RFR (S): Ensure BitMaps clearing is done with memset

[Aleksey Shipilev]

Hi,

On our large lab machines, we sometimes see rather high bitmap cleaning times:

 Concurrent Marking    =   353.84 s (a =  1259228 us) (n =   281)
     (lvls, us =   791016,  1132812,  1250000,  1328125,  3448034)

 Concurrent Evacuation =   192.62 s (a =   687937 us) (n =   280)
     (lvls, us =   398438,   609375,   687500,   744141,  1485181)

 Reset Bitmaps         =    53.37 s (a =   190614 us) (n =   280)
     (lvls, us =      684,   166016,   191406,   218750,   328977)


Not the least reason for this is that we are using per-word cleanup, while we
should instead use memset. BitMap provides the methods for that, with _large
suffixes. This patch ensures it:


GCC 4.8 has -ftree-loop-distribute-patterns at -O3, which seems to fold
clear_range into memset. This explains why we haven't seen this on our
up-to-date development machines. Disabling that optimization demonstrates the
benefits for the patch.

baseline:
 Reset Bitmaps         =     3.15 s (a =    11430 us) (n =   276)
    (lvls, us =       94,     8184,     9473,    13086,    35697)

patched:
 Reset Bitmaps         =     1.55 s (a =     5394 us) (n =   287)
    (lvls, us =       90,     4238,     4707,     5332,    19467)

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey