The mpfr-3.1.2 test suite fails against MPIR-2.7.0-alpha. Specifically atan
returns the wrong value.

With some help from Fredrik Johansson we've tracked it down to a bug in
mpn_divrem.

Specifically the quotient a/b is computed incorrectly with:

a=+0*2^0+0*2^64+0*2^128+0*2^192+0*2^256+0*2^320+0*2^384+0*2^448+9223372036854775808*2^512

b=+5441491000443887104*2^0+16890151831357120748*2^64+2531194513324792887*2^128+15023743366172631521*2^192

MPIR 2.6.0 returns the correct values with:

q=+2813613562306264048*2^0+18446744073709551575*2^64+18446744073709551615*2^128+18446744073709551615*2^192+11324819608111655951*2^256

r=+0*2^0+17436446882452594688*2^64+10431979954583400456*2^128+818554020547913635*2^192+217622286998165104*2^256

MPIR-2.7.0-alpha returns the incorrect values:

q=+2813613562306263966*2^0+18446744073709551575*2^64+18446744073709551615*2^128+4202895142513760287*2^192+11324819608111655953*2^256

r=+0*2^0+2470107076112546816*2^64+11918624597650930617*2^128+5462319302375862668*2^192+4800654167612485773*2^256

I will hopefully have some time later today to begin trcking down the bug,
which surely lies in the division basecase, probably the precomputed
inverse code.

Bill.

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