Improve packing of iseq_constant_body struct

By moving the two bools into a packing gap above the mark_bits
pointer/union we can save 8 bytes in the struct and avoid an extra cache
line (328 bytes vs 320 bytes).

Co-authored-by: Adam Hess <HParker@github.com>

mjit_c.rb

@@ -518,13 +518,13 @@ module RubyVM::MJIT
       icvarc_size: [CType::Immediate.parse("unsigned int"), Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), icvarc_size)")],
       ci_size: [CType::Immediate.parse("unsigned int"), Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), ci_size)")],
       stack_max: [CType::Immediate.parse("unsigned int"), Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), stack_max)")],
+      catch_except_p: [self._Bool, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), catch_except_p)")],
+      builtin_inline_p: [self._Bool, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), builtin_inline_p)")],
       mark_bits: [CType::Union.new(
         "", Primitive.cexpr!("SIZEOF(((struct rb_iseq_constant_body *)NULL)->mark_bits)"),
         list: CType::Pointer.new { self.iseq_bits_t },
         single: self.iseq_bits_t,
       ), Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), mark_bits)")],
-      catch_except_p: [self._Bool, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), catch_except_p)")],
-      builtin_inline_p: [self._Bool, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), builtin_inline_p)")],
       outer_variables: [CType::Pointer.new { self.rb_id_table }, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), outer_variables)")],
       mandatory_only_iseq: [CType::Pointer.new { self.rb_iseq_t }, Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), mandatory_only_iseq)")],
       jit_func: [CType::Immediate.parse("void *"), Primitive.cexpr!("OFFSETOF((*((struct rb_iseq_constant_body *)NULL)), jit_func)")],

vm_core.h

@@ -483,10 +483,6 @@ struct rb_iseq_constant_body {
     unsigned int icvarc_size; // Number of ICVARC caches
     unsigned int ci_size;
     unsigned int stack_max; /* for stack overflow check */
-    union {
-        iseq_bits_t * list; /* Find references for GC */
-        iseq_bits_t single;
-    } mark_bits;
 
     bool catch_except_p; // If a frame of this ISeq may catch exception, set true.
     // If true, this ISeq is leaf *and* backtraces are not used, for example,
@@ -495,6 +491,12 @@ struct rb_iseq_constant_body {
     // ObjectSpace#trace_object_allocations.
     // For more details, see: https://bugs.ruby-lang.org/issues/16956
     bool builtin_inline_p;
+
+    union {
+        iseq_bits_t * list; /* Find references for GC */
+        iseq_bits_t single;
+    } mark_bits;
+
     struct rb_id_table *outer_variables;
 
     const rb_iseq_t *mandatory_only_iseq;