GCC 12.1 Released

Posted May 8, 2022 9:38 UTC (Sun) by Sesse (subscriber, #53779)
In reply to: GCC 12.1 Released by wtarreau
Parent article: GCC 12.1 Released

If a struct only contains small types, its minimum alignment is not going to be 16, and GCC's autovectorization will of course not use instructions that expect such an alignment.

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GCC 12.1 Released

Posted May 9, 2022 23:27 UTC (Mon) by foom (subscriber, #14868) [Link]

Yeah, the usual case that trips folks up is when they lie to the compiler and claim larger alignment but then don't actually provide it. (e.g. using __attribute__((aligned(16))) but then use a custom allocator which only provides 8byte aligned memory.)

The compiler believes what you tell it, and will emit instructions or do optimizations that depend upon that claimed alignment. But it won't _always_ choose instructions that will trap, so depending on optimizations you can get away with the incorrectly specified alignment until a future compiler upgrade causes a different instruction choice.

GCC 12.1 Released

Posted May 15, 2022 9:57 UTC (Sun) by anton (subscriber, #25547) [Link]

The problem happens with code that, e.g. copies a block of memory with 64-byte accesses from one unaligned address to another unaligned address. The auto-vectorized code uses movdqa for one of the addresses, and extra code is generated to align this address to a 16-byte boundary assuming that the original address is 8-byte aligned. However, the original address is not 8-byte-aligned, and the movdqa then traps.

gcc could have used movdqu instead and achieved the same performance for this loop even in the 8-byte-aligned case (plus the intended non-trap in the unaligned case).