Encode link_id in tagged linkage

On 64-bit, we have enough space to encode (1) the tag, (2) the
`depmods` index, and (3) the offset all in a single 64-bit pointer
field. This means we don't need the external `link_id` arrays,
which reduces the size of many pkgimages by ~5%.

On 32-bit, we don't have enough bits to implement this strategy.
However, most linkages seem to be against the sysimage, and so
by giving that a separate tag we can achieve similar compression
because the `link_id` lists will be much shorter.

src/staticdata.c

@@ -356,6 +356,12 @@ typedef struct {
     arraylist_t fixup_types;    // a list of locations of types requiring (re)caching
     arraylist_t fixup_objs;     // a list of locations of objects requiring (re)caching
     arraylist_t ccallable_list; // @ccallable entry points to install
+#ifdef _P64
+    // On 64-bit, we can encode the "`depmods` index" in the reftag, so all we need is a mapping between
+    // the buildid_idx & depmods_idx:
+    jl_array_t *buildid_depmods_idxs;
+    // On 64bit, all the `link_ids_*` arrays below will be NULL.
+#endif
     // record of build_ids for all external linkages, in order of serialization for the current sysimg/pkgimg
     // conceptually, the base pointer for the jth externally-linked item is determined from
     //     i = findfirst(==(link_ids[j]), jl_build_ids)
@@ -386,7 +392,8 @@ enum RefTags {
     SymbolRef,          // symbols
     FunctionRef,        // generic functions
     BuiltinFunctionRef, // builtin functions
-    ExternalLinkage     // items defined externally (used when serializing packages)
+    SysimageLinkage,    // reference to the sysimage (from pkgimage)
+    ExternalLinkage     // reference to some other pkgimage
 };
 
 // calling conventions for internal entry points.
@@ -404,9 +411,11 @@ typedef enum {
 
 #ifdef _P64
 #define RELOC_TAG_OFFSET 61
+#define DEPS_IDX_OFFSET 40    // only on 64-bit can we encode the dependency-index as part of the tagged reloc
 #else
 // this supports up to 8 RefTags, 512MB of pointer data, and 4/2 (64/32-bit) GB of constant data.
 #define RELOC_TAG_OFFSET 29
+#define DEPS_IDX_OFFSET RELOC_TAG_OFFSET
 #endif
 
 #if RELOC_TAG_OFFSET <= 32
@@ -758,7 +767,7 @@ done_fields: ;
     }
     arraylist_push(&serialization_queue, (void*) v);
     size_t idx = serialization_queue.len - 1;
-    assert(serialization_queue.len < ((uintptr_t)1 << RELOC_TAG_OFFSET) && "too many items to serialize");
+    assert(serialization_queue.len < ((uintptr_t)1 << DEPS_IDX_OFFSET) && "too many items to serialize");
 
     *bp = (void*)((char*)HT_NOTFOUND + 1 + idx);
 }
@@ -863,29 +872,40 @@ static void write_pointer(ios_t *s) JL_NOTSAFEPOINT
 static uintptr_t add_external_linkage(jl_serializer_state *s, jl_value_t *v, jl_array_t *link_ids) {
     size_t i = external_blob_index(v);
     if (i < n_linkage_blobs()) {
+        // We found the sysimg/pkg that this item links against
+        // Compute the relocation code
+        size_t offset = (uintptr_t)v - (uintptr_t)jl_linkage_blobs.items[2*i];
+        offset /= sizeof(void*);
+        assert(offset < ((uintptr_t)1 << DEPS_IDX_OFFSET) && "offset to external image too large");
+
+        if (i == 0)
+            return ((uintptr_t)SysimageLinkage << RELOC_TAG_OFFSET) + offset;             // sysimage
         assert(link_ids && jl_is_array(link_ids));
+#ifdef _P64
+        uint64_t *link_id_data  = (uint64_t*)jl_array_data(link_ids);
+        return ((uintptr_t)ExternalLinkage << RELOC_TAG_OFFSET)
+            + (((uintptr_t)link_id_data[i]) << DEPS_IDX_OFFSET) + offset;  // on 64-bit, link_ids stores the mapping from build_id_idx -> deps_idx
+#else
+        // On 32bit, we store the image key in `link_ids`
         assert(jl_build_ids && jl_is_array(jl_build_ids));
         uint64_t *build_id_data = (uint64_t*)jl_array_data(jl_build_ids);
-        // We found the sysimg/pkg that this item links against
-        // Store the image key in `link_ids`
         jl_array_grow_end(link_ids, 1);
-        uint64_t *link_id_data  = (uint64_t*)jl_array_data(link_ids);
+        uint64_t *link_id_data  = (uint64_t*)jl_array_data(link_ids);  // wait until after the `grow`
         link_id_data[jl_array_len(link_ids)-1] = build_id_data[i];
-        // Compute the relocation code
-        size_t offset = (uintptr_t)v - (uintptr_t)jl_linkage_blobs.items[2*i];
-        offset /= sizeof(void*);
-        assert(offset < ((uintptr_t)1 << RELOC_TAG_OFFSET) && "offset to external image too large");
-        // jl_printf(JL_STDOUT, "External link %ld against blob %d with key %ld at position 0x%lx with offset 0x%lx to \n", jl_array_len(link_ids), i, build_id_data[i>>1], ios_pos(s->s), offset);
-        // jl_(v);
         return ((uintptr_t)ExternalLinkage << RELOC_TAG_OFFSET) + offset;
+#endif
     }
     return 0;
 }
 
