RFR: 8334230: Optimize C2 classes layout

[Neethu Prasad]

**Notes**

Rearrange C2 class fields to optimize footprint.


**Verification**

1. Ran tier2_compiler, hotspot_compiler, tier 1 & tier 2 tests.
2. Ran pahole on 64 bit machine post re-ordering and verified that there are no holes / reduction in total bytes.

| Class | Size | Cachelines | Sum Members | Holes | Sum holes | Last Cacheline | Padding |
| ----- | ----- | ---------- | --------------- | ----- | ---------- | --------------- | -------- |
| ArrayPointer | 56 -> 48 | 1 -> 1 | 45 -> 0 | 2 ->  0 | 11 -> 0  | 56 bytes -> 48 | 0 -> 3 |
| CallJavaNode | 152 -> 144 | 3 -> 3 | 12 -> 0 | 1 ->  0 | 5 -> 0  | 24 bytes -> 16 | 7 -> 4 |
| C2Access | 56 -> 48 | 1-> 1 | 42 -> 0 | 1 ->  0 | 7 -> 0  | 56 bytes -> 48 | 7 -> 6 |
| VectorSet| 32 -> 24 | 1-> 1 | 24 -> 0 | 1 ->  0 | 8 -> 0  | 32 bytes -> 24 | 1 -> 1 |

class ArrayPointer {
	const class Node  *        _pointer;             /*     0     8 */
	const class Node  *        _base;                /*     8     8 */
	const jlong                _constant_offset;     /*    16     8 */
	const class Node  *        _int_offset;          /*    24     8 */
	const class GrowableArray<Node*>  * _other_offsets; /*    32     8 */
	const jint                 _int_offset_shift;    /*    40     4 */
	const bool                 _is_valid;            /*    44     1 */
public:


	/* size: 48, cachelines: 1, members: 7 */
	/* padding: 3 */
	/* last cacheline: 48 bytes */
};



class CallJavaNode : public CallNode {
public:

	/* class CallNode            <ancestor>; */      /*     0   128 */
protected:

	/* --- cacheline 2 boundary (128 bytes) --- */
	class ciMethod *           _method;              /*   128     8 */
	bool                       _optimized_virtual;   /*   136     1 */
	bool                       _method_handle_invoke; /*   137     1 */
	bool                       _override_symbolic_info; /*   138     1 */
	bool                       _arg_escape;          /*   139     1 */
public:

protected:

public:


	/* size: 144, cachelines: 3, members: 6 */
	/* padding: 4 */
	/* last cacheline: 16 bytes */

	/* BRAIN FART ALERT! 144 bytes != 12 (member bytes) + 0 (member bits) + 0 (byte holes) + 0 (bit holes), diff = 1024 bits */
};



class C2Access : public StackObj {
public:

	/* class StackObj            <ancestor>; */      /*     0     0 */

	/* XXX last struct has 1 byte of padding */

	int ()(void) * *           _vptr.C2Access;       /*     0     8 */
protected:

	DecoratorSet               _decorators;          /*     8     8 */
	class Node *               _base;                /*    16     8 */
	class C2AccessValuePtr &   _addr;                /*    24     8 */
	class Node *               _raw_access;          /*    32     8 */
	enum BasicType             _type;                /*    40     1 */
	uint8_t                    _barrier_data;        /*    41     1 */
public:

protected:

public:


	/* size: 48, cachelines: 1, members: 8 */
	/* padding: 6 */
	/* paddings: 1, sum paddings: 1 */
	/* last cacheline: 48 bytes */
};



class VectorSet : public AnyObj {
public:

	/* class AnyObj              <ancestor>; */      /*     0     0 */

	/* XXX last struct has 1 byte of padding */

	static const uint                 word_bits;     /*     0     0 */
	static const uint                 bit_mask;      /*     0     0 */
	uint                       _size;                /*     0     4 */
	uint                       _data_size;           /*     4     4 */
	uint32_t *                 _data;                /*     8     8 */
	class Arena *              _set_arena;           /*    16     8 */

	/* size: 24, cachelines: 1, members: 5, static members: 2 */
	/* paddings: 1, sum paddings: 1 */
	/* last cacheline: 24 bytes */
};

I wrote simple program that just assigns integer value to a variable and observed the following - 
Number of ArrayPointer instances = 58. 
Number of C2Access instances = 1390.
Number of CallJavaNode instances = 1626.
58 * 8 byte + 1390 * 8 + 1626 * 8 = 24KB
24 KB space saving at the very least and significant memory footprint savings for much complex programs.

-------------

Commit messages:
 - 8334230: Keep constructor order same as before & optimize VectorSet
 - 8334230: Optimize C2 classes layout

Changes: https://git.openjdk.org/jdk/pull/19861/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19861&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334230
  Stats: 20 lines in 4 files changed: 8 ins; 8 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19861.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19861/head:pull/19861

PR: https://git.openjdk.org/jdk/pull/19861