On 11/07/17 05:16, Andrew Pinski wrote: > I was looking into some bitfield code for aarch64 and was wondering > why SLOW_BYTE_ACCESS is set to 0. I can't seem to figure out why > though. > The header says: > Although there's no difference in instruction count or cycles, > in AArch64 we don't want to expand to a sub-word to a 64-bit access > if we don't have to, for power-saving reasons. */ > > But that does not make sense because with SLOW_BYTE_ACCESS to 0, GCC > expands a sub-word access to a 64bit access. >> When I set to SLOW_BYTE_ACCESS to 1, I get between 38% to 208% speed > up for accesses of a bitfields inside a loop on ThunderX CN88xx.
What's the test case? > > Should we change SLOW_BYTE_ACCESS (or maybe better yet get rid of it)? > The documentation for SLOW_BYTE_ACCESS is just plain confusing, IMO. And your comment above seems to be contrary to the documentation as well. R.