 // Return the integer `id` for `v`. Generically this is looked up in `serialization_order`,
 // but symbols, small integers, and a couple of special items (`nothing` and the root Task)
 // have special handling.
+#ifdef _P64
+#define backref_id(s, v, link_ids) _backref_id(s, (jl_value_t*)(v), s->buildid_depmods_idxs)
+#else
 #define backref_id(s, v, link_ids) _backref_id(s, (jl_value_t*)(v), link_ids)
+#endif
 static uintptr_t _backref_id(jl_serializer_state *s, jl_value_t *v, jl_array_t *link_ids) JL_NOTSAFEPOINT
 {
     assert(v != NULL && "cannot get backref to NULL object");
@@ -898,7 +918,7 @@ static uintptr_t _backref_id(jl_serializer_state *s, jl_value_t *v, jl_array_t *
             write_uint32(s->symbols, l);
             ios_write(s->symbols, jl_symbol_name((jl_sym_t*)v), l + 1);
             size_t offset = ++nsym_tag;
-            assert(offset < ((uintptr_t)1 << RELOC_TAG_OFFSET) && "too many symbols");
+            assert(offset < ((uintptr_t)1 << DEPS_IDX_OFFSET) && "too many symbols");
             idx = (void*)((char*)HT_NOTFOUND + ((uintptr_t)SymbolRef << RELOC_TAG_OFFSET) + offset);
             *pidx = idx;
         }
@@ -1143,7 +1163,7 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
                 // write data and relocations
                 newa->data = NULL; // relocation offset
                 data /= sizeof(void*);
-                assert(data < ((uintptr_t)1 << RELOC_TAG_OFFSET) && "offset to constant data too large");
+                assert(data < ((uintptr_t)1 << DEPS_IDX_OFFSET) && "offset to constant data too large");
                 arraylist_push(&s->relocs_list, (void*)(reloc_offset + offsetof(jl_array_t, data))); // relocation location
                 arraylist_push(&s->relocs_list, (void*)(((uintptr_t)ConstDataRef << RELOC_TAG_OFFSET) + data)); // relocation target
                 if (jl_is_cpointer_type(et)) {
@@ -1246,7 +1266,7 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
             uintptr_t data = LLT_ALIGN(ios_pos(s->const_data), 8);
             write_padding(s->const_data, data - ios_pos(s->const_data));
             data /= sizeof(void*);
-            assert(data < ((uintptr_t)1 << RELOC_TAG_OFFSET) && "offset to constant data too large");
+            assert(data < ((uintptr_t)1 << DEPS_IDX_OFFSET) && "offset to constant data too large");
             arraylist_push(&s->relocs_list, (void*)(reloc_offset + 8)); // relocation location
             arraylist_push(&s->relocs_list, (void*)(((uintptr_t)ConstDataRef << RELOC_TAG_OFFSET) + data)); // relocation target
             void *pdata = jl_unbox_voidpointer(jl_get_nth_field(v, 2));
@@ -1533,6 +1553,8 @@ static uintptr_t get_reloc_for_item(uintptr_t reloc_item, size_t reloc_offset)
         case FunctionRef:
             assert(offset < JL_API_MAX && "unknown function pointer id");
             break;
+        case SysimageLinkage:
+            break;
         case ExternalLinkage:
             break;
         default:
@@ -1599,15 +1621,25 @@ static inline uintptr_t get_item_for_reloc(jl_serializer_state *s, uintptr_t bas
         //default:
             assert("corrupt relocation item id");
         }
+    case SysimageLinkage:
+        assert(jl_linkage_blobs.len > 0);
+        return (uintptr_t)jl_linkage_blobs.items[0] + offset*sizeof(void*);
     case ExternalLinkage:
         assert(link_ids);
-        assert(link_index);
         assert(jl_build_ids);
         uint64_t *link_id_data  = (uint64_t*)jl_array_data(link_ids);
         uint64_t *build_id_data = (uint64_t*)jl_array_data(jl_build_ids);
+#ifdef _P64
+        size_t depsidx = offset >> DEPS_IDX_OFFSET;
+        offset &= ((size_t)1 << DEPS_IDX_OFFSET) - 1;
+        assert(depsidx < jl_array_len(link_ids));    // on 64-bit, here link_ids is a build_id lookup by depmod index
+        uint64_t build_id = link_id_data[depsidx];
+#else
+        assert(link_index);
         assert(0 <= *link_index && *link_index < jl_array_len(link_ids));
         uint64_t build_id = link_id_data[*link_index];
         *link_index += 1;
+#endif
         size_t i = 0, nids = jl_array_len(jl_build_ids);
         while (i < nids) {
             if (build_id == build_id_data[i])
@@ -1799,8 +1831,12 @@ static jl_value_t *jl_delayed_reloc(jl_serializer_state *s, uintptr_t offset) JL
     uintptr_t base = (uintptr_t)&s->s->buf[0];
     size_t size = s->s->size;
     int link_index = 0;
+#ifdef _P64
+    jl_value_t *ret = (jl_value_t*)get_item_for_reloc(s, base, size, offset, s->buildid_depmods_idxs, &link_index);
+#else
     jl_value_t *ret = (jl_value_t*)get_item_for_reloc(s, base, size, offset, s->link_ids_relocs, &link_index);
     assert(link_index < jl_array_len(s->link_ids_relocs));
+#endif
     return ret;
 }
 
