Re: GMP code committed -- watch for bugs.

From: Mikael Djurfeldt <djurfeldt@nada.kth.se>
Cc: djurfeldt@nada.kth.se
Subject: Re: GMP code committed -- watch for bugs.
Date: Sun, 06 Apr 2003 11:16:44 +0200	[thread overview]
Message-ID: <xy7of3kkmtf.fsf@nada.kth.se> (raw)
In-Reply-To: <xy7u1dclij6.fsf@nada.kth.se> (Mikael Djurfeldt's message of "Sat, 05 Apr 2003 23:51:41 +0200")

Mikael Djurfeldt <djurfeldt@nada.kth.se> writes:

> 2. My first attempt to build Guile has failed.  check_sanity aborts at
>    the long_long LLONG_MAX test.  Unfortunately, I can't look into
>    that right now.

OK.  There seems to be two bugs preventing the long_long tests from
succeeding:

1. num2integral.i.c:INTEGRAL2BIG had a bug in computing the absolute
   value of n when preparing arguments to mpz_import.  I've committed
   a fix for this.

2. The determination of required storage size for a bignum is buggy:

ITYPE
NUM2INTEGRAL (SCM num, unsigned long int pos, const char *s_caller)
{

     [...]

              numbits = mpz_sizeinbase (SCM_I_BIG_MPZ (num), 2);
              if (UNSIGNED) numbits++;
              scm_remember_upto_here_1 (num);
              if (numbits > (sizeof (ITYPE) * 8))
                scm_out_of_range (s_caller, num);

Firstly, you probably intended to write "if (!UNSIGNED)" (to make room
for the sign bit).  Secondly, that doesn't work either.  For example,
the absolute value of LLONG_MIN requires 64 bits, even though it is a
signed number.  So, regardless if we have "if (UNSIGNED)" or "if
(!UNSIGNED)" there will always be a case where a valid long_long or
ulong_long will cause an out_of_range error.

Unfortunately, I don't have time to get enough overview of the
situation to fix this bug.  I'll leave that to you.

Best regards,
Mikael

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