enable inline allocation of structs with pointers (#34126)

Author: JeffBezanson

NEWS.md

@@ -70,6 +70,16 @@ Language changes
 * The line number of function definitions is now added by the parser as an
   additional `LineNumberNode` at the start of each function body ([#35138]).
 
+Compiler/Runtime improvements
+-----------------------------
+
+* Immutable structs (including tuples) that contain references can now be allocated
+  on the stack, and allocated inline within arrays and other structs ([#33886]).
+  This significantly reduces the number of heap allocations in some workloads.
+  Code that requires assumptions about object layout and addresses (usually for
+  interoperability with C or other languages) might need to be updated; for
+  example any object that needs a stable address should be a `mutable struct`.
+
 Command-line option changes
 ---------------------------
 
@@ -79,9 +89,6 @@ Command-line option changes
 
 * Color now defaults to on when stdout and stderr are TTYs ([#34347])
 
-Command-line option changes
----------------------------
-
   * `-t N`, `--threads N` starts Julia with `N` threads. This option takes precedence over
     `JULIA_NUM_THREADS`. The specified number of threads also propagates to worker
     processes spawned using the `-p`/`--procs` or `--machine-file` command line arguments.

src/datatype.c

@@ -506,11 +506,10 @@ void jl_compute_field_offsets(jl_datatype_t *st)
     // now finish deciding if this instantiation qualifies for special properties
     assert(!isbitstype || st->layout->npointers == 0); // the definition of isbits
     if (isinlinealloc && st->layout->npointers > 0) {
-        //if (st->ninitialized != nfields)
-        //    isinlinealloc = 0;
-        //else if (st->layout->fielddesc_type != 0) // GC only implements support for this
-        //    isinlinealloc = 0;
-        isinlinealloc = 0;
+        if (st->ninitialized != nfields)
+            isinlinealloc = 0;
+        else if (st->layout->fielddesc_type != 0) // GC only implements support for this
+            isinlinealloc = 0;
     }
     st->isbitstype = isbitstype;
     st->isinlinealloc = isinlinealloc;