From: mka@chromium•org (Matthias Kaehlcke)
To: linux-arm-kernel@lists•infradead.org
Subject: arm64: Question about warnings due to unspecified ASM operand width
Date: Tue, 18 Apr 2017 11:12:25 -0700 [thread overview]
Message-ID: <20170418181225.GB128305@google.com> (raw)
In-Reply-To: <20170418113849.GC27592@e104818-lin.cambridge.arm.com>
Hi Catalin,
El Tue, Apr 18, 2017 at 12:38:49PM +0100 Catalin Marinas ha dit:
> On Mon, Apr 17, 2017 at 06:31:56PM -0700, Matthias Kaehlcke wrote:
> > During my work on improving support for kernel builds with clang I
> > came across a bunch of warnings on arm64 builds about the width of
> > operands in assembly not being specified:
> >
> > arch/arm64/include/asm/arch_timer.h:92:46: error: value size does
> > not match register size specified by the constraint and modifier [-Werror,-Wasm-operand-widths]
> > asm volatile("mrs %0, cntfrq_el0" : "=r" (val));
>
> I think the first step would be to test it against a newer kernel. The
> above code disappeared in 4.9 in favour of dedicated read_sysreg()
> macros which shouldn't give this warning (u64 __val).
Sure, it was just one example from the 4.4 kernel my project currently
uses. There are others in more recent kernel versions.
> > I understand that this is usually not a problem and might even be
> > desired to give the compiler more flexiblity in the use of the
> > available registers.
>
> I don't think arm64 would benefit from such ambiguity, so we should
> rather fix them. In practice there is no issue since the compiler cannot
> allocate two 32-bit variables in a 64-bit register.
Thanks for the clarification!
According to Ard there is the case of the msr/mrs instructions to
consider, but lets talk about this in the subthread of his reply.
Matthias
next prev parent reply other threads:[~2017-04-18 18:12 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-18 1:31 arm64: Question about warnings due to unspecified ASM operand width Matthias Kaehlcke
2017-04-18 11:38 ` Catalin Marinas
2017-04-18 18:12 ` Matthias Kaehlcke [this message]
2017-04-18 14:29 ` Ard Biesheuvel
2017-04-18 19:23 ` Matthias Kaehlcke
2017-04-18 19:54 ` Ard Biesheuvel
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