Merge adjacent implicit binding partitions (JuliaLang#57995)

After:
```
julia> convert(Core.Binding, GlobalRef(Base, :Intrinsics))
Binding Base.Intrinsics
   617:∞ - implicit `using` resolved to constant Core.Intrinsics
   0:616 - undefined binding - guard entry

julia> convert(Core.Binding, GlobalRef(Base, :Math))
Binding Base.Math
   22128:∞ - constant binding to Base.Math
   0:22127 - backdated constant binding to Base.Math
```

There is a bit of trickiness here. In particular, the question is, "when
do we check" whether the partition next to the one we currently looked
at happens to have the same implicit resolution as our current one. The
most obvious answer is that we should do it on access, but in practice
that would require essentially scanning back and considering every
possible world age state at every lookup. This is undesirable - the
lookup is not crazy expensive, but it can add up and most world ages we
never touch, so it is also wasteful.

This instead implements a different approach where we only perform the
resolution for world ages that somebody actually asked about, but can
then subsequently merge partitions if we do find that they are
identical. The logic for that is a bit involved, since we need to be
careful to keep the datastructure valid at every point, but does address
the issue.

Fixes JuliaLang#57923

Author: Keno

src/jltypes.c

@@ -3266,8 +3266,10 @@ void jl_init_types(void) JL_GC_DISABLED
                         jl_svec(5, jl_any_type,
                         jl_ulong_type, jl_ulong_type, jl_any_type/*jl_binding_partition_type*/, jl_ulong_type),
                         jl_emptysvec, 0, 1, 0);
-    const static uint32_t binding_partition_atomicfields[] = { 0b01101 }; // Set fields 1, 3, 4 as atomic
+    const static uint32_t binding_partition_atomicfields[] = { 0b01111 }; // Set fields 1, 2, 3, 4 as atomic
     jl_binding_partition_type->name->atomicfields = binding_partition_atomicfields;
+    const static uint32_t binding_partition_constfields[] = { 0x100001 }; // Set fields 1, 5 as constant
+    jl_binding_partition_type->name->constfields = binding_partition_constfields;
 
     jl_binding_type =
         jl_new_datatype(jl_symbol("Binding"), core, jl_any_type, jl_emptysvec,

src/julia.h

@@ -760,7 +760,7 @@ typedef struct JL_ALIGNED_ATTR(8) _jl_binding_partition_t {
      * } restriction;
      */
     jl_value_t *restriction;
-    size_t min_world;
+    _Atomic(size_t) min_world;
     _Atomic(size_t) max_world;
     _Atomic(struct _jl_binding_partition_t *) next;
     size_t kind;
@@ -1942,8 +1942,8 @@ JL_DLLEXPORT jl_sym_t *jl_get_root_symbol(void);
 JL_DLLEXPORT jl_value_t *jl_get_binding_value(jl_binding_t *b JL_PROPAGATES_ROOT);
 JL_DLLEXPORT jl_value_t *jl_get_binding_value_in_world(jl_binding_t *b JL_PROPAGATES_ROOT, size_t world);
 JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_const(jl_binding_t *b JL_PROPAGATES_ROOT);
-JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
-JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved_and_const(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
+JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved_debug_only(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
+JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_latest_resolved_and_const_debug_only(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
 JL_DLLEXPORT jl_value_t *jl_declare_const_gf(jl_module_t *mod, jl_sym_t *name);
 JL_DLLEXPORT jl_method_t *jl_method_def(jl_svec_t *argdata, jl_methtable_t *mt, jl_code_info_t *f, jl_module_t *module);
 JL_DLLEXPORT jl_code_info_t *jl_code_for_staged(jl_method_instance_t *linfo, size_t world, jl_code_instance_t **cache);

src/module.c

@@ -579,7 +579,7 @@ JL_DLLEXPORT jl_value_t *jl_stderr_obj(void) JL_NOTSAFEPOINT
     if (jl_base_module == NULL)
         return NULL;
     jl_binding_t *stderr_obj = jl_get_module_binding(jl_base_module, jl_symbol("stderr"), 0);
-    return stderr_obj ? jl_get_binding_value_if_resolved(stderr_obj) : NULL;
+    return stderr_obj ? jl_get_binding_value_if_resolved_debug_only(stderr_obj) : NULL;
 }
 
 // toys for debugging ---------------------------------------------------------
@@ -674,7 +674,7 @@ static int is_globname_binding(jl_value_t *v, jl_datatype_t *dv) JL_NOTSAFEPOINT
     jl_sym_t *globname = dv->name->mt != NULL ? dv->name->mt->name : NULL;
     if (globname && dv->name->module) {
         jl_binding_t *b = jl_get_module_binding(dv->name->module, globname, 0);
-        jl_value_t *bv = jl_get_binding_value_if_resolved_and_const(b);
+        jl_value_t *bv = jl_get_binding_value_if_latest_resolved_and_const_debug_only(b);
         // The `||` makes this function work for both function instances and function types.
         if (bv && (bv == v || jl_typeof(bv) == v))
             return 1;

src/rtutils.c

@@ -1749,7 +1749,7 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
                     write_uint(f, 0);
                 }
             } else {
-                write_uint(f, bpart->min_world);
+                write_uint(f, jl_atomic_load_relaxed(&bpart->min_world));
                 write_uint(f, max_world);
             }
             write_pointerfield(s, (jl_value_t*)jl_atomic_load_relaxed(&bpart->next));
@@ -3618,7 +3618,7 @@ static int jl_validate_binding_partition(jl_binding_t *b, jl_binding_partition_t
         // and allocate a fresh bpart?
         jl_update_loaded_bpart(b, bpart);
         bpart->kind |= (raw_kind & PARTITION_MASK_FLAG);
-        if (bpart->min_world > jl_require_world)
+        if (jl_atomic_load_relaxed(&bpart->min_world) > jl_require_world)
             goto invalidated;
     }
     if (!jl_bkind_is_some_explicit_import(kind) && kind != PARTITION_KIND_IMPLICIT_GLOBAL)
@@ -3629,7 +3629,8 @@ static int jl_validate_binding_partition(jl_binding_t *b, jl_binding_partition_t
     jl_binding_partition_t *latest_imported_bpart = jl_atomic_load_relaxed(&imported_binding->partitions);
     if (!latest_imported_bpart)
         return 1;
-    if (latest_imported_bpart->min_world <= bpart->min_world) {
+    if (jl_atomic_load_relaxed(&latest_imported_bpart->min_world) <=
+        jl_atomic_load_relaxed(&bpart->min_world)) {
 add_backedge:
         // Imported binding is still valid
         if ((kind == PARTITION_KIND_EXPLICIT || kind == PARTITION_KIND_IMPORTED) &&
@@ -3640,8 +3641,9 @@ static int jl_validate_binding_partition(jl_binding_t *b, jl_binding_partition_t
     }
     else {
         // Binding partition was invalidated
-        assert(bpart->min_world == jl_require_world);
-        bpart->min_world = latest_imported_bpart->min_world;
+        assert(jl_atomic_load_relaxed(&bpart->min_world) == jl_require_world);
+        jl_atomic_store_relaxed(&bpart->min_world,
+            jl_atomic_load_relaxed(&latest_imported_bpart->min_world));
     }
 invalidated:
     // We need to go through and re-validate any bindings in the same image that

src/staticdata.c

@@ -309,7 +309,7 @@ void jl_declare_global(jl_module_t *m, jl_value_t *arg, jl_value_t *set_type, in
                     goto check_type;
                 }
                 check_safe_newbinding(gm, gs);
-                if (bpart->min_world == new_world) {
+                if (jl_atomic_load_relaxed(&bpart->min_world) == new_world) {
                     bpart->kind = new_kind | (bpart->kind & PARTITION_MASK_FLAG);
                     bpart->restriction = global_type;
                     if (global_type)
@@ -730,7 +730,7 @@ JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val2(
     JL_LOCK(&world_counter_lock);
     size_t new_world = jl_atomic_load_relaxed(&jl_world_counter) + 1;
     jl_binding_partition_t *bpart = jl_declare_constant_val3(b, mod, var, val, constant_kind, new_world);
-    if (bpart->min_world == new_world)
+    if (jl_atomic_load_relaxed(&bpart->min_world) == new_world)
         jl_atomic_store_release(&jl_world_counter, new_world);
     JL_UNLOCK(&world_counter_lock);
     return bpart;

src/toplevel.c

@@ -318,3 +318,68 @@ module UndefinedTransitions
         @test Base.Compiler.is_nothrow(Base.Compiler.decode_effects(ci.ipo_purity_bits))
     end
 end
+
+# Identical implicit partitions should be merge (#57923)
+for binding in (convert(Core.Binding, GlobalRef(Base, :Math)),
+                convert(Core.Binding, GlobalRef(Base, :Intrinsics)))
+    # Test that these both only have two partitions
+    @test isdefined(binding, :partitions)
+    @test isdefined(binding.partitions, :next)
+    @test !isdefined(binding.partitions.next, :next)
+end
+
+# Test various scenarios for implicit partition merging
+module MergeStress
+    for i = 1:5
+        @eval module $(Symbol("M$i"))
+            export x, y
+            const x = 1
+            const y = 2
+        end
+    end
+    const before = Base.get_world_counter()
+    using .M1
+    const afterM1 = Base.get_world_counter()
+    using .M2
+    const afterM2 = Base.get_world_counter()
+    using .M3
+    const afterM3 = Base.get_world_counter()
+    using .M4
+    const afterM4 = Base.get_world_counter()
+    using .M5
+    const afterM5 = Base.get_world_counter()
+end
+
+function count_partitions(b::Core.Binding)
+    n = 0
+    isdefined(b, :partitions) || return n
+    bpart = b.partitions
+    while true
+        n += 1
+        isdefined(bpart, :next) || break
+        bpart = bpart.next
+    end
+    return n
+end
+using Base: invoke_in_world
+
+const xbinding = convert(Core.Binding, GlobalRef(MergeStress, :x))
+function access_and_count(point)
+    invoke_in_world(getglobal(MergeStress, point), getglobal, MergeStress, :x)
+    count_partitions(xbinding)
+end
+
+@test count_partitions(xbinding) == 0
+@test access_and_count(:afterM1) == 1
+# M2 is the first change to the `usings` table after M1. The partitions
+# can and should be merged
+@test access_and_count(:afterM2) == 1
+
+# There is a gap between M2 and M5 - the partitions should not be merged
+@test access_and_count(:afterM5) == 2
+
+# M4 and M5 are adjacent, these partitions should also be merged (in the opposite direction)
+@test access_and_count(:afterM4) == 2
+
+# M3 connects all, so we should have a single partition
+@test access_and_count(:afterM3) == 1