RFR (XS): Bad shift coalescing in C2 matrix update barrier
rkennke at redhat.com
Wed Mar 15 14:01:37 UTC 2017
Am 15.03.2017 um 14:56 schrieb Aleksey Shipilev:
> So I succumbed by Roman's nagging that something is wrong with C2 matrix update
> barriers, and by staring into the code discovered an absolutely moronic
> optimization I did there!
> I assumed:
> [(x >> A) << B] is [X >> (A-B)]
> ...but you also need to mask out the lower B bits in the result!
> Let's remove that optimization:
> I will figure out separately if shift coalescing + mask is still beneficial.
> Testing: failing tests with matrix verification
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