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Warning for converting (possibly) negative float/double to unsigned int

Perhaps this question is appropriate for the gcc mail list.

Converting a float/double to unsigned int is undefined if the result would be negative when converted to a signed int.

x86-64 and arm treat this condition differently---x86-64 returns a value whose bit pattern is the same as the bit pattern for converting to signed int, and arm returns zero. So it would be nice to have a warning that this will (or could) happen.

I couldn't find such a warning in the GCC manual or in the GCC code base. Looking through the code, it seemed it might go in this code in force_operand() in expr.c on mainline:

  if (UNARY_P (value))
      if (!target)
        target = gen_reg_rtx (GET_MODE (value));
      op1 = force_operand (XEXP (value, 0), NULL_RTX);
      switch (code)
        case ZERO_EXTEND:
        case SIGN_EXTEND:
        case TRUNCATE:
        case FLOAT_EXTEND:
        case FLOAT_TRUNCATE:
          convert_move (target, op1, code == ZERO_EXTEND);
          return target;

        case FIX:
        case UNSIGNED_FIX:
          expand_fix (target, op1, code == UNSIGNED_FIX);
          return target;

        case FLOAT:
        case UNSIGNED_FLOAT:
          expand_float (target, op1, code == UNSIGNED_FLOAT);
          return target;

return expand_simple_unop (GET_MODE (value), code, op1, target, 0);

But maybe not.

Any advice on how to proceed? I'd be willing to write and test the few lines of code myself if I knew where to put them.


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