@@ -1890,13 +1926,24 @@ static void jl_update_all_gvars(jl_serializer_state *s, jl_image_t *image, uint3
     reloc_t *gvars = (reloc_t*)&s->gvar_record->buf[0];
     int gvar_link_index = 0;
     int external_fns_link_index = 0;
+    jl_array_t *link_ids = NULL;
     for (i = 0; i < l; i++) {
         uintptr_t offset = gvars[i];
         uintptr_t v = 0;
         if (i < external_fns_begin) {
-            v = get_item_for_reloc(s, base, size, offset, s->link_ids_gvars, &gvar_link_index);
+#ifdef _P64
+            link_ids = s->buildid_depmods_idxs;
+#else
+            link_ids = s->link_ids_gvars;
+#endif
+            v = get_item_for_reloc(s, base, size, offset, link_ids, &gvar_link_index);
         } else {
-            v = get_item_for_reloc(s, base, size, offset, s->link_ids_external_fnvars, &external_fns_link_index);
+#ifdef _P64
+            link_ids = s->buildid_depmods_idxs;
+#else
+            link_ids = s->link_ids_external_fnvars;
+#endif
+            v = get_item_for_reloc(s, base, size, offset, link_ids, &external_fns_link_index);
         }
         uintptr_t *gv = sysimg_gvars(image->gvars_base, image->gvars_offsets, i);
         *gv = v;
@@ -2217,7 +2264,7 @@ static void jl_prepare_serialization_data(jl_array_t *mod_array, jl_array_t *new
 }
 
 // In addition to the system image (where `worklist = NULL`), this can also save incremental images with external linkage
-static void jl_save_system_image_to_stream(ios_t *f,
+static void jl_save_system_image_to_stream(ios_t *f, jl_array_t *udeps,
                                            jl_array_t *worklist, jl_array_t *extext_methods,
                                            jl_array_t *new_specializations, jl_array_t *method_roots_list,
                                            jl_array_t *ext_targets, jl_array_t *edges) JL_GC_DISABLED
@@ -2265,10 +2312,15 @@ static void jl_save_system_image_to_stream(ios_t *f,
     arraylist_new(&s.fixup_types, 0);
     arraylist_new(&s.fixup_objs, 0);
     arraylist_new(&s.ccallable_list, 0);
+#ifdef _P64
+    s.buildid_depmods_idxs = image_to_depmodidx(jl_build_ids, udeps);
+    s.link_ids_relocs = s.link_ids_gctags = s.link_ids_gvars = s.link_ids_external_fnvars = NULL;
+#else
     s.link_ids_relocs = jl_alloc_array_1d(jl_array_uint64_type, 0);
     s.link_ids_gctags = jl_alloc_array_1d(jl_array_uint64_type, 0);
     s.link_ids_gvars = jl_alloc_array_1d(jl_array_uint64_type, 0);
     s.link_ids_external_fnvars = jl_alloc_array_1d(jl_array_uint64_type, 0);
+#endif
     htable_new(&s.callers_with_edges, 0);
     jl_value_t **const*const tags = get_tags(); // worklist == NULL ? get_tags() : NULL;
 
