From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>,
Richard Henderson <rth@twiddle.net>
Cc: QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] target-i386: guest variable shift by 0 provokes shift by -1
Date: Tue, 18 Mar 2014 17:34:35 +0100 [thread overview]
Message-ID: <5328759B.5010406@redhat.com> (raw)
In-Reply-To: <CAFEAcA-aE6HqMx64Uw=NWXdj+OP94MS7ys2gF0xxJ+HGodY0yg@mail.gmail.com>
Il 18/03/2014 16:01, Peter Maydell ha scritto:
> By whether the backend or the frontend has trivial
> access to the value to be able to avoid doing bad
> things if it's out of range. If either do, they can
> do a translate-time check to avoid issues. If neither
> does then we need the host CPU architecture to be
> OK with the out of range value, or we need to insert
> an explicit mask op in the backend (which is likely
> to be a duplicate of an explicit mask op already
> inserted at the frontend to get the frontend-target
> mandated behaviour for out of range ops).
Isn't that exactly what SPARC does? But the optimizer should be able to
remove a redundant AND (not sure it is able to optimize & 255 & 31 to a
single instruction, but it certainly can remove the second AND in "& 31
& 255").
Paolo
prev parent reply other threads:[~2014-03-18 16:34 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-18 12:33 [Qemu-devel] target-i386: guest variable shift by 0 provokes shift by -1 Peter Maydell
2014-03-18 14:20 ` Richard Henderson
2014-03-18 14:25 ` Peter Maydell
2014-03-18 14:39 ` Richard Henderson
2014-03-18 14:47 ` Peter Maydell
2014-03-18 14:52 ` Peter Maydell
2014-03-18 14:56 ` Richard Henderson
2014-03-18 15:01 ` Peter Maydell
2014-03-18 15:19 ` Richard Henderson
2014-03-18 16:34 ` Paolo Bonzini [this message]
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