On 03/18/2015 10:01 PM, Andy Lutomirski wrote: > On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <[email protected]> wrote: >> We lose a number of large insns there: >> >> text data bss dec hex filename >> 9863 0 0 9863 2687 entry_64_before.o >> 9671 0 0 9671 25c7 entry_64.o >> >> What's more important, we convert two "MOVQ $imm,off(%rsp)" to "PUSH $imm" >> (the ones which fill pt_regs->cs,ss). >> >> Before this patch, placing them on fast path was slowing it down by two >> cycles: >> this form of MOV is very large, 12 bytes, and this probably reduces decode >> bandwidth >> to one insn per cycle when it meets them. >> Therefore they were living in FIXUP_TOP_OF_STACK instead (away from hot >> path). > > Does that mean that this has zero performance impact, or is it > actually a speedup?
No, it's not a speedup because those big bad instructions weren't on hot path to begin with. We want them to be there. Inserting them in a form of MOVs into hot path (say, in order to eliminate FIXUP_TOP_OF_STACK) *would be* a slowdown. But we switch to PUSH method, and then inserting them _as PUSHes_ seems to be a wash. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
