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
>
