Re: [patch] Fix segfault caused by spurious constant overflow

[Eric Botcazou <ebotcazou@adacore.com>]

> Hmm, ISTR we had such mitigations in place (or have) elsewhere keying
> on the most significant bit set instead of power-of-two.

fold_plusminus_mult_expr only factors out for power-of-two:

      if (exact_log2 (absu_hwi (int11)) > 0 && int01 % int11 == 0
	  /* The remainder should not be a constant, otherwise we
	     end up folding i * 4 + 2 to (i * 2 + 1) * 2 which has
	     increased the number of multiplications necessary.  */
	  && TREE_CODE (arg10) != INTEGER_CST)

> But your case
> likely recurses and runs into the extract_multiv limiting to eventually
> stop, even for (N + 4) * 8, right?

Yes, it oscillates between extract_multiv and fold_plusminus_mult_expr until 
reaching the maximal depth.

> If so shouldn't we prevent this even for !TYPE_OVERFLOW_WRAPS?  Also
> 
> +         && !(tree_fits_shwi_p (c)
> +              && exact_log2 (absu_hwi (tree_to_shwi (c))) > 0))

The code only distributes for TYPE_OVERFLOW_WRAPS though:

      /* The last case is if we are a multiply.  In that case, we can
	 apply the distributive law to commute the multiply and addition
	 if the multiplication of the constants doesn't overflow
	 and overflow is defined.  With undefined overflow
	 op0 * c might overflow, while (op0 + orig_op1) * c doesn't.  */
      if (code == MULT_EXPR && TYPE_OVERFLOW_WRAPS (ctype))
	return fold_build2 (tcode, ctype,
			    fold_build2 (code, ctype,
					 fold_convert (ctype, op0),
					 fold_convert (ctype, c)),
			    op1);

> is better written as
> 
>            && exact_log2 (wi::to_wide (c)) > 0
> 
> not sure why the sizetype constant for you fits in a signed HWI
> or you need to compute its absolute value.  Eventually you
> need to use wi::abs(wide_int::from (wi::to_wide (c), TYPE_PRECISION
> (TREE_TYPE (c)), SIGNED))
> or so.

This is just mirrored on what fold_plusminus_mult_expr does.

-- 
Eric Botcazou