Running Atmel Studio with its provided gcc 6.3.1 to build firmware for an Atmel/Microchip SAMV70 (ARM Cortex-M7) chip. I have code that compares a 4-byte input array to a 4-byte local array using memcmp(). When compiled with -O0 to disable optimizations it works fine. When compiled with -Os to optimize for size or with -O3 for max optimization, the compiler is replacing the memcmp() call with a direct 4-byte comparison (verified by examining the disassembly). Unfortunately the optimization also sometimes moves the local 4-byte array to an unaligned starting address, so while memcmp() would work fine the direct comparison triggers a HardFault due to unaligned access.
In my opinion this is 100% a compiler optimization bug (possibly gcc, possibly something Atmel added), but I'm stuck with the provided compiler so updating isn't an option. So here's my actual question: Is there a way to keep optimizations enabled but disable this particular optimization? Otherwise I'm stuck forcing the local 4-byte arrays to be 4-byte aligned or finding some other workaround.
Compiler version:
gcc version 6.3.1 20170620 (release) [ARM/embedded-6-branch revision 249437] (Atmel build: 508)
Here's an example function that could trigger the fault:
bool example(uint8_t *input_data)
{
uint8_t local_data[4] = { 0x00, 0x01, 0x02, 0x03 };
return (memcmp(input_data, local_data, 4) == 0);
}
My code is always passing in a 4-byte-aligned input_data so that's not an issue, but once again it's bad form for the compiler optimizations to take that for granted.