Re: [PATCH] Fix overzealous folding

[Eric Botcazou <ebotcazou at adacore dot com>]

> Even if so, there's no reason to make the problem worse!

I think it depends by how much, say in percentage.  First of all, the 
"equivalence" principle was already violated, just compile at -O2

eric@linux:~/build/gcc/native32> cat t.cc
extern void abort (void);

#ifdef TRUE_ENUM

typedef enum { FIRST = 1, SECOND = 2, THIRD = 3 } my_enum;

#else

typedef int my_enum;

#define FIRST  1
#define SECOND 2
#define THIRD  3

#endif

int valid(my_enum v)
{
  if (FIRST <= v && v <= THIRD)
    return 1;
  else
    abort();
}

with or without defining TRUE_ENUM.  I think that we can agree that it's not a 
small violation, so let's say the principle is already violated in 50% of the 
cases for typical C++ code using enumeration types.

As I previously said, I'd think that the case at hand (casting back to an 
enumeration type the result of a bitwise manipulation) is pretty rare, so 
let's the additional violation is 1%.

So the trade off is: 51% vs 50% of violation, or non-negligible work in the 
middle-end or VRP that may have far reaching consequences.  In my opinion 
it's worth at least considering.

-- 
Eric Botcazou