From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
ruscur@russell.cc, linux-kernel@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org
Subject: Re: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection
Date: Tue, 21 Jan 2020 13:55:01 -0600 [thread overview]
Message-ID: <20200121195501.GJ3191@gate.crashing.org> (raw)
In-Reply-To: <a008a182-f1db-073c-7d38-27bfd1fd8676@c-s.fr>
On Tue, Jan 21, 2020 at 05:22:32PM +0000, Christophe Leroy wrote:
> g1() should return 3, not 5.
What makes you say that?
"A return of 0 does not indicate that the
value is _not_ a constant, but merely that GCC cannot prove it is a
constant with the specified value of the '-O' option."
(And the rules it uses for this are *not* the same as C "constant
expressions" or C "integer constant expression" or C "arithmetic
constant expression" or anything like that -- which should be already
obvious from that it changes with different -Ox).
You can use builtin_constant_p to have the compiler do something better
if the compiler feels like it, but not anything more. Often people
want stronger guarantees, but when they see how much less often it then
returns "true", they do not want that either.
Segher
next prev parent reply other threads:[~2020-01-21 19:55 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-11 8:30 [PATCH v2 00/10] Kernel Userspace protection for PPC32 Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 01/10] powerpc/6xx: fix setup and use of SPRN_SPRG_PGDIR for hash32 Christophe Leroy
2019-03-20 13:04 ` [v2, " Michael Ellerman
2019-03-11 8:30 ` [PATCH v2 02/10] powerpc/mm: Detect bad KUAP faults (Squash of v5 series) Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 03/10] powerpc/32: Remove MSR_PR test when returning from syscall Christophe Leroy
2019-04-21 14:18 ` [v2, " Michael Ellerman
2019-03-11 8:30 ` [PATCH v2 04/10] powerpc/32: Prepare for Kernel Userspace Access Protection Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 05/10] powerpc/8xx: Only define APG0 and APG1 Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 06/10] powerpc/8xx: Add Kernel Userspace Execution Prevention Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 07/10] powerpc/8xx: Add Kernel Userspace Access Protection Christophe Leroy
2019-04-18 6:53 ` Michael Ellerman
2019-03-11 8:30 ` [PATCH v2 08/10] powerpc/32s: Implement Kernel Userspace Execution Prevention Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 09/10] powerpc/32s: Prepare Kernel Userspace Access Protection Christophe Leroy
2019-03-11 8:30 ` [PATCH v2 10/10] powerpc/32s: Implement " Christophe Leroy
2019-04-18 6:55 ` Michael Ellerman
2019-04-23 9:26 ` Christophe Leroy
2020-01-21 17:22 ` GCC bug ? " Christophe Leroy
2020-01-21 19:55 ` Segher Boessenkool [this message]
2020-01-22 6:52 ` Christophe Leroy
2020-01-22 13:36 ` Segher Boessenkool
2020-01-22 14:45 ` Christophe Leroy
2020-01-22 6:57 ` Christophe Leroy
2020-01-22 13:18 ` Segher Boessenkool
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