InvertSqrt not working for higher precision

[hiddely]

Hi Marcel,

The InvertSqrt implementation seems to have stopped working for custom precisions. I used the same script when reporting the correctness bug in InvertSqrt last May.

InvertSqrt example script

# test mpc_math.InvertSqrt and compare with ground truth

import numpy as np
from Compiler import mpc_math, util

# with these two commented out the script works
cfix.set_precision(32, 62)
sfix.set_precision(32, 62)

def InvertSqrtGroundTruth(x):
    return 1.0 / np.sqrt(x)

input_value = 0.001

# ground truth
ground_truth = InvertSqrtGroundTruth(input_value)
test = mpc_math.InvertSqrt(sfix(input_value))
test_sfix = 1 / mpc_math.sqrt(sfix(input_value))

print_ln("ground_truth %s == InvertSqrt %s", ground_truth, test.reveal())
print_ln("1 / sqrt() %s == InvertSqrt %s", test_sfix.reveal(), test.reveal())

The script runs successfully for the version after the fix (a44132e), but in a newer version of MP-SPDZ (6b2f4e54) it gives the following compiler error:

Traceback (most recent call last):
  File "./Scripts/compile-run.py", line 14, in <module>
    prog = compiler.compile_file()
  File "./Scripts/../Compiler/compilerLib.py", line 496, in compile_file
    return self.finalize_compile()
  File "./Scripts/../Compiler/compilerLib.py", line 529, in finalize_compile
    self.prog.finalize()
  File "./Scripts/../Compiler/program.py", line 558, in finalize
    tape.optimize(self.options)
  File "./Scripts/../Compiler/program.py", line 1038, in wrapper
    return function(self, *args, **kwargs)
  File "./Scripts/../Compiler/program.py", line 1127, in optimize
    self.expand_cisc()
  File "./Scripts/../Compiler/program.py", line 1038, in wrapper
    return function(self, *args, **kwargs)
  File "./Scripts/../Compiler/program.py", line 1291, in expand_cisc
    expanded = block.expand_cisc()
  File "./Scripts/../Compiler/program.py", line 966, in expand_cisc
    inst.expand_merged(skip)
  File "./Scripts/../Compiler/instructions_base.py", line 650, in expand_merged
    self.expand_to_tape(size, new_regs)
  File "./Scripts/../Compiler/instructions_base.py", line 605, in expand_to_tape
    res = util.tuplify(f(*(arg.load() for arg in in_args)))
  File "./Scripts/../Compiler/library.py", line 459, in __call__
    self.on_first_call(wrapped_function, key, my_args)
  File "./Scripts/../Compiler/library.py", line 469, in on_first_call
    assert tape_handle == program.new_tape(
  File "./Scripts/../Compiler/program.py", line 351, in new_tape
    function(*args)
  File "./Scripts/../Compiler/library.py", line 452, in wrapped_function
    self.result = self.function(*actual_call_args,
  File "./Scripts/../Compiler/instructions_base.py", line 599, in f
    self.new_instructions(size, my_args)
  File "./Scripts/../Compiler/instructions_base.py", line 502, in new_instructions
    self.function(*args, **self.kwargs)
  File "./Scripts/../Compiler/instructions_base.py", line 735, in instruction
    res.mov(res, function(sfix._new(arg, k=k, f=f), *args).v)
  File "./Scripts/../Compiler/mpc_math.py", line 943, in InvertSqrt
    u, z = Sep(x, sfix=my_sfix)
  File "./Scripts/../Compiler/mpc_math.py", line 908, in Sep
    bb.insert(0, type(b[0])(0))
  File "./Scripts/../Compiler/GC/types.py", line 460, in __init__
    bits.__init__(self, *args, **kwargs)
  File "./Scripts/../Compiler/GC/types.py", line 148, in __init__
    self.load_other(value)
  File "./Scripts/../Compiler/GC/types.py", line 519, in load_other
    super(sbits, self).load_other(other)
  File "./Scripts/../Compiler/GC/types.py", line 168, in load_other
    self.set_length(self.n or util.int_len(other))
  File "./Scripts/../Compiler/GC/types.py", line 152, in set_length
    if n > self.n:
TypeError: '>' not supported between instances of 'int' and 'NoneType'

Thanks in advance, any pointers would be greatly appreciated!