llvm/utils/generate-apfloat-pow5-table.py

#!/usr/bin/env python3
# ===----------------------------------------------------------------------===#
#
# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
# See https://llvm.org/LICENSE.txt for license information.
# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#
# ===----------------------------------------------------------------------===#
"""Generate the powers-of-five table used by APFloat.cpp.

Run from the llvm source directory:

  utils/generate-apfloat-pow5-table.py > lib/Support/APFloatPowerOfFiveTable.inc
"""

MAX_EXPONENT = 16383
WORD_BITS = 64


def words(value):
    """Return VALUE as little-endian 64-bit words."""
    result = []
    while value:
        result.append(value & ((1 << WORD_BITS) - 1))
        value >>= WORD_BITS
    return result


def main():
    table = []
    exponent = 8
    # Python integers have arbitrary precision, so this and the repeated
    # squaring below compute each power exactly.
    value = 5**exponent
    while exponent <= MAX_EXPONENT:
        value_words = words(value)
        table.append((exponent, value_words))
        exponent *= 2
        value *= value

    print("// clang-format off")
    print(
        "// This file is auto-generated via "
        "llvm/utils/generate-apfloat-pow5-table.py."
    )
    print("// Do not edit manually.")
    for exponent, value_words in table:
        print(f"// 5^{exponent}")
        for word in value_words:
            print(f"UINT64_C(0x{word:016X}),")
    print("    // clang-format on")


if __name__ == "__main__":
    main()