Binary search for pkgimage metadata (JuliaLang#48940)

Co-authored-by: Jameson Nash <[email protected]>

Author: pchintalapudi

src/gc.c

@@ -173,6 +173,13 @@ pagetable_t memory_map;
 // List of marked big objects.  Not per-thread.  Accessed only by master thread.
 bigval_t *big_objects_marked = NULL;
 
+// Eytzinger tree of images. Used for very fast jl_object_in_image queries during gc
+// See https://algorithmica.org/en/eytzinger
+static arraylist_t eytzinger_image_tree;
+static arraylist_t eytzinger_idxs;
+static uintptr_t gc_img_min;
+static uintptr_t gc_img_max;
+
 // -- Finalization --
 // `ptls->finalizers` and `finalizer_list_marked` might have tagged pointers.
 // If an object pointer has the lowest bit set, the next pointer is an unboxed c function pointer.
@@ -183,6 +190,118 @@ arraylist_t finalizer_list_marked;
 arraylist_t to_finalize;
 JL_DLLEXPORT _Atomic(int) jl_gc_have_pending_finalizers = 0;
 
+static int ptr_cmp(const void *l, const void *r)
+{
+    uintptr_t left = *(const uintptr_t*)l;
+    uintptr_t right = *(const uintptr_t*)r;
+    // jl_safe_printf("cmp %p %p\n", (void*)left, (void*)right);
+    return (left > right) - (left < right);
+}
+
+// Build an eytzinger tree from a sorted array
+static int eytzinger(uintptr_t *src, uintptr_t *dest, size_t i, size_t k, size_t n)
+{
+    if (k <= n) {
+        i = eytzinger(src, dest, i, 2 * k, n);
+        dest[k-1] = src[i];
+        i++;
+        i = eytzinger(src, dest, i, 2 * k + 1, n);
+    }
+    return i;
+}
+
+static size_t eyt_obj_idx(jl_value_t *obj) JL_NOTSAFEPOINT
+{
+    size_t n = eytzinger_image_tree.len - 1;
+    if (n == 0)
+        return n;
+    assert(n % 2 == 0 && "Eytzinger tree not even length!");
+    uintptr_t cmp = (uintptr_t) obj;
+    if (cmp <= gc_img_min || cmp > gc_img_max)
+        return n;
+    uintptr_t *tree = (uintptr_t*)eytzinger_image_tree.items;
+    size_t k = 1;
+    // note that k preserves the history of how we got to the current node
+    while (k <= n) {
+        int greater = (cmp > tree[k - 1]);
+        k <<= 1;
+        k |= greater;
+    }
+    // Free to assume k is nonzero, since we start with k = 1
+    // and cmp > gc_img_min
+    // This shift does a fast revert of the path until we get
+    // to a node that evaluated less than cmp.
+    k >>= (__builtin_ctzll(k) + 1);
+    assert(k != 0);
+    assert(k <= n && "Eytzinger tree index out of bounds!");
+    assert(tree[k - 1] < cmp && "Failed to find lower bound for object!");
+    return k - 1;
+}
+
+//used in staticdata.c after we add an image
+void rebuild_image_blob_tree(void)
+{
+    size_t inc = 1 + jl_linkage_blobs.len - eytzinger_image_tree.len;
+    assert(eytzinger_idxs.len == eytzinger_image_tree.len);
+    assert(eytzinger_idxs.max == eytzinger_image_tree.max);
+    arraylist_grow(&eytzinger_idxs, inc);
+    arraylist_grow(&eytzinger_image_tree, inc);
+    eytzinger_idxs.items[eytzinger_idxs.len - 1] = (void*)jl_linkage_blobs.len;
+    eytzinger_image_tree.items[eytzinger_image_tree.len - 1] = (void*)1; // outside image
+    for (size_t i = 0; i < jl_linkage_blobs.len; i++) {
+        assert((uintptr_t) jl_linkage_blobs.items[i] % 4 == 0 && "Linkage blob not 4-byte aligned!");
+        // We abuse the pointer here a little so that a couple of properties are true:
+        // 1. a start and an end are never the same value. This simplifies the binary search.
+        // 2. ends are always after starts. This also simplifies the binary search.
+        // We assume that there exist no 0-size blobs, but that's a safe assumption
+        // since it means nothing could be there anyways
+        uintptr_t val = (uintptr_t) jl_linkage_blobs.items[i];
+        eytzinger_idxs.items[i] = (void*)(val + (i & 1));
+    }
+    qsort(eytzinger_idxs.items, eytzinger_idxs.len - 1, sizeof(void*), ptr_cmp);
+    gc_img_min = (uintptr_t) eytzinger_idxs.items[0];
+    gc_img_max = (uintptr_t) eytzinger_idxs.items[eytzinger_idxs.len - 2] + 1;
+    eytzinger((uintptr_t*)eytzinger_idxs.items, (uintptr_t*)eytzinger_image_tree.items, 0, 1, eytzinger_idxs.len - 1);
+    // Reuse the scratch memory to store the indices
+    // Still O(nlogn) because binary search
+    for (size_t i = 0; i < jl_linkage_blobs.len; i ++) {
+        uintptr_t val = (uintptr_t) jl_linkage_blobs.items[i];
+        // This is the same computation as in the prior for loop
+        uintptr_t eyt_val = val + (i & 1);
+        size_t eyt_idx = eyt_obj_idx((jl_value_t*)(eyt_val + 1)); assert(eyt_idx < eytzinger_idxs.len - 1);
+        assert(eytzinger_image_tree.items[eyt_idx] == (void*)eyt_val && "Eytzinger tree failed to find object!");
+        if (i & 1)
+            eytzinger_idxs.items[eyt_idx] = (void*)n_linkage_blobs();
+        else
+            eytzinger_idxs.items[eyt_idx] = (void*)(i / 2);
+    }
+}
+
+static int eyt_obj_in_img(jl_value_t *obj) JL_NOTSAFEPOINT
+{
+    assert((uintptr_t) obj % 4 == 0 && "Object not 4-byte aligned!");
+    int idx = eyt_obj_idx(obj);
+    // Now we use a tiny trick: tree[idx] & 1 is whether or not tree[idx] is a
+    // start (0) or an end (1) of a blob. If it's a start, then the object is
+    // in the image, otherwise it is not.
+    int in_image = ((uintptr_t)eytzinger_image_tree.items[idx] & 1) == 0;
+    return in_image;
+}
+
+size_t external_blob_index(jl_value_t *v) JL_NOTSAFEPOINT
+{
+    assert((uintptr_t) v % 4 == 0 && "Object not 4-byte aligned!");
+    int eyt_idx = eyt_obj_idx(v);
+    // We fill the invalid slots with the length, so we can just return that
+    size_t idx = (size_t) eytzinger_idxs.items[eyt_idx];
+    return idx;
+}
+
+uint8_t jl_object_in_image(jl_value_t *obj) JL_NOTSAFEPOINT
+{
+    return eyt_obj_in_img(obj);
+}
+
 NOINLINE uintptr_t gc_get_stack_ptr(void)
 {
     return (uintptr_t)jl_get_frame_addr();
@@ -2270,7 +2389,8 @@ FORCE_INLINE void gc_mark_outrefs(jl_ptls_t ptls, jl_gc_markqueue_t *mq, void *_
         uint8_t bits = (gc_old(o->header) && !mark_reset_age) ? GC_OLD_MARKED : GC_MARKED;
         int update_meta = __likely(!meta_updated && !gc_verifying);
         int foreign_alloc = 0;
-        if (update_meta && jl_object_in_image(new_obj)) {
+        // directly point at eyt_obj_in_img to encourage inlining
+        if (update_meta && eyt_obj_in_img(new_obj)) {
             foreign_alloc = 1;
             update_meta = 0;
         }
@@ -3245,6 +3365,10 @@ void jl_gc_init(void)
 
