[Bug tree-optimization/56360] New: Loop invariant motion can introduce speculative store

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56360

             Bug #: 56360
           Summary: Loop invariant motion can introduce speculative store
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ian@airs.com


When I compile this C++ program with -O3 -std=gnu+11 on x86_64:

#include<mutex>
#include<thread>

extern int a;
std::mutex m;

void nop2(int*);

// this function is ONLY called with i=0.
void foo(int i)
{
  if (i) m.lock();
  nop2(&i); // Only here to prevent optimiser from creating one big if().
  for (int c = 0; c < 10000; ++c) {
    if (i) {
      ++a; // Thus this is never executed.
    }
  }
  nop2(&i); // Only here to prevent optimiser from creating one big if().
  if (i) m.unlock();
}

I see this in _Z3fooi:

        movl    a(%rip), %eax
        movl    12(%rsp), %ecx
        leaq    12(%rsp), %rdi
        leal    10000(%rax), %edx
        testl   %ecx, %ecx
        cmovne  %edx, %eax
        movl    %eax, a(%rip)

This unconditionally stores a value in a.  This is a speculative store, which
is invalid according to the C++ memory model.

This does not happen with -O2.

>From looking at the dumps, the bug appears to be in the loop invariant motion
pass.