[Bug tree-optimization/63185] Improve DSE with branches

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63185

--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
Another example:

#include <time.h>
void f(){
  const int n=1<<14;
  double a[n];
  double b[n];
  double c[n];
  __builtin_memset(a,0,n);
  __builtin_memset(b,0,n);
  __builtin_memset(c,0,n);
  for(int i=0;i<n;++i)
    c[i]=a[i]+b[i];
  time(0);
}

If I make n a constant (using #define), DCE knows that c is not
ref_may_be_aliased (that test could probably be weakened) and removes almost
everything. We don't replace alloca_with_align with an array because the size
is larger than 256 (with a more limited scope the limit would even be 25...).
DSE is likely confused by the loop. And PRE and others don't know that a[i] is
always 0. (llvm removes everything but the final call)