On Fri, Nov 14, 2014 at 10:42:27AM +0000, Andrew Pinski wrote: > On Fri, Nov 14, 2014 at 2:35 AM, James Greenhalgh > <james.greenha...@arm.com> wrote: > > > > Hi, > > > > We currently do not set any interesting default values for jump and function > > alignment in AArch64. I've made the formula for these values derive from > > the issue rate of the processor as so: > > > > jumps: 4 * processor issue-rate (rounded down to nearest power of two) > > functions: 4 * processor issue-rate (rounded up to nearest power of two) > > > > This is sensible for the ARMv8-a implementations I tested on. An > > alternative patch would make these values new fields in the tuning > > tables. > > I had submitted an alternative patch a few hours ago which allows the > tuning structure say what alignment is wanted for all three: > https://gcc.gnu.org/ml/gcc-patches/2014-11/msg01615.html
D'oh! I should have flicked through gcc-patches before hitting send! I imagine I'm encoding similar logic to that you used when writing this patch. I'm happy with either approach, so I'll leave it to the maintainers to decide which they prefer. > > > > This happens to work well for some benchmarks and doesn't harm others. > > The benefit swings depending on the existing alignment and the knock-on > > effects. > > > > Bootstrapped on aarch64-none-linux-gnu with no issues. > > > > Does anyone have any thoughts or preferences as to how we set these > > values in future? If not, OK For trunk? > > Also I notice you don't align loops, was that an oversight or just you > did not think it was needed? I didn't think it was needed, and saw some performance issues when benchmarking with it on. I found these parameters to be very fickle in the way they change performance. It is easy to end up with a bunch of cold loops needlessly padding out the binary and therefore the cache. It might be that hard-coding loop alignment to 8 for all cores is sensible, but I don't have data either way. Cheers, James > > --- > > 2014-11-14 James Greenhalgh <james.greenha...@arm.com> > > > > * config/aarch64/aarch64.c (aarch64_override_options): Set default > > alignments for functions and jumps. >
