Re: [PATCH PR c/71699] Handle pointer arithmetic in nonzero tree checks

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 30 Jun 2016, Manish Goregaokar wrote:

> Alright, the following patch was tested and it works
>
> diff --git a/gcc/fold-const.c b/gcc/fold-const.c
> index 3b9500d..0d82018 100644
> --- a/gcc/fold-const.c
> +++ b/gcc/fold-const.c
> @@ -13199,6 +13199,9 @@ tree_binary_nonzero_warnv_p (enum tree_code code,
>   switch (code)
>     {
>     case POINTER_PLUS_EXPR:
> +      return flag_delete_null_pointer_checks
> +            && (tree_expr_nonzero_warnv_p (op0, strict_overflow_p)
> +                || tree_expr_nonzero_warnv_p (op1, strict_overflow_p));
>     case PLUS_EXPR:
>       if (ANY_INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_UNDEFINED (type))
>     {

So, what prevents us from deciding that p+(0-p) is nonzero when p is? Not 
sure if it is strictly legal in all languages, but I wouldn't be surprised 
if there was at least one language where optimizations could lead to such 
expressions.

I think this is an exciting optimization, but I was always too scared to 
try anything like this.

--
Marc Glisse