linux-kernel - Re: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection

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Date:   Tue, 21 Jan 2020 13:55:01 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, ruscur@...sell.cc,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection

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