log.dat, log.c: analyse infinite loop for test added in r1192

   as a preliminary workaround, add assertion and disable test


git-svn-id: svn://scm.gforge.inria.fr/svn/mpc/trunk@1198 211d60ee-9f03-0410-a15a-8952a2c7a4e4

4 files changed, 16 insertions, 8 deletions

diff --git a/TODO b/TODO
index 900a728..498e0fa 100644
--- a/TODO
+++ b/TODO
@@ -1,3 +1,7 @@
+From Andreas Enge 27 June 2012:
+implement over- and underflow resistant log(x^2+y^2) in log.c;
+uncomment corresponding tests in log.dat
+
 From Andreas Enge 31 August 2011:
 implement mul_karatsuba with three multiplications at precision around p,
 instead of two at precision 2*p and one at precision p
diff --git a/src/log.c b/src/log.c
index 2927f68..5a91fb4 100644
--- a/src/log.c
+++ b/src/log.c
@@ -28,7 +28,7 @@ mpc_log (mpc_ptr rop, mpc_srcptr op, mpc_rnd_t rnd){
    mpfr_prec_t prec;
    int loops = 0;
    int re_cmp, im_cmp;
-   int inex_re, inex_im;
+   int inex_re, inex_im, inex;
 
    /* special values: NaN and infinities */
    if (!mpc_fin_p (op)) {
@@ -121,10 +121,13 @@ mpc_log (mpc_ptr rop, mpc_srcptr op, mpc_rnd_t rnd){
       mpfr_set_prec (w, prec);
 
       /* w is rounded down */
-      mpc_norm (w, op, GMP_RNDD);
+      inex = mpc_norm (w, op, GMP_RNDD);
          /* error 1 ulp */
       MPC_ASSERT (!mpfr_inf_p (w));
          /* FIXME: intermediate overflow; the logarithm may be representable */
+      MPC_ASSERT (!(inex && mpfr_cmp_ui (w, 1) == 0));
+         /* FIXME: this is like an underflow; the following call to log will
+            compute 0 instead of a positive result                           */
       
       mpfr_log (w, w, GMP_RNDD);
       /* generic error of log: (2^(2 - exp(w)) + 1) ulp */
diff --git a/tests/div.dat b/tests/div.dat
index 2cffc7e..dbdc2b1 100644
--- a/tests/div.dat
+++ b/tests/div.dat
@@ -1,6 +1,6 @@
 # Data file for mpc_div.
 #
-# Copyright (C) 2008, 2009, 2010, 2011 INRIA
+# Copyright (C) 2008, 2009, 2010, 2011, 2012 INRIA
 #
 # This file is part of GNU MPC.
 #
diff --git a/tests/log.dat b/tests/log.dat
index 57e3b35..0f590d0 100644
--- a/tests/log.dat
+++ b/tests/log.dat
@@ -1,13 +1,13 @@
 # Data test file for mpc_log.
 #
-# Copyright (C) 2008, 2009, 2010 INRIA
+# Copyright (C) 2008, 2009, 2010, 2012 INRIA
 #
 # This file is part of GNU MPC.
 #
 # GNU MPC is free software; you can redistribute it and/or modify it under
 # the terms of the GNU Lesser General Public License as published by the
 # Free Software Foundation; either version 3 of the License, or (at your
-#o ption) any later version.
+# option) any later version.
 #
 # GNU MPC is distributed in the hope that it will be useful, but WITHOUT ANY
 # WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
@@ -170,6 +170,7 @@
 # instead of the correct result. Since this may happen in other parts of the
 # library as well, we do not consider it a bug for the moment.
 # + + 53 0x58B90BFD4BCBFp-22   2  0x1p0       2 0x1p536870912 2 0x1p536870912 U U
-
-# log(-1 + i*eps) : infinite loop ?
-# 0 0 2 0 2 0b11 2 -1 2 0x1p-1073741813 N N
+# Due to intermediate "underflow" (see FIXME in log.c), the following example
+# loops.
+# log(-1 + i*eps)
+# - - 2 0 2 3 2 -1 2 0x1p-1073741813 N N