[AArch64] Improve getUsefulBitsForUse for narrow stores.
For narrow stores (e.g., strb, srth) we know the upper bits of the register are
unused/not useful. In some cases we can use this information to eliminate
unnecessary instructions.
For example, without this patch we generate (from the 2nd test case):
ldr w8, [x0]
and w8, w8, #0xfff0
bfxil w8, w2, #16, #4
strh w8, [x1]
and after the patch the 'and' is removed:
ldr w8, [x0]
bfxil w8, w2, #16, #4
strh w8, [x1]
ret
During the lowering of the bitfield insert instruction the 'and' is eliminated
because we know the upper 16-bits that are masked off are unused and the lower
4-bits that are masked off are overwritten by the insert itself. Therefore, the
'and' is unnecessary.
Differential Revision: http://reviews.llvm.org/D20175
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269226
91177308-0d34-0410-b5e6-
96231b3b80d8