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Date: Fri, 7 Jan 2011 23:48:56 +0000
From: Stuart Brady <sdb@zubnet.me.uk>
Subject: Re: [Qemu-devel] [PATCH 1/7] tcg: Define "deposit" as an optional
	operation.
Message-ID: <20110107234856.GA20900@zubnet.me.uk>
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To: Richard Henderson <rth@twiddle.net>
Cc: qemu-devel@nongnu.org, Aurelien Jarno <aurelien@aurel32.net>, Alexander Graf <agraf@suse.de>

On Fri, Jan 07, 2011 at 02:42:57PM -0800, Richard Henderson wrote:
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/README    |   14 ++++++++++++++
>  tcg/tcg-op.h  |   40 ++++++++++++++++++++++++++++++++++++++++
>  tcg/tcg-opc.h |    6 ++++++
>  tcg/tcg.c     |   13 +++++++++++++
>  4 files changed, 73 insertions(+), 0 deletions(-)
> 
> diff --git a/tcg/README b/tcg/README
> index 68d27ff..ef59070 100644
> --- a/tcg/README
> +++ b/tcg/README
> @@ -285,6 +285,20 @@ the four high order bytes are set to zero.
>  Indicate that the value of t0 won't be used later. It is useful to
>  force dead code elimination.
>  
> +* deposit_i32/i64 dest, t1, t2, loc
> +
> +Deposit T2 as a bitfield into T1, placing the result in DEST.
> +The bitfield is described by LOC, an immediate value:
> +
> +  bits 0:7  - the length of the bitfield
> +  bits 8:15 - the position of the first bit
> +
> +For example, 0x101 indicates a 1-bit field at bit 1.
> +This operation would be equivalent to
> +
> +  dest = (t1 & ~2) | ((t2 << 1) & 2)

I'm being rather picky, but would something like this be better? :-

  For example, 0x804 indicates a 4-bit field starting from bit 8.
  This operation would be equivalent to:

    dest = (t1 & ~(0xf << 8)) | ((t2 << 8) & (0xf << 8))

OTOH, the code in your version was simpler... so maybe 0x201 or 0x102
as a compromise?

I suppose it's unlikely that anyone's really going to need the example
though, so I'm probably fussing too much. :-)

Cheers,
-- 
Stuart Brady