UInt256 returns wrong value

notooth · June 11, 2025, 8:13pm

Can anyone let me know why this number returns a wrong value?

number = UInt256(1) << 243
print(number)

$ mojo life.mojo
1434776518227074636666380005943348126619871175004951664972849610340958208

duck_tape · June 11, 2025, 8:59pm

I think that is correct?

❯ python
Python 3.9.11 (main, Mar 29 2022, 14:04:34) 
[Clang 12.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print(1<<243)
14134776518227074636666380005943348126619871175004951664972849610340958208

notooth · June 11, 2025, 9:03pm

They are 2 different numbers:

1434776518227074636666380005943348126619871175004951664972849610340958208
14134776518227074636666380005943348126619871175004951664972849610340958208

duck_tape · June 11, 2025, 9:11pm

What mojo version / system are you running on?

I’m getting the same result:

# scratch.mojo
def main():
    var number = UInt256(1) << 243
    print(number)

❯ magic run mojo run scratch.mojo
14134776518227074636666380005943348126619871175004951664972849610340958208
❯ python
Python 3.9.11 (main, Mar 29 2022, 14:04:34) 
[Clang 12.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 14134776518227074636666380005943348126619871175004951664972849610340958208 == 14134776518227074636666380005943348126619871175004951664972849610340958208
True
>>> 14134776518227074636666380005943348126619871175004951664972849610340958208 == 1 << 243
True

notooth · June 11, 2025, 9:18pm

AlmaLinux 10
Mojo 25.4.0.dev2025061116 (26345f4f)

sora · June 11, 2025, 9:36pm

I think we simply didn’t update SIMD.write_to after introducing the larger integer types. Change 64 to max(64, dtype.bitwidth()) and it works.

github.com/modular/modular

mojo/stdlib/stdlib/builtin/format_int.mojo

e6523da95


      
                  ptr=zero_buf.unsafe_ptr(), length=1
              )
              writer.write(zero)
          
              return None
          
          # Create a buffer to store the formatted value
          
          # Stack allocate enough bytes to store any formatted 64-bit integer
          # TODO: use a dynamic size when #2194 is resolved
          alias CAPACITY: Int = 64 + 1  # +1 for storing NUL terminator.
          
          var buf = InlineArray[UInt8, CAPACITY](uninitialized=True)
          
          # Start the buf pointer at the end. We will write the least-significant
          # digits later in the buffer, and then decrement the pointer to move
          # earlier in the buffer as we write the more-significant digits.
          var offset = CAPACITY - 1
          
          buf.unsafe_ptr().offset(offset).init_pointee_copy(
              0

sora · June 11, 2025, 9:40pm

Since it’s a very simple fix, are you willing to maybe contribute a patch?

notooth · June 11, 2025, 11:49pm

I do not really understand the source code. Would you please commit this patch?

sora · June 12, 2025, 2:26am

No worries. I will get it fixed soonish. Thanks for reporting!

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UInt256 returns wrong value

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