Messages in this thread | | | Date | Wed, 18 Mar 2015 22:12:50 +0100 | From | Denys Vlasenko <> | Subject | Re: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs on stack |
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On 03/18/2015 10:01 PM, Andy Lutomirski wrote: > On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@redhat.com> 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.
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