Hi Gabe, Thanks for the info! This is a bit helpful. Although, I'm still not sure what the next steps would be or how to even start on (1) or (2) that you listed.
Is it possible to focus on functional correctness first and then work on the timing correctness? The problem is that modern applications assume these SSE instructions exist. Specifically, we can't get Ubuntu to boot with systemd enabled without SSE instructions (specifically PSHUFB). >From your description here, it sounds like this could be weeks+ of development effort for us. Do we think this is worth it? There are like 4 cases of this instruction executing in the workloads we've looked at so far. Getting the timing *perfect* doesn't seem important. As you say, we need to overhaul the x86 SIMD instructions completely. However, this is a months long project for someone very familiar with gem5. It's infeasible to do that right now with our current resources. Additionally, since I am not an expert on this code, we would really need someone like you, Gabe, to mentor whoever is working on this project. Cheers, Jason On Mon, Oct 12, 2020 at 10:34 PM Gabe Black via gem5-dev <gem5-dev@gem5.org> wrote: > Hi Hao. The shuffle microop is implemented in > arch/x86/isa/microops/mediaop.isa. It looks like you'll need to do three > things to implement PSHUFB. > > 1. Figure out a realistic way to get all three register operands into the > instruction. The current version takes a destination register, and both > halves of one of the source registers, and then finally takes the 8 bit > immediate value in the ext field of the microop. With larger registers > which won't fit into two 64 bit slices, that scheme won't work for the > source operands. You'll need to figure out how to get all the information > you need into a single instruction, or in other words all the data you'll > need to generate one 64 bit chunk of the destination. It looks like instead > of always passing the shuffle instructions in an 8 bit immediate, the > PSHUFB takes yet another register to hold that value. You'll need to figure > out how to get that in there too. > 2. The shuffle microop seems to expect assume there are two flavors of > behavior, size = 8, and otherwise. You may need to add more logic to handle > additional situations this new instruction needs. > 3. Actually implement the PSHUFB macroop using the revised version of > shuffle. > > The two important requirements for this sort of modification are that the > microops still behave realistically and are constrained. If they could just > do whatever whacky, one off thing a particular instruction needed, then > there wouldn't be much value in microops, we could (sort of) just do > everything with regular instructions directly. That's not entirely true, > but the idea is true. Also we need to make sure not to break any current > instructions, so the existing uses of shuffle need to either work as is, or > be updated to continue working with the revised version of shuffle. > > Note that unlike many of the other microops gem5 uses, I actually made up > the ones that implement SSE. I went through all the SSE instructions that > existed at that time and made up a set of microops which seemed realistic > and could implement all the instructions. The instructions have changed > over time (AVX didn't exist then for instance), and so we may need to > update those microops to match the new instructions. Whatever we do though, > we still need to make things realistic, not break any existing > instructions, and not just hack in a magic microop that does what we need > in this one case without considering the design as a whole. > > Gabe > > On Mon, Oct 12, 2020 at 10:08 PM Hoa Nguyen via gem5-dev < > gem5-dev@gem5.org> wrote: > >> Hi all, >> >> I've been trying to implement the PSHUFB instruction and I need some >> help with this? While I found some documentation about this >> instruction as well as I found a similar(?) instruction implemented in >> gem5 (PSHUFD), I don't know how to implement PSHUFB in gem5. >> >> I saw that PSHUFD is broken down into 3 microps, two of which are >> `shuffle` instructions. I don't really understand and not able to find >> any documentation about this shuffle instruction. I wonder whether >> PSHUFB could also be broken into shuffle instructions. >> >> Any help or suggestions would be appreciated! >> >> Regards, >> Hoa Nguyen >> _______________________________________________ >> gem5-dev mailing list -- gem5-dev@gem5.org >> To unsubscribe send an email to gem5-dev-le...@gem5.org >> %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s >> > _______________________________________________ > gem5-dev mailing list -- gem5-dev@gem5.org > To unsubscribe send an email to gem5-dev-le...@gem5.org > %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s
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