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858a0bfae9
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 older ARM, misaligned
accesses are emulated by the kernel, and support for them is optional.
So we may end up with a fault.
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 runtime
warning will be issued. This check is disabled unless
--enable-alignment-checks is passed at configure time
- 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.
Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
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| build-aux | ||
| common | ||
| docs | ||
| m4 | ||
| python_modules | ||
| tests | ||
| .gitmodules | ||
| autogen.sh | ||
| configure.ac | ||
| COPYING | ||
| git.mk | ||
| Makefile.am | ||
| spice1.proto | ||
| spice_codegen.py | ||
| spice.proto | ||