Question on secutiry check of ifcombine pass

[ywgrit via Gcc-bugs]

I have the following question:

// Way.h
struct waymapt
{
  int fillnum;
  int num;
};
typedef waymapt* waymappt;

class wayobj
{
public:
    int bound;
    int *maparp;
    waymappt waymap;
    int makebound2(int fillnum, int iters);
    void create(int size);
};


// WayInit.cpp
#include <cstring>
#include "Way.h"

void wayobj::create(int size)
{
  maparp=new int[size];
  waymap=new waymapt[size];
}


// Way.cpp
#include "Way.h"
int wayobj::makebound2(int fillnum, int iters)
{
  for (int i = 0; i < iters; i++)
  {
    if (waymap[i].fillnum!=fillnum)
      if (maparp[i]!=0)
        bound++;
  }
  return bound;
}


// main.cpp
#include <cstdio>
#include "Way.h"

#define SIZE 16
int main() {
  wayobj *woj = new wayobj;
  woj->create(SIZE);
  woj->makebound2(10, SIZE);

  printf("bound == %d\n", woj->bound);
  return 0;
}

Both maparp and waymap of wayobj are arrays, and in the function
wayobj::create, the same number of elements are allocated for both arrays.
In function makebound2, if (maparp[i]! = 0) is within if
(waymap[i].fillnum!=fillnum). So if "if (waymap[i].fillnum!=fillnum)" won't
trap, then "if (maparp[i]! = 0)" won't trap either. Is there a way we can
tell that the inner branch won't trap by the relationship between the outer
branch and the inner branch?  If this is possible then ifcombine pass can
do a merge on this nested branch. In spec2006's 473.astar program, this
nested branch is a hotspot with poor prediction accuracy, so the
performance improvement after merging this nested branches is very
significant. Icc merges this nested branch, but gcc doesn't.


We modified the following source code in spec2006.473.astar
if (waymap[i].fillnum!=fillnum)
      if (maparp[i]!=0)

into

a = (waymap[index1].fillnum == fillnum) | maparp[index1] ;
if(!a)

After the modification, gcc was able to merge the two branches, and we can
see from the generated assembly that there is only one jmp instruction
There is significant change in the number of branch-misses:
gcc-not-combine: 1.5483E+10
gcc-combine: 8.4778E+9