@@ -2367,21 +2419,21 @@ static void jl_save_system_image_to_stream(ios_t *f,
         jl_write_relocations(&s);
     }
 
-    if (sysimg.size > ((uintptr_t)1 << RELOC_TAG_OFFSET)) {
+    if (sysimg.size > ((uintptr_t)1 << DEPS_IDX_OFFSET)) {
         jl_printf(
             JL_STDERR,
             "ERROR: system image too large: sysimg.size is %jd but the limit is %" PRIxPTR "\n",
             (intmax_t)sysimg.size,
-            ((uintptr_t)1 << RELOC_TAG_OFFSET)
+            ((uintptr_t)1 << DEPS_IDX_OFFSET)
         );
         jl_exit(1);
     }
-    if (const_data.size / sizeof(void*) > ((uintptr_t)1 << RELOC_TAG_OFFSET)) {
+    if (const_data.size / sizeof(void*) > ((uintptr_t)1 << DEPS_IDX_OFFSET)) {
         jl_printf(
             JL_STDERR,
             "ERROR: system image too large: const_data.size is %jd but the limit is %" PRIxPTR "\n",
             (intmax_t)const_data.size,
-            ((uintptr_t)1 << RELOC_TAG_OFFSET)*sizeof(void*)
+            ((uintptr_t)1 << DEPS_IDX_OFFSET)*sizeof(void*)
         );
         jl_exit(1);
     }
@@ -2457,6 +2509,7 @@ static void jl_save_system_image_to_stream(ios_t *f,
             jl_write_value(&s, ext_targets);
             jl_write_value(&s, edges);
         }
+#ifndef _P64
         write_uint32(f, jl_array_len(s.link_ids_gctags));
         ios_write(f, (char*)jl_array_data(s.link_ids_gctags), jl_array_len(s.link_ids_gctags)*sizeof(uint64_t));
         write_uint32(f, jl_array_len(s.link_ids_relocs));
@@ -2465,6 +2518,7 @@ static void jl_save_system_image_to_stream(ios_t *f,
         ios_write(f, (char*)jl_array_data(s.link_ids_gvars), jl_array_len(s.link_ids_gvars)*sizeof(uint64_t));
         write_uint32(f, jl_array_len(s.link_ids_external_fnvars));
         ios_write(f, (char*)jl_array_data(s.link_ids_external_fnvars), jl_array_len(s.link_ids_external_fnvars)*sizeof(uint64_t));
+#endif
         write_uint32(f, external_fns_begin);
         jl_write_arraylist(s.s, &s.ccallable_list);
     }
@@ -2591,7 +2645,7 @@ JL_DLLEXPORT void jl_create_system_image(void **_native_data, jl_array_t *workli
     }
     if (_native_data != NULL)
         native_functions = *_native_data;
-    jl_save_system_image_to_stream(ff, worklist, extext_methods, new_specializations, method_roots_list, ext_targets, edges);
+    jl_save_system_image_to_stream(ff, *udeps, worklist, extext_methods, new_specializations, method_roots_list, ext_targets, edges);
     if (_native_data != NULL)
         native_functions = NULL;
     // make sure we don't run any Julia code concurrently before this point
@@ -2764,6 +2818,9 @@ static void jl_restore_system_image_from_stream_(ios_t *f, jl_image_t *image, jl
         offset_ext_targets = jl_read_offset(&s);
         offset_edges = jl_read_offset(&s);
     }
+#ifdef _P64
+    s.buildid_depmods_idxs = depmod_to_imageidx(depmods);
+#else
     size_t nlinks_gctags = read_uint32(f);
     if (nlinks_gctags > 0) {
         s.link_ids_gctags = jl_alloc_array_1d(jl_array_uint64_type, nlinks_gctags);
@@ -2784,6 +2841,7 @@ static void jl_restore_system_image_from_stream_(ios_t *f, jl_image_t *image, jl
         s.link_ids_external_fnvars = jl_alloc_array_1d(jl_array_uint64_type, nlinks_external_fnvars);
         ios_read(f, (char*)jl_array_data(s.link_ids_external_fnvars), nlinks_external_fnvars * sizeof(uint64_t));
     }
+#endif
     uint32_t external_fns_begin = read_uint32(f);
     jl_read_arraylist(s.s, ccallable_list ? ccallable_list : &s.ccallable_list);
     if (s.incremental) {
@@ -2810,10 +2868,19 @@ static void jl_restore_system_image_from_stream_(ios_t *f, jl_image_t *image, jl
         *base = image_base;
 
