[Bug target/56538] New: No opiton to disable slow 'lock' instr. one does not need

["jan.kratochvil at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56538

             Bug #: 56538
           Summary: No opiton to disable slow 'lock' instr. one does not
                    need
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jan.kratochvil@redhat.com
            Target: x86_64-unknown-linux-gnu


When no threads / shared mem is in use 'lock' has no benefits.
But it still has heavy performance hit.

#include <memory>
#include <unistd.h>
#include <sys/wait.h>
int main() {
  const int jobs=8;
  pid_t pid[jobs];
  std::shared_ptr<int> p(new int());
  for (int job=0;job<jobs;job++)
    if ((pid[job]=fork())==0) {
      for (int i=0;i<1000000000;i++)
    std::shared_ptr<int> q(p);
      _exit(0);
    }
  for (int job=0;job<jobs;job++)
    waitpid(pid[job],NULL,0);
}

g++ -o bench1 bench1.C -Wall -g -std=gnu++11 -O3
7.816s

g++ bench1.C -Wall -g -std=gnu++11 -O3 -S -o -|g++ -Dlock= -o bench1 -x
assembler-with-cpp - -lstdc++
7.150s

= 9.3% improvement

fork() has to be there to see an improvement - Intel i7-920 probably ignores
'lock' when only single core runs.

gcc-4.8.0-0.14.fc19.x86_64