From: Peter Maydell <peter.maydell@linaro.org>
To: Richard Henderson <richard.henderson@linaro.org>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Alex Bennée" <alex.bennee@linaro.org>,
"QEMU Developers" <qemu-devel@nongnu.org>
Subject: Re: [PATCH v2 4/7] exec: Use const alias for TARGET_PAGE_BITS_VARY
Date: Fri, 25 Oct 2019 22:01:15 +0100 [thread overview]
Message-ID: <CAFEAcA99ABj9LU4fox-7Zaz4NG-yKQ7cD21M9xv=AwtSSOr1mA@mail.gmail.com> (raw)
In-Reply-To: <2d65342e-ed48-1fe6-7e6c-97f51ac21a76@linaro.org>
On Fri, 25 Oct 2019 at 21:43, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 10/25/19 10:51 AM, Peter Maydell wrote:
> >> + * We want to declare the "target_page" variable as const, which tells
> >> + * the compiler that it can cache any value that it reads across calls.
> >> + * This avoids multiple assertions and multiple reads within any one user.
> >> + *
> >> + * This works because we initialize the target_page data very early, in a
> >> + * location far removed from the functions that require the final results.
> >
> > I have to say that this feels like a worryingly large amount
> > of magic. Is this actually guaranteed to work by the compiler?
>
> Yes.
I'm curious to know how the compiler engineers define
"very early" and "far removed" -- in my experience they
usually prefer to be more precise than that :-)
thanks
-- PMM
next prev parent reply other threads:[~2019-10-25 21:04 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-23 15:44 [PATCH v2 0/7] exec: Improve code for TARGET_PAGE_BITS_VARY Richard Henderson
2019-10-23 15:44 ` [PATCH v2 1/7] cpu: use ROUND_UP() to define xxx_PAGE_ALIGN Richard Henderson
2019-10-24 11:52 ` Philippe Mathieu-Daudé
2019-10-24 12:04 ` Paolo Bonzini
2019-10-24 14:06 ` Richard Henderson
2019-10-24 14:14 ` Paolo Bonzini
2019-10-25 11:48 ` Richard Henderson
2019-10-23 15:45 ` [PATCH v2 2/7] exec: Split out variable page size support to exec-vary.c Richard Henderson
2019-10-25 14:02 ` Alex Bennée
2019-10-23 15:45 ` [PATCH v2 3/7] configure: Detect compiler support for __attribute__((alias)) Richard Henderson
2019-10-25 14:04 ` Alex Bennée
2019-11-08 16:01 ` Thomas Huth
2019-10-23 15:45 ` [PATCH v2 4/7] exec: Use const alias for TARGET_PAGE_BITS_VARY Richard Henderson
2019-10-25 14:28 ` Alex Bennée
2019-10-25 14:51 ` Peter Maydell
2019-10-25 20:43 ` Richard Henderson
2019-10-25 21:01 ` Peter Maydell [this message]
2019-10-25 21:16 ` Richard Henderson
2019-10-23 15:45 ` [PATCH v2 5/7] exec: Restrict TARGET_PAGE_BITS_VARY assert to CONFIG_DEBUG_TCG Richard Henderson
2019-10-25 14:44 ` Alex Bennée
2019-10-23 15:45 ` [PATCH v2 6/7] exec: Promote TARGET_PAGE_MASK to target_long Richard Henderson
2019-10-23 15:45 ` [PATCH v2 7/7] exec: Cache TARGET_PAGE_MASK for TARGET_PAGE_BITS_VARY Richard Henderson
2019-10-24 9:29 ` [PATCH v2 0/7] exec: Improve code " no-reply
2019-10-25 13:57 ` Alex Bennée
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