On 11 Mar 2015, at 11:23, Roland King wrote:
On 11 Mar 2015, at 18:13, Jonas Maebe <[email protected]>
wrote:
On 06 Mar 2015, at 02:50, Jens Alfke wrote:
Shouldn’t C alignment rules dictate that the ‘value’ union and the
surrounding ‘atomic_val_t’ type be 8-byte-aligned, since they
contain a 64-bit integer?
Alignment rules are not dictated by the programming language, but
by the ABI. The ARM ABI, and also Apple's variant, says that 64 bit
integers are only guaranteed 4 byte alignment: https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARMv6FunctionCallingConventions.html
If that's true, and read it too, the the compiler shouldn't be
emitting a double word load which requires 8-byte alignment on a
platform which doesn't guarantee it.
The compiler will, under normal circumstances, never by itself emit an
atomic 64 bit load. My guess is that Jens is passing the address of
that field to a function that is hardcoded (possibly using inline
assembly) to perform a 64 bit atomic load.
Jonas
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