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armclang and memcpy: Weird behavior

42Bastian Schick

Just for info:

armclang optimizes memcpy() in a weird way (seen for ARMv7e-M), not respecting the order of access.

So a memcpy(RBAR, mputable,...) failes :(

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  • 0 in reply to 42Bastian Schick

    Following your hint, a similar search found the Linux kernel documentation about its IO access APIs. It mentions "Do not use memset or memcpy on IO addresses; they are not guaranteed to copy data in order."

    The doc suggests using memcpy_toio and memcpy_fromio to copy data between IO memory and the RAM. Although these versions copy data in order, their knowledge about specific IO access patterns is limited - they only attempt aligned (alignment of the IO address) transfers, by copying in small chunks around the largest possible (4 bytes max on x86, it seems) alignment.

    The Linux source for memset_io comments: "memset can mangle the IO patterns quite a bit."

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  • 0 in reply to 42Bastian Schick

    Following your hint, a similar search found the Linux kernel documentation about its IO access APIs. It mentions "Do not use memset or memcpy on IO addresses; they are not guaranteed to copy data in order."

    The doc suggests using memcpy_toio and memcpy_fromio to copy data between IO memory and the RAM. Although these versions copy data in order, their knowledge about specific IO access patterns is limited - they only attempt aligned (alignment of the IO address) transfers, by copying in small chunks around the largest possible (4 bytes max on x86, it seems) alignment.

    The Linux source for memset_io comments: "memset can mangle the IO patterns quite a bit."

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