src/org/nutz/repo/org/objectweb/asm/Frame.java
private static boolean merge(final ClassWriter cw, int t,
            final int[] types, final int index) {
        int u = types[index];
        if (u == t) {
            // if the types are equal, merge(u,t)=u, so there is no change
            return false;
        }
        if ((t & ~DIM) == NULL) {
            if (u == NULL) {
                return false;
            }
            t = NULL;
        }
        if (u == 0) {
            // if types[index] has never been assigned, merge(u,t)=t
            types[index] = t;
            return true;
        }
        int v;
        if ((u & BASE_KIND) == OBJECT || (u & DIM) != 0) {
            // if u is a reference type of any dimension
            if (t == NULL) {
                // if t is the NULL type, merge(u,t)=u, so there is no change
                return false;
            } else if ((t & (DIM | BASE_KIND)) == (u & (DIM | BASE_KIND))) {
                // if t and u have the same dimension and same base kind
                if ((u & BASE_KIND) == OBJECT) {
                    // if t is also a reference type, and if u and t have the
                    // same dimension merge(u,t) = dim(t) | common parent of the
                    // element types of u and t
                    v = (t & DIM) | OBJECT
                            | cw.getMergedType(t & BASE_VALUE, u & BASE_VALUE);
                } else {
                    // if u and t are array types, but not with the same element
                    // type, merge(u,t) = dim(u) - 1 | java/lang/Object
                    int vdim = ELEMENT_OF + (u & DIM);
                    v = vdim | OBJECT | cw.addType("java/lang/Object");
                }
            } else if ((t & BASE_KIND) == OBJECT || (t & DIM) != 0) {
                // if t is any other reference or array type, the merged type
                // is min(udim, tdim) | java/lang/Object, where udim is the
                // array dimension of u, minus 1 if u is an array type with a
                // primitive element type (and similarly for tdim).
                int tdim = (((t & DIM) == 0 || (t & BASE_KIND) == OBJECT) ? 0
                        : ELEMENT_OF) + (t & DIM);
                int udim = (((u & DIM) == 0 || (u & BASE_KIND) == OBJECT) ? 0
                        : ELEMENT_OF) + (u & DIM);
                v = Math.min(tdim, udim) | OBJECT
                        | cw.addType("java/lang/Object");
            } else {
                // if t is any other type, merge(u,t)=TOP
                v = TOP;
            }
        } else if (u == NULL) {
            // if u is the NULL type, merge(u,t)=t,
            // or TOP if t is not a reference type
            v = (t & BASE_KIND) == OBJECT || (t & DIM) != 0 ? t : TOP;
        } else {
            // if u is any other type, merge(u,t)=TOP whatever t
            v = TOP;
        }
        if (u != v) {
            types[index] = v;
            return true;
        }
        return false;
    }
private static boolean merge(final ClassWriter cw, int t,
            final int[] types, final int index) {
        int u = types[index];
        if (u == t) {
            // if the types are equal, merge(u,t)=u, so there is no change
            return false;
        }
        if ((t & ~DIM) == NULL) {
            if (u == NULL) {
                return false;
            }
            t = NULL;
        }
        if (u == 0) {
            // if types[index] has never been assigned, merge(u,t)=t
            types[index] = t;
            return true;
        }
        int v;
        if ((u & BASE_KIND) == OBJECT || (u & DIM) != 0) {
            // if u is a reference type of any dimension
            if (t == NULL) {
                // if t is the NULL type, merge(u,t)=u, so there is no change
                return false;
            } else if ((t & (DIM | BASE_KIND)) == (u & (DIM | BASE_KIND))) {
                // if t and u have the same dimension and same base kind
                if ((u & BASE_KIND) == OBJECT) {
                    // if t is also a reference type, and if u and t have the
                    // same dimension merge(u,t) = dim(t) | common parent of the
                    // element types of u and t
                    v = (t & DIM) | OBJECT
                            | cw.getMergedType(t & BASE_VALUE, u & BASE_VALUE);
                } else {
                    // if u and t are array types, but not with the same element
                    // type, merge(u,t) = dim(u) - 1 | java/lang/Object
                    int vdim = ELEMENT_OF + (u & DIM);
                    v = vdim | OBJECT | cw.addType("java/lang/Object");
                }
            } else if ((t & BASE_KIND) == OBJECT || (t & DIM) != 0) {
                // if t is any other reference or array type, the merged type
                // is min(udim, tdim) | java/lang/Object, where udim is the
                // array dimension of u, minus 1 if u is an array type with a
                // primitive element type (and similarly for tdim).
                int tdim = (((t & DIM) == 0 || (t & BASE_KIND) == OBJECT) ? 0
                        : ELEMENT_OF) + (t & DIM);
                int udim = (((u & DIM) == 0 || (u & BASE_KIND) == OBJECT) ? 0
                        : ELEMENT_OF) + (u & DIM);
                v = Math.min(tdim, udim) | OBJECT
                        | cw.addType("java/lang/Object");
            } else {
                // if t is any other type, merge(u,t)=TOP
                v = TOP;
            }
        } else if (u == NULL) {
            // if u is the NULL type, merge(u,t)=t,
            // or TOP if t is not a reference type
            v = (t & BASE_KIND) == OBJECT || (t & DIM) != 0 ? t : TOP;
        } else {
            // if u is any other type, merge(u,t)=TOP whatever t
            v = TOP;
        }
        if (u != v) {
            types[index] = v;
            return true;
        }
        return false;
    }
Merges the type at the given index in the given type array with the given
type. Returns <tt>true</tt> if the type array has been modified by this
operation.

@param cw
the ClassWriter to which this label belongs.
@param t
the type with which the type array element must be merged.
@param types
an array of types.
@param index
the index of the type that must be merged in 'types'.
@return <tt>true</tt> if the type array has been modified by this
operation.