
instructions to const null pointer. So do it manually during
interpretatin, as a somewhat ugly hack.

This fixes https://github.com/tinygo-org/tinygo/issues/363

interp/frame.go

@@ -184,6 +184,40 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 			lhs := fr.getLocal(inst.Operand(0)).(*LocalValue).Underlying
 			rhs := fr.getLocal(inst.Operand(1)).(*LocalValue).Underlying
 			predicate := inst.IntPredicate()
+			if predicate == llvm.IntEQ && lhs.Type().TypeKind() == llvm.PointerTypeKind {
+				// Unfortunately, the const propagation in the IR builder
+				// doesn't handle pointer compares of inttoptr values. So we
+				// implement it manually here.
+				isNil := func(v llvm.Value) (result bool, ok bool) {
+					if !v.IsAConstantExpr().IsNil() && v.Opcode() == llvm.IntToPtr {
+						// Whether a constant inttoptr is nil is easy to
+						// determine.
+						operand := v.Operand(0)
+						if operand.IsConstant() {
+							return operand.ZExtValue() == 0, true
+						}
+					}
+					if !v.IsAConstantPointerNull().IsNil() {
+						// A constant pointer null is always null, of course.
+						return true, true
+					}
+					return false, false // not valid
+				}
+				lhsNil, ok1 := isNil(lhs)
+				rhsNil, ok2 := isNil(rhs)
+				if ok1 && ok2 {
+					if lhsNil && rhsNil {
+						// Both are nil, so this icmp is always evaluated to true.
+						fr.locals[inst] = &LocalValue{fr.Eval, llvm.ConstInt(fr.Mod.Context().Int1Type(), 1, false)}
+						continue
+					}
+					if lhsNil != rhsNil {
+						// Only one of them is nil, so this comparison must return false.
+						fr.locals[inst] = &LocalValue{fr.Eval, llvm.ConstInt(fr.Mod.Context().Int1Type(), 0, false)}
+						continue
+					}
+				}
+			}
 			fr.locals[inst] = &LocalValue{fr.Eval, fr.builder.CreateICmp(predicate, lhs, rhs, "")}
 		case !inst.IsAFCmpInst().IsNil():
 			lhs := fr.getLocal(inst.Operand(0)).(*LocalValue).Underlying