Hi, On Thu, 11 Aug 2016, Denys Vlasenko wrote:
> This change makes it possible to align function to 64-byte boundaries > *IF* this does not introduce huge amount of padding. > > Patch drops forced alignment to 8 if requested alignment is higher than > 8: before the patch, -falign-functions=9 was generating > > .p2align 4,,8 > .p2align 3 > > which means: "align to 16 if the skip is 8 bytes or less; else align to 8". > After this change, ".p2align 3" is not emitted. > > It is dropped because I ultimately want to do something like > -falign-functions=64,8 - IOW, I want to align functions to 64 bytes, but > only if that generates padding of less than 8 bytes - otherwise I want > *no alignment at all*. Have you tested the performance impact of your patch? Note that the macro you changed is used for function and code label alignment. So, unless I misunderstand something that means that if the large alignment can't be achieved for e.g. a loop start label, you won't align it at all anymore. This should be fairly catastrophic for any loopy benchmark, so anything like this would have to be checked on a couple benchmarks from cpu2000 (possibly cpu2006), which has some that are extremely alignment sensitive. Even for function labels I'd find no alignment at all strange, and I don't see why you'd want this. > The forced ".p2align 3" interferes with that intention. Well, yes, but _why_ don't you want 8 byte alignment if 64-byte can't be achieved? Ciao, Michael.