     s.s = &sysimg;
-    jl_read_reloclist(&s, s.link_ids_gctags, GC_OLD); // gctags
+#ifdef _P64
+    jl_array_t *link_ids = NULL;
+#else
+    jl_array_t *link_ids = s.link_ids_gctags;
+#endif
+    jl_read_reloclist(&s, link_ids, GC_OLD); // gctags
     size_t sizeof_tags = ios_pos(&relocs);
     (void)sizeof_tags;
-    jl_read_reloclist(&s, s.link_ids_relocs, 0); // general relocs
+#ifdef _P64
+#else
+    link_ids = s.link_ids_relocs;
+#endif
+    jl_read_reloclist(&s, link_ids, 0); // general relocs
     // s.link_ids_gvars will be processed in `jl_update_all_gvars`
     // s.link_ids_external_fns will be processed in `jl_update_all_gvars`
     jl_update_all_gvars(&s, image, external_fns_begin); // gvars relocs

src/staticdata_utils.c

@@ -1231,3 +1231,56 @@ JL_DLLEXPORT uint64_t jl_read_verify_header(ios_t *s, uint8_t *pkgimage, int64_t
     }
     return checksum;
 }
+
+// Returns `depmodidxs` where `udeps[depmodidxs[i]]` corresponds to `build_ids[i]`
+static jl_array_t *image_to_depmodidx(jl_array_t *build_ids,  jl_array_t *udeps)
+{
+    if (!udeps || !build_ids)
+        return NULL;
+    size_t j = 0, lbids = jl_array_len(build_ids), ldeps = jl_array_len(udeps);
+    uint64_t *bids = (uint64_t*)jl_array_data(build_ids);
+    jl_array_t *depmodidxs = jl_alloc_array_1d(jl_array_int32_type, lbids);
+    int32_t *dmidxs = (int32_t*)jl_array_data(depmodidxs);
+    for (size_t i = 0; i < lbids; i++) {
+        dmidxs[i] = -1;
+        uint64_t bid = bids[i];
+        j = 0;   // sad that this is of O(M*N)
+        while (j < ldeps) {
+            jl_value_t *deptuple = jl_array_ptr_ref(udeps, j);
+            jl_module_t *depmod = (jl_module_t*)jl_fieldref(deptuple, 0);  // evaluating module
+            jl_module_t *depmod_top = depmod;
+            while (depmod_top->parent != jl_main_module && depmod_top->parent != depmod_top)
+                depmod_top = depmod_top->parent;
+            if (depmod_top == jl_base_module) {
+                dmidxs[i] = 0;
+                break;
+            }
+            if (depmod_top->build_id.lo == bid) {
+                dmidxs[i] = j;
+                break;
+            }
+            j++;
+        }
+        assert(dmidxs[i] >= 0);
+    }
+    return depmodidxs;
+}
+
+// Returns `imageidxs` where `imageidxs[i]` is the blob corresponding to `depmods[i]`
+static jl_array_t *depmod_to_imageidx(jl_array_t *depmods)
+{
+    if (!depmods)
+        return NULL;
+    size_t ldeps = jl_array_len(depmods);
+    jl_array_t *imageidxs = jl_alloc_array_1d(jl_array_int32_type, ldeps);
+    int32_t *imgidxs = (int32_t*)jl_array_data(imageidxs);
+    for (size_t i = 0; i < ldeps; i++) {
+        imgidxs[i] = -1;
+        jl_value_t *depmod = jl_array_ptr_ref(depmods, i);
+        assert(jl_is_module(depmod));
+        size_t j = external_blob_index(depmod);
+        assert(j < 1<<31);
+        imgidxs[i] = (int32_t)j;
+    }
+    return imageidxs;
+}