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Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour

From: Jeff Law <law at redhat dot com>
To: Richard Biener <richard dot guenther at gmail dot com>, Mikael Morin <mikael dot morin at sfr dot fr>, Markus Trippelsdorf <markus at trippelsdorf dot de>
Cc: gcc-patches <gcc-patches at gcc dot gnu dot org>
Date: Wed, 12 Aug 2015 12:07:13 -0600
Subject: Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
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References: <55C33636 dot 7020907 at sfr dot fr> <55CA51C4 dot 3050601 at redhat dot com> <CAFiYyc1pM=BT06yLY=L9EDzDCSiUHeXL0D26ic-g8bnYBmqYNA at mail dot gmail dot com> <20150812110724 dot GB403 at x4> <CAFiYyc3G2NM2iqM5b5TKs+yWZJZ5-S13n30Bz51a2uMYL7N6KA at mail dot gmail dot com> <55CB4B40 dot 6040902 at sfr dot fr> <55CB7C0C dot 7030203 at redhat dot com> <F89F5949-3938-4F58-9C31-8BC0B217533B at gmail dot com>

On 08/12/2015 11:12 AM, Richard Biener wrote:


Prec is almost never a constant and is heavily used from wide-int.

We are not exploiting this undefined ness in C so I object to making this so much slower.

Can we instead do what we do for abs_hwi and add a checking assert so we can move the tests to the callers that misbehave instead?

Given that ISO C++ is moving away from making shifting 1 into the signbit undefined behaviour, maybe we should make UBSan less strict in itswarning. That may eliminate the need for Mikael's patch.


jeff

Follow-Ups:
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Richard Biener
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Mike Stump

References:
- [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Mikael Morin
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Jeff Law
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Richard Biener
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Markus Trippelsdorf
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Richard Biener
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Mikael Morin
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Jeff Law
- Re: [Patch] sext_hwi: Avoid left shift of negative value undefined behaviour
  - From: Richard Biener

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