linux-kernel - RE: [PATCH v5] coresight: etm4x: avoid build failure with unrolled loops

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Date:   Tue, 28 Jun 2022 09:40:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Suzuki K Poulose' <suzuki.poulose@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Arnd Bergmann <arnd@...nel.org>,
        Tao Zhang <quic_taozha@...cinc.com>,
        "Mike Leach" <mike.leach@...aro.org>, Leo Yan <leo.yan@...aro.org>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        "coresight@...ts.linaro.org" <coresight@...ts.linaro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH v5] coresight: etm4x: avoid build failure with unrolled
 loops

...
> > Regardless of which compiler or compiler options determine whether a
> > loop can or can't be unrolled, which determines whether
> > __builtin_constant_p evaluates to true when passed an expression using a
> > loop induction variable, it is NEVER safe to allow the preprocessor to
> > construct inline asm like:
> >    asm volatile (".inst (0x160 + (i * 4))" : "=r"(__val));
> >                                   ^ expected constant expression

Can't you use (IIRC) an "=i" constraint with the C expression
so that the compiler evaluates the expression and passes the
correct constant value to the assembler?

	David

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