Re: [EXTERNAL] Re: [PATCH] [tree-optimization] Fix for PR97223

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Oct 29, 2020 at 8:45 PM Eugene Rozenfeld
<Eugene.Rozenfeld@microsoft.com> wrote:
>
> Thank you for the review Richard!
>
> I re-worked the patch based on your suggestions. I combined the two patterns. Neither one requires a signedness check as long as the type of the 'add' has overflow wrap semantics.
>
> I had to modify the regular expression in no-strict-overflow-4.c test. In that test the following function is compiled with -fno-strict-overflow :
>
> int
> foo (int i)
> {
>   return i + 1 > i;
> }
>
> We now optimize this function so that the tree-optimized dump has
>
> ;; Function foo (foo, funcdef_no=0, decl_uid=1931, cgraph_uid=1, symbol_order=0)
>
> foo (int i)
> {
>   _Bool _1;
>   int _3;
>
>   <bb 2> [local count: 1073741824]:
>   _1 = i_2(D) != 2147483647;
>   _3 = (int) _1;
>   return _3;
> }
>
> This is a correct optimization since -fno-strict-overflow implies -fwrapv.

OK.

Thanks,
Richard.

> Eugene
>
> -----Original Message-----
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Tuesday, October 27, 2020 2:23 AM
> To: Eugene Rozenfeld <Eugene.Rozenfeld@microsoft.com>
> Cc: gcc-patches@gcc.gnu.org
> Subject: [EXTERNAL] Re: [PATCH] [tree-optimization] Fix for PR97223
>
> On Sat, Oct 24, 2020 at 2:20 AM Eugene Rozenfeld via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> >
> > This patch adds a pattern for folding
> >                 x < (short) ((unsigned short)x + const) to
> >          x <= SHORT_MAX - const
> > (and similarly for other integral types) if const is not 0.
> > as described in PR97223.
> >
> > For example, without this patch the x86_64-pc-linux code generated for
> > this function
> >
> > bool f(char x)
> > {
> >     return x < (char)(x + 12);
> > }
> >
> > is
> >
> > lea    eax,[rdi+0xc]
> > cmp    al,dil
> > setg   al
> > ret
> >
> > With the patch the code is
> >
> > cmp    dil,0x73
> > setle  al
> > ret
> >
> > Tested on x86_64-pc-linux.
>
> +/* Similar to the previous pattern but with additional casts. */ (for
> +cmp (lt le ge gt)
> +     out (gt gt le le)
> + (simplify
> +  (cmp:c (convert@3 (plus@2 (convert@4 @0) INTEGER_CST@1)) @0)
> +  (if (!TYPE_UNSIGNED (TREE_TYPE (@0))
> +       && types_match (TREE_TYPE (@0), TREE_TYPE (@3))
> +       && types_match (TREE_TYPE (@4), unsigned_type_for (TREE_TYPE (@0)))
> +       && TYPE_OVERFLOW_WRAPS (TREE_TYPE (@4))
> +       && wi::to_wide (@1) != 0
> +       && single_use (@2))
> +   (with { unsigned int prec = TYPE_PRECISION (TREE_TYPE (@0)); }
> +    (out @0 { wide_int_to_tree (TREE_TYPE (@0),
> +                               wi::max_value (prec, SIGNED)
> +                               - wi::to_wide (@1)); })))))
>
> I think it's reasonable but the comment can be made more precise.
> In particular I wonder why we require a signed comparison here while the previous pattern requires an unsigned comparison.  It might be an artifact and the restriction instead only applies to the plus?
>
> Note that
>
> +       && types_match (TREE_TYPE (@4), unsigned_type_for (TREE_TYPE
> + (@0)))
>
> unsigned_type_for should be avoided since it's quite expensive.  May I suggest
>
>           && TYPE_UNSIGNED (TREE_TYPE (@4))
>           && tree_nop_conversion_p (TREE_TYPE (@4), TREE_TYPE (@0))
>
> instead?
>
> I originally wondered if "but with additional casts" could be done in a single pattern via (convert? ...) uses but then I noticed the strange difference in the comparison signedness requirement ...
>
> Richard.
>
> > Eugene
> >