royger added a comment.
In https://reviews.llvm.org/D20561#663006, @kimgr wrote:
> ... and both revisions should be in the 4.0 branch (taken from r291814)
>
> I was looking forward to this warning to help iron out alignment issues at
> compile-time instead of runtime (our ARM CPUs don't like unaligned access).
> @royger, can you expand a little on what you mean by
>
> > For once, it doesn't really check whether the access is actually unaligned
> > or no
>
> ? Isn't that exactly what it does? Or do you mean in relation to whether the
> target allows unaligned access or not?
Take the following example structure:
typedef union __attribute__((__packed__)) segment_attributes {
uint16_t bytes;
struct
{
uint16_t type:4; /* 0; Bit 40-43 */
uint16_t s: 1; /* 4; Bit 44 */
uint16_t dpl: 2; /* 5; Bit 45-46 */
uint16_t p: 1; /* 7; Bit 47 */
uint16_t avl: 1; /* 8; Bit 52 */
uint16_t l: 1; /* 9; Bit 53 */
uint16_t db: 1; /* 10; Bit 54 */
uint16_t g: 1; /* 11; Bit 55 */
uint16_t pad: 4;
} fields;
} segment_attributes_t;
struct __attribute__((__packed__)) segment_register {
uint16_t sel;
segment_attributes_t attr;
uint32_t limit;
uint64_t base;
};
clang is complaining about:
vmx.c:973:38: error: taking address of packed member 'base' of class or
structure 'segment_register' may result in an unaligned pointer value
[-Werror,-Waddress-of-packed-member]
__vmread(GUEST_CS_BASE, ®->base);
^~~~~~~~~
But "base" is indeed aligned (it would be at the same offset without the
__packed__ attribute), so I'm not sure why it complains.
Repository:
rL LLVM
https://reviews.llvm.org/D20561
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