     arraylist_new(&finalizer_list_marked, 0);
     arraylist_new(&to_finalize, 0);
+    arraylist_new(&eytzinger_image_tree, 0);
+    arraylist_new(&eytzinger_idxs, 0);
+    arraylist_push(&eytzinger_idxs, (void*)0);
+    arraylist_push(&eytzinger_image_tree, (void*)1); // outside image
 
     gc_num.interval = default_collect_interval;
     last_long_collect_interval = default_collect_interval;

src/julia_internal.h

@@ -954,28 +954,9 @@ STATIC_INLINE size_t n_linkage_blobs(void) JL_NOTSAFEPOINT
     return jl_image_relocs.len;
 }
 
-// TODO: Makes this a binary search
-STATIC_INLINE size_t external_blob_index(jl_value_t *v) JL_NOTSAFEPOINT {
-    size_t i, nblobs = n_linkage_blobs();
-    assert(jl_linkage_blobs.len == 2*nblobs);
-    for (i = 0; i < nblobs; i++) {
-        uintptr_t left = (uintptr_t)jl_linkage_blobs.items[2*i];
-        uintptr_t right = (uintptr_t)jl_linkage_blobs.items[2*i + 1];
-        if (left < (uintptr_t)v && (uintptr_t)v <= right) {
-            // the last object may be a singleton (v is shifted by a type tag, so we use exclusive bounds here)
-            break;
-        }
-    }
-    return i;
-}
+size_t external_blob_index(jl_value_t *v) JL_NOTSAFEPOINT;
 
-STATIC_INLINE uint8_t jl_object_in_image(jl_value_t* v) JL_NOTSAFEPOINT {
-    size_t blob = external_blob_index(v);
-    if (blob == n_linkage_blobs()) {
-        return 0;
-    }
-    return 1;
-}
+uint8_t jl_object_in_image(jl_value_t* v) JL_NOTSAFEPOINT;
 
 typedef struct {
     LLVMOrcThreadSafeModuleRef TSM;

src/staticdata.c

@@ -2366,6 +2366,10 @@ static void jl_save_system_image_to_stream(ios_t *f, jl_array_t *mod_array,
         jl_write_relocations(&s);
     }
 
+    // This ensures that we can use the low bit of addresses for
+    // identifying end pointers in gc's eytzinger search.
+    write_padding(&sysimg, 4 - (sysimg.size % 4));
+
     if (sysimg.size > ((uintptr_t)1 << RELOC_TAG_OFFSET)) {
         jl_printf(
             JL_STDERR,
@@ -2658,6 +2662,8 @@ JL_DLLEXPORT void jl_set_sysimg_so(void *handle)
 // }
 #endif
 
+extern void rebuild_image_blob_tree(void);
+
 static void jl_restore_system_image_from_stream_(ios_t *f, jl_image_t *image, jl_array_t *depmods, uint64_t checksum,
                                 /* outputs */    jl_array_t **restored,         jl_array_t **init_order,
                                                  jl_array_t **extext_methods,
@@ -3151,6 +3157,7 @@ static void jl_restore_system_image_from_stream_(ios_t *f, jl_image_t *image, jl
     arraylist_push(&jl_linkage_blobs, (void*)image_base);
     arraylist_push(&jl_linkage_blobs, (void*)(image_base + sizeof_sysimg + sizeof(uintptr_t)));
     arraylist_push(&jl_image_relocs, (void*)relocs_base);
+    rebuild_image_blob_tree();
 
     // jl_printf(JL_STDOUT, "%ld blobs to link against\n", jl_linkage_blobs.len >> 1);
     jl_gc_enable(en);