[Spice-devel] [PATCH v4 1/3] Avoid clang warnings on casts with stricter alignment requirements

[Christophe Fergeau]

On Tue, May 30, 2017 at 02:51:49PM +0200, Christophe de Dinechin wrote:
> From: Christophe de Dinechin <dinechin at redhat.com>
> 
> For example, something like this:
>     uint8_t  *p8;
>     uint32_t *p32 = (uint32_t *) p8;
> 
> generates a warning like this:
>   spice-channel.c:1350:10: error: cast from 'uint8_t *' (aka 'unsigned char *') to
>       'uint32_t *' (aka 'unsigned int *') increases required alignment from 1 to
>       4 [-Werror,-Wcast-align]
> 
> The warning indicates that we end up with a pointer to data that
> should be 4-byte aligned, but its value may be misaligned. On x86,
> this does not make much of a difference, except a relatively minor
> performance penalty. However, on platforms such as ARM, misaligned
> accesses are emulated by the kernel, and support for them is optional.
> So we may end up with a fault.

Nit: such as *older* ARM, since ARMv6, unaligned access are supported in
hardware (no idea about actual performance of such accesses though), see
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka15414.html


> The intent of the fix here is to make it easy to identify and rework
> places where actual mis-alignment occurs. Wherever casts raise the warning,
> they are replaced with a macro:
> 
> - SPICE_ALIGNED_CAST(type, value) casts value to type, and indicates that
>   we believe the resulting pointer is aligned. If it is not, a warning will

"a runtime warning will be issued. This check is disabled if NDEBUG is
defined at compile-time" (in other words, this is not an env var but a
preprocessor define)

>   be issued. This runtime check is disabled if NDEBUG is defined
>   (much like assert() as defined in <assert.h>)
> 
> - SPICE_UNALIGNED_CAST(type, value) casts value to type, and indicates that
>   we believe the resulting pointer is not always aligned.
> 
> Any code using SPICE_UNALIGNED_CAST may need to be revisited in order
> to improve performance, e.g. by using memcpy.
> 
> There are normally no warnings for SPICE_UNALIGNED_CAST, but it is possible
> to emit debug messages for mis-alignment in SPICE_UNALIGNED_CAST
> by configuring with CFLAGS=-DSPICE_DEBUG_ALIGNMENT.

Do we want both -DSPICE_DEBUG_ALIGNMENT and -DNDEBUG to enable/disable
slightly different things at compile time?

Christophe
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