Incorrect result with bigNatPowMod

Summary

When trying to use bigNatPowMod to replace powModInteger I notice the function sometimes returns incorrect result.

In the GMP backend, bignat_powmod ignores the result of integer_gmp_powm#, so does not set the size of the output, possibly leaving uninitialized limbs in the word array.

Steps to reproduce

The following program:

{-# LANGUAGE MagicHash #-}

import GHC.Num.BigNat
import GHC.Num.Integer

main :: IO ()
main =
    let b = integerToBigNatClamp# 251943445928310882947152017889649234
        e = integerToBigNatClamp# 503886891856621765894304035779298468
        m = integerToBigNatClamp# 503886891856621765894304035779298469
        r = integerFromBigNat# (bigNatPowMod b e m)
     in print r

when executed with runghc displays a non-constant result like 85639447066539974357520744449.

Expected behavior

Result displayed should be always 1.

Environment

• GHC version used: branch master, commit e1dc3d7b89ea79aea158ee487234d3730e857f04.

Optional:

• Operating System: macOS
• System Architecture: x86_64