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Date: 16 Oct 2015 17:30:26 -0400
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From: "George Spelvin" <linux@horizon.com>
To: akpm@linux-foundation.org, dvlasenk@redhat.com, linux@horizon.com,
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Subject: Re: [PATCH 5/5] arch: x86: use sign_extend64() for sign extension
Cc: linux-kernel@vger.kernel.org, martin.kepplinger@theobroma-systems.com
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> --- a/arch/x86/kernel/cpu/perf_event_msr.c
> +++ b/arch/x86/kernel/cpu/perf_event_msr.c
> @@ -163,10 +163,9 @@ again:
>  		goto again;
>  
>  	delta = now - prev;
> -	if (unlikely(event->hw.event_base == MSR_SMI_COUNT)) {
> -		delta <<= 32;
> -		delta >>= 32; /* sign extend */
> -	}
> +	if (unlikely(event->hw.event_base == MSR_SMI_COUNT))
> +		delta = sign_extend64(delta, 31);
> +
>  	local64_add(now - prev, &event->count);
>  }

GCC can figure it out wither way, but wouldn't

	delta = (s32)delta;

be simpler than either?