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Date: Mon, 18 Dec 2023 15:58:59 +0100
From: Claudio Imbrenda <imbrenda@linux.ibm.com>
To: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Shuah Khan <shuah@kernel.org>,
        Janosch Frank <frankja@linux.ibm.com>,
        Christian Borntraeger
 <borntraeger@linux.ibm.com>,
        kvm@vger.kernel.org, linux-kselftest@vger.kernel.org,
        David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: s390: selftest: memop: Fix undefined behavior
Message-ID: <20231218153358.14acb611@p-imbrenda>
In-Reply-To: <59621e88c5c29bdff8bc06f68b02b2c7a420a09a.camel@linux.ibm.com>
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	<20231215180206.740df738@p-imbrenda>
	<59621e88c5c29bdff8bc06f68b02b2c7a420a09a.camel@linux.ibm.com>
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On Mon, 18 Dec 2023 13:18:14 +0100
Nina Schoetterl-Glausch <nsg@linux.ibm.com> wrote:

[...]

> > > diff --git a/tools/testing/selftests/kvm/s390x/memop.c b/tools/testing/selftests/kvm/s390x/memop.c
> > > index bb3ca9a5d731..2eba9575828e 100644
> > > --- a/tools/testing/selftests/kvm/s390x/memop.c
> > > +++ b/tools/testing/selftests/kvm/s390x/memop.c
> > > @@ -485,11 +485,13 @@ static bool popcount_eq(__uint128_t a, __uint128_t b)
> > >  
> > >  static __uint128_t rotate(int size, __uint128_t val, int amount)
> > >  {
> > > -	unsigned int bits = size * 8;
> > > +	unsigned int left, right, bits = size * 8;
> > >    
> > 
> > ...why not just:
> > 
> > if (!amount)
> > 	return val;
> > 
> > ?  
> 
> That works if you move it one statement down (128 would also trigger UB).

oops, yes it has to be after

> % 128 does the trick, is branchless and there is a bit of a symmetry going
> on between right and left.
> But I can use an early return if you want.

I think it's more readable, and furthermore...

> 
> >   
> > > -	amount = (amount + bits) % bits;
> > > +	right = (amount + bits) % bits;
> > > +	/* % 128 prevents left shift UB if size == 16 && right == 0 */
> > > +	left = (bits - right) % 128;
> > >  	val = cut_to_size(size, val);
> > > -	return (val << (bits - amount)) | (val >> amount);

...this is a more idiomatic syntax for a rotate operation 

> > > +	return (val << left) | (val >> right);
> > >  }
> > >  
> > >  const unsigned int max_block = 16;
> > > 
> > > base-commit: 305230142ae0637213bf6e04f6d9f10bbcb74af8  
> >   
>