[PPL-devel] [GIT] ppl/ppl(floating_point): Avoid dangerous casts when computing errors.

[Fabio Bossi]

Module: ppl/ppl
Branch: floating_point
Commit: 5a827fc7a229ed6474101c2d951da3aa14c2d358
URL:    http://www.cs.unipr.it/git/gitweb.cgi?p=ppl/ppl.git;a=commit;h=5a827fc7a229ed6474101c2d951da3aa14c2d358

Author: Fabio Bossi <bossi at cs.unipr.it>
Date:   Wed Jul 28 11:51:32 2010 +0200

Avoid dangerous casts when computing errors.

---

 src/Float.templates.hh       |   12 ++++++------
 src/Linear_Form.templates.hh |    8 +++++---
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/src/Float.templates.hh b/src/Float.templates.hh
index a784723..6b911ac 100644
--- a/src/Float.templates.hh
+++ b/src/Float.templates.hh
@@ -109,12 +109,12 @@ const FP_Interval_Type& compute_absolute_error(
 
   PPL_ASSERT(to_compute != NULL);
 
-  // FIXME: the inner cast may be dangerous.
-  analyzer_format omega = std::max(
-  static_cast<analyzer_format>(pow(f_base,
-                               static_cast<analyzer_format>(1) -
-                               f_exponent_bias - f_mantissa_bits)),
-  std::numeric_limits<analyzer_format>::denorm_min());
+  // We assume that f_base is a power of 2.
+  analyzer_format omega;
+  int power = static_cast<int>(log2(f_base)) *
+              (1 - f_exponent_bias - f_mantissa_bits);
+  omega = std::max(static_cast<analyzer_format>(ldexpl(1.0, power)),
+                   std::numeric_limits<analyzer_format>::denorm_min());
 
   to_compute->build(i_constraint(GREATER_OR_EQUAL, -omega),
                     i_constraint(LESS_OR_EQUAL, omega));
diff --git a/src/Linear_Form.templates.hh b/src/Linear_Form.templates.hh
index fcb57c7..5bb64c0 100644
--- a/src/Linear_Form.templates.hh
+++ b/src/Linear_Form.templates.hh
@@ -418,9 +418,11 @@ Linear_Form<C>::relative_error(
   }
 
   C error_propagator;
-  // FIXME: this cast may be dangerous.
-  analyzer_format lb = -pow(f_base,
-  -static_cast<analyzer_format>(f_mantissa_bits));
+  // We assume that f_base is a power of 2.
+  int power = static_cast<int>(log2(f_base)) *
+              (-f_mantissa_bits);
+  analyzer_format lb = -static_cast<analyzer_format>(ldexpl(1.0, power));
+
   error_propagator.build(i_constraint(GREATER_OR_EQUAL, lb),
                          i_constraint(LESS_OR_EQUAL, -lb));