Kazu Hirata cdc2a0473e [Mips] Simplify loadImmediate (NFC) ...

loadImmediate computes ShiftAmount in an unnecessarily complicated
manner.  We just need to know the minimum right shift amount to bring
the immediate down to an unsigned 16-bit value, so

  unsigned ShiftAmount = llvm::bit_width((uint64_t)ImmValue) - 16;

is sufficient.  In other words, the following are all equivalent:

  unsigned ShiftAmount = FirstSet - (15 - (LastSet - FirstSet));
  unsigned ShiftAmount = llvm::countr_zero(IV) - (15 - (63 - llvm::countl_zero(IV) - llvm::countr_zero(IV)));
  unsigned ShiftAmount = llvm::countr_zero(IV) - 15 + (63 - llvm::countl_zero(IV) - llvm::countr_zero(IV));
  unsigned ShiftAmount = 48 - llvm::countl_zero(IV);
  unsigned ShiftAmount = 64 - llvm::countl_zero(IV) - 16;
  unsigned ShiftAmount = llvm::bit_width(IV) - 16;

where IV represents (uint64)ImmValue.  I've also checked the
equivalence empirically up to 2u << 32.

2023-01-25 20:54:43 -08:00

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