From: Eric Botcazou <botcazou@adacore.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] x86: Replace ne:CCC/ne:CCO with UNSPEC_CC_NE in neg patterns
Date: Tue, 01 Nov 2022 22:28:57 +0100 [thread overview]
Message-ID: <3131485.5fSG56mABF@fomalhaut> (raw)
In-Reply-To: <20221101203431.GM25951@gate.crashing.org>
> Yes. But it is all the same: neither signed overflow nor unsigned
> overflow (of an addition, say) can be described as the result of an
> RTL comparison.
I disagree, see for example the implementation of the addvdi4_sp3 pattern (for
which we indeed use an UNSPEC) and of the uaddvdi4_sp32 pattern (for which we
describe the overflow with a COMPARE) in the SPARC back-end. And that's even
simpler for an unsigned subtraction, where we do not need a special CC mode.
Sure there is a technical difficulty for unsigned negation because of the
canonicalization rules, hence the trick used in the SPARC back-end, but
unsigned overflow is much easier to deal with than signed overflow.
--
Eric Botcazou
next prev parent reply other threads:[~2022-11-01 21:29 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-26 18:58 H.J. Lu
2022-10-27 6:06 ` Uros Bizjak
2022-10-28 5:56 ` Hongtao Liu
2022-10-28 6:08 ` Hongtao Liu
2022-10-28 8:35 ` Eric Botcazou
2022-10-28 15:51 ` H.J. Lu
2022-10-28 17:58 ` Eric Botcazou
2022-10-28 21:43 ` Segher Boessenkool
2022-10-28 21:55 ` Eric Botcazou
2022-11-01 20:34 ` Segher Boessenkool
2022-11-01 21:28 ` Eric Botcazou [this message]
2022-10-28 21:32 ` Segher Boessenkool
2022-10-28 22:28 ` H.J. Lu
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