On 10/29/20 1:45 PM, Eugene Rozenfeld via Gcc-patches 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.
>
> Eugene
>
> -----Original Message-----
> From: Richard Biener <richard.guent...@gmail.com>
> Sent: Tuesday, October 27, 2020 2:23 AM
> To: Eugene Rozenfeld <eugene.rozenf...@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
>>
>>
>> 0001-Add-a-tree-optimization-described-in-PR97223.patch
>>
>> From 973942122522bbf2e9de54cff17de59de5955547 Mon Sep 17 00:00:00 2001
>> From: Eugene Rozenfeld <ero...@microsoft.com>
>> Date: Fri, 23 Oct 2020 16:47:01 -0700
>> Subject: [PATCH] Add a tree optimization described in PR97223.
>> MIME-Version: 1.0
>> Content-Type: text/plain; charset=UTF-8
>> Content-Transfer-Encoding: 8bit
>>
>> Convert
>> x < (short) ((unsigned short)x + const)
>> to
>> x <= SHORT_MAX – const
>> (and similarly for other integral types) if const is not 0.
>>
>> 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
>> ---
>> gcc/match.pd | 16 ++++++++++------
>> gcc/testsuite/gcc.dg/no-strict-overflow-4.c | 5 +++--
>> 2 files changed, 13 insertions(+), 8 deletions(-)
Committed to the trunk. Thanks.
jeff
