[Bug c++/61621] New: Normal enum switch slower than test case.

[holger.seelig at yahoo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61621

            Bug ID: 61621
           Summary: Normal enum switch slower than test case.
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: holger.seelig at yahoo dot de

I detected that a normal enum switch is 1.5 up to 3 times slower than a
'double' enum switch. Below I created a test case for you. Unkomment the marked
lines to get better performace results.


### Compiler options
/usr/bin/g++ -O3 -Wall -std=c++11 test-case.cpp

###  test-case.cpp

#include <iostream>
#include <vector>
#include <chrono>


enum InstructionType
{
   UNDEFINED,
   INT32,
   UINT32,
   NUMBER,
   STRING,
   NULL_OBJECT,
   OBJECT,
   BOOLEAN_OBJECT,
   NUMBER_OBJECT,
   STRING_OBJECT,
   ARRAY_OBJECT,
   DATE_OBJECT,
   REGEX_OBJECT,
   FUNCTION_OBJECT,
   ADDRESS,
   NOP,
   RESULT,
   JUMP_IF_ZERO,
   MULTIPLICATION_EXPRESSION,
   DIVISION_EXPRESSION,
   REMAINDER_EXPRESSION,
   ADDITION_EXPRESSION,
   SUBTRACTION_EXPRESSION,
   LESS_EXPRESSION,
   RETURN_STATEMENT,

};

int
run (const std::vector <InstructionType> & instructions)
{
   int value = 0;

   for (size_t i = 0, size = instructions .size (); i < size; ++ i)
   {
      switch (instructions [i])
      {
         case UNDEFINED:
         case INT32:
         case UINT32:
         case NUMBER:
         case STRING:
         case NULL_OBJECT:
         case OBJECT:
         case BOOLEAN_OBJECT:
         case NUMBER_OBJECT:
         case STRING_OBJECT:
         case ARRAY_OBJECT:
         case DATE_OBJECT:
         case REGEX_OBJECT:
         case FUNCTION_OBJECT:
         case ADDRESS:
            value += 1;
            break;
// UNCOMMENT THE FOLLOWING LINES TO GET BETTER PERFORMANCE
//         default:
//         {
//            switch (instructions [i])
//            {
               case NOP:
                  break;
               case RESULT:
                  value += 2;
                  break;
               case JUMP_IF_ZERO:
                  value ++;
                  break;
               case MULTIPLICATION_EXPRESSION:
                  value += 3;
                  break;
               case DIVISION_EXPRESSION:
                  value += 4;
                  break;
               case REMAINDER_EXPRESSION:
                  value += 5;
                  break;
               case ADDITION_EXPRESSION:
                  value += 6;
                  break;
               case SUBTRACTION_EXPRESSION:
                  value += 7;
                  break;
               case LESS_EXPRESSION:
                  value += 8;
                  break;
               case RETURN_STATEMENT:
                  value += 9;
                  break;
// UNCOMMENT THE FOLLOWING LINES TO GET BETTER PERFORMANCE
//               default:
//                  break;
//            }
//         }
      }
   }

   return value;
}

inline
double
now ()
{
   using namespace std::chrono;

   return duration_cast <duration <double>> (high_resolution_clock::now ()
.time_since_epoch ()) .count ();
}

int
main (int argc, char **argv)
{
   std::cout << "Enum switch performance test!" << std::endl;

   std::vector <InstructionType> instructions = {
      UNDEFINED,
      INT32,
      UINT32,
      NUMBER,
      STRING,
      NULL_OBJECT,
      OBJECT,
      BOOLEAN_OBJECT,
      NUMBER_OBJECT,
      STRING_OBJECT,
      ARRAY_OBJECT,
      DATE_OBJECT,
      REGEX_OBJECT,
      FUNCTION_OBJECT,
      ADDRESS,
      NOP,
      RESULT,
      JUMP_IF_ZERO,
      MULTIPLICATION_EXPRESSION,
      DIVISION_EXPRESSION,
      REMAINDER_EXPRESSION,
      ADDITION_EXPRESSION,
      SUBTRACTION_EXPRESSION,
      LESS_EXPRESSION,
      RETURN_STATEMENT,
   };

   int  value = 0;
   auto t0    = now ();

   for (int i = 0; i < 100000000; ++ i)
      value += run (instructions);

   std::cout << "value: " << value << std::endl;
   std::cout << "time:  " << now () - t0 << " s" << std::endl;

   return 0;
}


################

holger@qualle:~$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1'
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)