[X86] LowerBITREVERSE - use AND+CMPEQ+MOVMSK trick to lower scalar types #92236
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RKSimon
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…2 scalar types By splitting each byte of a scalar type into 8 copies, in reverse order, we can then test each one to see if the bit is set and then use PMOVMSKB to pack them back together to get the bit reversal. So far I've only managed to make this worth it for i8/i16 with SSSE3 (PSHUFB), i32 with AVX2 (AVX1 would be worth it if we can force all 256-bit operations to be split), and i64 with AVX512BW (which can use VPTESTMB). Fixes llvm#79794
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By splitting each byte of a scalar type into 8 copies, in reverse order, we can then test each one to see if the bit is set and then use PMOVMSKB/VPTESTMB to pack them back together to get the bit reversal.
So far I've only managed to make this worth it for i8/i16 with SSSE3 (PSHUFB), i32 with AVX2 (AVX1 would be worth it if we can force all 256-bit operations to be split), and i64 with AVX512BW (which can use VPTESTMB).
Fixes #79794