arm64: Question about warnings due to unspecified ASM operand width
Catalin Marinas
catalin.marinas at arm.com
Tue Apr 18 07:38:49 EDT 2017
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).
> 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.
> Before delving into 'fixing' these 'asm-operand-widths' warnings I'd
> be interested to know if this is actually desirable or if it is
> preferred to keep the operand width unspecified in certain cases.
I'm not aware of a desirable ambiguity here.
Thanks.
--
Catalin
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