[lmi-commits] [lmi] master d9bdcd4d 5/5: Remove UB

[Greg Chicares]

branch: master
commit d9bdcd4d7f4821bf51619c2b948e5083757e2f4b
Author: Gregory W. Chicares <gchicares@sbcglobal.net>
Commit: Gregory W. Chicares <gchicares@sbcglobal.net>

    Remove UB
    
    Comparing the original (with UB) to this commit (using unsigned int)
    and also to a less elegant alternative using an int64_t instead of
    unsigned int, unit-test timings are:
    
    //           original  unsigned   int64_t
    // GNU/Linux   13300     15350     16400   [default flags]
    // GNU/Linux   13450     12300     13400   -O3 -fno-omit-frame-pointer
    //    msw      13400     12400     12400   [default flags]
    //    msw      13400     12400     12400   -O3 -fno-omit-frame-pointer
    
    For the gcc-10 msw builds, either non-UB alternative is faster. For the
    gcc-11 GNU/Linux builds, this commit is discernibly slower with '-O2'
    (lmi's usual optimization level), but faster with '-O3'.
    
    Oddly, the usual lmi speed test shows a small but opposite effect:
    GNU/Linux became about half a percent faster, while msw became two or
    three percent slower.
---
 Speed_gcc_x86_64-pc-linux-gnu | 12 ++++++------
 Speed_gcc_x86_64-w64-mingw32  | 12 ++++++------
 bin_exp.hpp                   |  8 +++++---
 3 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/Speed_gcc_x86_64-pc-linux-gnu b/Speed_gcc_x86_64-pc-linux-gnu
index 8f5e8c83..d25f5d37 100644
--- a/Speed_gcc_x86_64-pc-linux-gnu
+++ b/Speed_gcc_x86_64-pc-linux-gnu
@@ -1,7 +1,7 @@
 Test speed:
-  naic, no solve      : 1.301e-02 s mean;      12880 us least of  77 runs
-  naic, specamt solve : 2.722e-02 s mean;      26775 us least of  37 runs
-  naic, ee prem solve : 2.395e-02 s mean;      23725 us least of  42 runs
-  finra, no solve     : 4.972e-03 s mean;       4781 us least of 100 runs
-  finra, specamt solve: 1.807e-02 s mean;      17653 us least of  56 runs
-  finra, ee prem solve: 1.603e-02 s mean;      15718 us least of  63 runs
+  naic, no solve      : 1.285e-02 s mean;      12692 us least of  78 runs
+  naic, specamt solve : 2.694e-02 s mean;      26677 us least of  38 runs
+  naic, ee prem solve : 2.364e-02 s mean;      23478 us least of  43 runs
+  finra, no solve     : 4.963e-03 s mean;       4773 us least of 100 runs
+  finra, specamt solve: 1.782e-02 s mean;      17609 us least of  57 runs
+  finra, ee prem solve: 1.585e-02 s mean;      15495 us least of  64 runs
diff --git a/Speed_gcc_x86_64-w64-mingw32 b/Speed_gcc_x86_64-w64-mingw32
index b6a47327..c0af4c79 100644
--- a/Speed_gcc_x86_64-w64-mingw32
+++ b/Speed_gcc_x86_64-w64-mingw32
@@ -1,7 +1,7 @@
 Test speed:
-  naic, no solve      : 2.752e-02 s mean;      26034 us least of  37 runs
-  naic, specamt solve : 4.784e-02 s mean;      46855 us least of  21 runs
-  naic, ee prem solve : 4.305e-02 s mean;      42312 us least of  24 runs
-  finra, no solve     : 1.590e-02 s mean;      15481 us least of  63 runs
-  finra, specamt solve: 3.408e-02 s mean;      33505 us least of  30 runs
-  finra, ee prem solve: 3.130e-02 s mean;      30774 us least of  32 runs
+  naic, no solve      : 2.833e-02 s mean;      27762 us least of  36 runs
+  naic, specamt solve : 4.861e-02 s mean;      47763 us least of  21 runs
+  naic, ee prem solve : 4.412e-02 s mean;      43033 us least of  23 runs
+  finra, no solve     : 1.642e-02 s mean;      14582 us least of  61 runs
+  finra, specamt solve: 3.501e-02 s mean;      34512 us least of  29 runs
+  finra, ee prem solve: 3.233e-02 s mean;      31589 us least of  31 runs
diff --git a/bin_exp.hpp b/bin_exp.hpp
index ba337a61..8188d608 100644
--- a/bin_exp.hpp
+++ b/bin_exp.hpp
@@ -24,6 +24,8 @@
 
 #include "config.hpp"
 
+#include "math_functions.hpp"           // u_abs()
+
 #include <type_traits>                  // is_floating_point_v
 
 /// Binary method for exponentiation.
@@ -59,11 +61,11 @@
 /// unsigned values anyway.
 
 template<typename T>
-constexpr T bin_exp(T x, int n)
+constexpr T bin_exp(T x, int exponent)
 {
     static_assert(std::is_floating_point_v<T>);
-    bool negative_exponent {n < 0};
-    if(negative_exponent) n = -n;
+    bool negative_exponent {exponent < 0};
+    unsigned int n = u_abs(exponent);
     T y = 1;
     for(;;)
         {