From: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
To: Richard Sandiford <Richard.Sandiford@arm.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
Subject: Re: [PATCH][AArch64] Improve bit tests [PR105773]
Date: Thu, 13 Oct 2022 13:40:44 +0000 [thread overview]
Message-ID: <AS4PR08MB7901DEEE5CD0D784BC6ED39E83259@AS4PR08MB7901.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <mptczaxc9k5.fsf@arm.com>
Hi Richard,
> Maybe pre-existing, but are ordered comparisons safe for the
> ZERO_EXTRACT case? If we extract the top 8 bits (say), zero extend,
> and compare with zero, the result should be >= 0, whereas TST would
> set N to the top bit.
Yes in principle zero extract should always be positive assuming we never
extract all bits (= nop). GCC never generates a zero extend of the top bits
(it becomes a shift), so I don't think it can be generated.
However I'll change it to use CC_Z in the commit since signed comparisons
of zero extend seem to be folded to equality (or true/false), so there is no
point in supporting anything but equality comparisons anyway.
Cheers,
Wilco
prev parent reply other threads:[~2022-10-13 13:41 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-05 12:30 Wilco Dijkstra
2022-10-05 16:54 ` Richard Sandiford
2022-10-12 14:38 ` Wilco Dijkstra
2022-10-12 16:04 ` Richard Sandiford
2022-10-13 13:40 ` Wilco Dijkstra [this message]
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