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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 17 Jul 2015 07:30:52 +0100
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Subject: Re: [Qemu-devel] [PATCH RFC 7/9] tcg: replace ext/u_i32_i64 by a
 mov when not implemented
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To: Aurelien Jarno <aurelien@aurel32.net>, qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>

On 07/15/2015 12:03 PM, Aurelien Jarno wrote:
> When ext_i32_i64 and extu_i32_i64 ops are not implemented, this means
> that the register is already properly zero/sign extended, so we can
> simply replace it by a mov.
>
> In practice it means at least one of the two ops should always be
> implemented on 64-bit targets.
>
> Cc: Paolo Bonzini<pbonzini@redhat.com>
> Cc: Richard Henderson<rth@twiddle.net>
> Signed-off-by: Aurelien Jarno<aurelien@aurel32.net>
> ---
>   tcg/tcg-op.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

If we're going to do this (and of course pick a solution for all of the other 
backends), I think perhaps x86 should choose trunc + exts as the two that 
should be implemented, leaving extu the one that can be folded away.

Something to experiment with...


r~