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Not sure if this is a gcc bug or some weird corner of UB or what...

But I have a packed struct containing a uint32 as the first field. I'm running on ARMv7-M so 32-bit unaligned loads are allowed (but not 64-bit).

This struct is being read directly via casting from a network RX buffer that is likely not aligned to any particular byte boundary. It's a) packed and b) has 32-bit fields in it.

So silly me assumed that gcc would generate either bytewise reads (assuming no alignment at all) or a ldr instruction (accepting that 32-bit unaligned loads are OK).

But for some reason at -O3 it generates a 64-bit read with ldrd, which promptly hard faults. I have no idea why it's doing that given that I was just __builtin_bswap32'ing a single 32-bit field.

Was able to work around the issue with memcpy, but seriously WTF? If I'm using a packed struct I'm explicitly telling the compiler not to make any assumptions about alignment because I'm directly serializing the data from somewhere. Where did it magically get the idea that my packed 32-bit field had 64-bit alignment?