[all-commits] [llvm/llvm-project] cdc2a0: [Mips] Simplify loadImmediate (NFC)

[kazutakahirata via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cdc2a0473e90732245387344248b5487778f9b6f
      https://github.com/llvm/llvm-project/commit/cdc2a0473e90732245387344248b5487778f9b6f
  Author: Kazu Hirata <kazu at google.com>
  Date:   2023-01-25 (Wed, 25 Jan 2023)

  Changed paths:
    M llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp

  Log Message:
  -----------
  [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.