current code location encoding doesn't work in aarch64

[Jie He]

Hi

We know jdk/tsan uses a 64-bit value to identify code location, lowest 16-bit for bci, the next 44-bit for method id.

In aarch64 current tsan memory mapping, method id allocated from heap is typically like 0xffffxxxxxxxx, which
Couldn't be encoded by 47-bit directly. However, there are only 3 application memory regions, they are 

static const uptr kLoAppMemBeg   = 0x0000000001000ull;
static const uptr kLoAppMemEnd   = 0x0000200000000ull;
static const uptr kMidAppMemBeg  = 0x0aaaa00000000ull;
static const uptr kMidAppMemEnd  = 0x0aaaf00000000ull;
static const uptr kHiAppMemBeg   = 0x0ffff00000000ull;
static const uptr kHiAppMemEnd   = 0x1000000000000ull;

I think in aarch64, we don't need 47-bit to encode the method id, because highest 12 bits are fixed, and could be encoded by at most 2 bits.
e.g. 00 means LoAppMemRange, 01 means MidRange, 10 means HighRange.
Or simpler, just check the value of the highest 4 bits, 0x0 means LoRange, 0x2 means MidRange, 0x7 means HighRange.
Like below

307  static jmethodID tsan_method_id_from_code_location(u8 loc) {
308     u8 id = (u8)(
309         (loc & ~(tsan_fake_pc_bit | tsan_bci_mask)) >> tsan_method_id_shift);
310
311     #ifdef AARCH64
312     u8 ms4bits = id >> 44;
313     if (ms4bits == 0x7ULL || ms4bits == 0x2ULL )
314       id = id | 0x800000000000ULL;
315     #endif
316
317     return (jmethodID)id;
318   }

What do you think?

Thanks
Jie He