[libcxx] r268354 - Don't use std::__clz in 'test/support/hexfloat.h'.

[Eric Fiselier via cfe-commits]

Author: ericwf
Date: Mon May  2 21:04:26 2016
New Revision: 268354

URL: http://llvm.org/viewvc/llvm-project?rev=268354&view=rev
Log:
Don't use std::__clz in 'test/support/hexfloat.h'.

std::__clz is a libc++ specific function so it can't be used in the test suite.
This patch implements a dumb "count leading zeros" implementation within
hexfloat itself.

This patch also fixes UB since the output of `__builtin_clz(0)` is undefined
according to the GCC docs.

Modified:
    libcxx/trunk/test/support/hexfloat.h

Modified: libcxx/trunk/test/support/hexfloat.h
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/support/hexfloat.h?rev=268354&r1=268353&r2=268354&view=diff
==============================================================================
--- libcxx/trunk/test/support/hexfloat.h (original)
+++ libcxx/trunk/test/support/hexfloat.h Mon May  2 21:04:26 2016
@@ -15,7 +15,6 @@
 #ifndef HEXFLOAT_H
 #define HEXFLOAT_H
 
-#include <algorithm>
 #include <cmath>
 #include <climits>
 
@@ -23,13 +22,26 @@ template <class T>
 class hexfloat
 {
     T value_;
+
+    static int CountLeadingZeros(unsigned long long n) {
+        const std::size_t Digits = sizeof(unsigned long long) * CHAR_BIT;
+        const unsigned long long TopBit = 1ull << (Digits - 1);
+        if (n == 0) return Digits;
+        int LeadingZeros = 0;
+        while ((n & TopBit) == 0) {
+            ++LeadingZeros;
+            n <<= 1;
+        }
+        return LeadingZeros;
+    }
+
 public:
     hexfloat(long long m1, unsigned long long m0, int exp)
     {
-        const std::size_t n = sizeof(unsigned long long) * CHAR_BIT;
+        const std::size_t Digits = sizeof(unsigned long long) * CHAR_BIT;
         int s = m1 < 0 ? -1 : 1;
-        value_ = std::ldexp(m1 + s * std::ldexp(T(m0), -static_cast<int>(n -
-                                                     std::__clz(m0)/4*4)), exp);
+        int exp2 = -static_cast<int>(Digits - CountLeadingZeros(m0)/4*4);
+        value_ = std::ldexp(m1 + s * std::ldexp(T(m0), exp2), exp);
     }
 
     operator T() const {return value_;}