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From: Wilco Dijkstra <>
To: Richard Sandiford <>
Cc: GCC Patches <>,
	Kyrylo Tkachov <>
Subject: Re: [PATCH][AArch64] Improve bit tests [PR105773]
Date: Thu, 13 Oct 2022 13:40:44 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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.


      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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