Module: ppl/ppl Branch: floating_point Commit: f6167277a1078a7891e62097d4e034830001f373 URL: http://www.cs.unipr.it/git/gitweb.cgi?p=ppl/ppl.git;a=commit;h=f6167277a1078...
Author: Fabio Bossi bossi@cs.unipr.it Date: Tue Aug 3 11:09:28 2010 +0200
Do not use log2 to compute the base 2 logarithm of an integer.
---
src/Float.defs.hh | 9 +++++++++ src/Float.inlines.hh | 33 +++++++++++++++++++++++++++++++++ src/Float.templates.hh | 3 +-- src/Linear_Form.templates.hh | 3 +-- 4 files changed, 44 insertions(+), 4 deletions(-)
diff --git a/src/Float.defs.hh b/src/Float.defs.hh index 4e66c05..b437184 100644 --- a/src/Float.defs.hh +++ b/src/Float.defs.hh @@ -369,6 +369,15 @@ public: }; #endif
+// FIXME: is this the right place for this function? +#ifdef PPL_DOXYGEN_INCLUDE_IMPLEMENTATION_DETAILS +//! Returns the position of the most significative bit in \p a. +#endif // defined(PPL_DOXYGEN_INCLUDE_IMPLEMENTATION_DETAILS) +unsigned int ld2(unsigned long long a); + +/* FIXME: some of the following documentation should probably be + under PPL_DOXYGEN_INCLUDE_IMPLEMENTATION_DETAILS */ + /*! \brief Returns <CODE>true</CODE> if and only if there is some floating point number that is representable by \p f2 but not by \p f1. diff --git a/src/Float.inlines.hh b/src/Float.inlines.hh index 219797b..2b9c3f2 100644 --- a/src/Float.inlines.hh +++ b/src/Float.inlines.hh @@ -451,6 +451,39 @@ is_less_precise_than(Floating_Point_Format f1, Floating_Point_Format f2) { return f1 < f2; }
+#if defined(__GNUC__) +inline unsigned int ld2(unsigned long long a) { + return __builtin_clzll(a) ^ (sizeof(a)*8 - 1); +} +#else +unsigned int ld2(unsigned long long v) { + unsigned r = 0; + if (v >= 0x100000000ULL) { + v >>= 32; + r += 32; + } + if (v >= 0x10000) { + v >>= 16; + r += 16; + } + if (v >= 0x100) { + v >>= 8; + r += 8; + } + if (v >= 0x10) { + v >>= 4; + r += 4; + } + if (v >= 4) { + v >>= 2; + r += 2; + } + if (v >= 2) + r++; + return r; +} +#endif + template <typename FP_Interval_Type> inline void affine_form_image(std::map<dimension_type, diff --git a/src/Float.templates.hh b/src/Float.templates.hh index f062fde..287fb4f 100644 --- a/src/Float.templates.hh +++ b/src/Float.templates.hh @@ -113,8 +113,7 @@ const FP_Interval_Type& compute_absolute_error(
// 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); + int power = ld2(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());
diff --git a/src/Linear_Form.templates.hh b/src/Linear_Form.templates.hh index 5bb64c0..596e095 100644 --- a/src/Linear_Form.templates.hh +++ b/src/Linear_Form.templates.hh @@ -419,8 +419,7 @@ Linear_Form<C>::relative_error(
C error_propagator; // We assume that f_base is a power of 2. - int power = static_cast<int>(log2(f_base)) * - (-f_mantissa_bits); + int power = ld2(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),