<div dir="ltr"> <div class="gmail_extra"> <div class="gmail_quote">On Fri, Feb 20, 2015 at 1:09 PM, Francisco Jerez <<a href="mailto:currojerez@riseup.net" target="_blank">currojerez@riseup.net</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> writes: > On Fri, Feb 20, 2015 at 4:11 AM, Francisco Jerez <<a href="mailto:currojerez@riseup.net">currojerez@riseup.net</a>> > wrote: > >> Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> writes: >> >> > I'm still a little pensive. But >> > >> > Reviewed-by: Jason Ekstrand <<a href="mailto:jason.ekstrand@intel.com">jason.ekstrand@intel.com</a>> >> > >> Thanks. >> >> > Now for a little aside. I have come to the conclusion that I made a >> grave >> > mistake when I did the LOAD_PAYLOAD stuff. In retrospect, I should have >> > just subclassed fs_inst for load_payload. The problem is that we need to >> > snag a bunch of information for the sources when we create the >> > load_payload. In particular, we need to know the width of the source so >> > that we know how much space it consumes in the payload and we need to >> know >> > the information required to properly re-create the mov such as >> > force_sechalf and force_writemask_all. Really, in order to do things >> > properly, we need to gather this information *before* we do any >> > optimizations. The nasty pile of code that you're editing together with >> > the "effective_width" parameter is a lame attempt to capture/reconstruct >> > this information. Really, we should just subclass, capture the >> information >> > up-front, and do it properly. >> > >> Yes, absolutely, this lowering pass is a real mess. There are four more >> unreviewed patches in the mailing list fixing bugs of the metadata >> guessing of lower_load_payload() [1][2][3][4], you may want to take a >> look at them. There are more bugs I'm aware of but it didn't seem >> practical to fix them. >> > > Yeah, Matt pointed me at those. I'll give them a look later today. > > >> That said, I don't think it would be worth subclassing fs_inst. >> >> My suggestion would have been to keep it simple and lower LOAD_PAYLOAD >> into a series of MOVs with force_writemask_all enabled in all cases, >> simply rely on the optimizer to eliminate those where possible. Then >> get rid of the metadata and effective_width guessing. Instead agree on >> immediates and uniforms being exec_size-wide by convention >> (i.e. LOAD_PAYLOAD's exec_size rather than the original instruction's), >> then prevent constant propagation from propagating an immediate load >> into a LOAD_PAYLOAD if it would lead to a change in the semantics of the >> program, and maybe just run constant propagation after lowering so we >> can be sure those cases are cleaned up properly where register coalesce >> isn't enough. >> > > First off, simply setting force_writemask_all isn't an option. Consider > the following scenario: > > a = foo; > if (bar) { > b = 7; > } else { > use(a); > } > use(b); > > If "use(a)" is the last use of the variable a, then the live ranges of a > and b don't actually over-lap and we can assign a and b to the same > register. However, if force_writemask_all is set on b, it will destroy the > value in a before its last use. Right now, we don't actually do this > because our liveness analysis pass flattens everything so it will think > that b and a over-lap even though they don't. However, if we ever decide > to make the liveness information more accurate, having a pile of > force_writemask_all instructions will be a major problem. So, while it is > *technically* safe for now, it's a really bad idea in the long-term. > </div></div>The thing is we *will* have to deal with that scenario. Building a message payload inherently involves crossing channel boundaries (because of headers, unsupported SIMD modes by some shared functions, etc.). I'd say it's a bug in the liveness analysis pass if it wouldn't consider the liveness interval of a and b to overlap in your example if the assignment of b had force_writemask_all set. </blockquote><div> </div><div>Yes, I'm aware of that. However, the part of that register that has to squash everything is usually only a couple of registers while the entire payload may be up to 13 (if I remmeber correctly). We'd rather not have to squash everything if we can. All that being said, our liveness/register allocation can't handle this and making register allocation handle parts of registers interfering but not other bits is going to be a real pain. Maybe not even worth it. If you'd rather force_writemask_all everything, I'll sign off on that for now. I just wanted to point out that it may not be a good permanent solution. </div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> A reasonable compromise might be to leave it up to the caller whether to set the force_writemask_all and force_sechalf flags or not. I.e. just copy the same flags set on the LOAD_PAYLOAD instruction to the MOV instructions. That would still allow reducing the liveness intervals in cases where we know that the payload respects channel boundaries. Tracking the flag information per-register in cases where the same payload has well- and ill-behaved values seems rather premature and rather useless to me because the optimizer is likely to be able to get rid of redundant copies with force_writemask_all -- register coalesce is doing this already AFAIK, maybe by accident. <div class="HOEnZb"><div class="h5"></div></div></blockquote><div> </div><div>Sure. I'm not worried about our ability to optimize. I'm primarily worried about register pressure. Like I said, it's an OK solution in the temporary. I think we'll want to give it more thought in the long-run but that's going to interact a lot with how we do register allocation etc. </div><div>--Jason </div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">> Regarding the other suggestion of just requiring width == exec_size for > immediates and uniforms, that seems pretty reasonable to me. I'd like to > know what it will do to optimizations, but it seems ok initially. I'm > still a bigger fan of just subclassing and stashing everything we need to > know up-front. If we do it right, the only things that will actually need > to know about the subclass are the function for creating a LOAD_PAYLOAD and > lower_load_payloads. > > --Jason > > >> >> [1] >> <a href="http://lists.freedesktop.org/archives/mesa-dev/2015-January/074614.html" target="_blank">http://lists.freedesktop.org/archives/mesa-dev/2015-January/074614.html</a> >> [2] >> <a href="http://lists.freedesktop.org/archives/mesa-dev/2015-February/076094.html" target="_blank">http://lists.freedesktop.org/archives/mesa-dev/2015-February/076094.html</a> >> [3] >> <a href="http://lists.freedesktop.org/archives/mesa-dev/2015-February/076097.html" target="_blank">http://lists.freedesktop.org/archives/mesa-dev/2015-February/076097.html</a> >> [4] >> <a href="http://lists.freedesktop.org/archives/mesa-dev/2015-February/076098.html" target="_blank">http://lists.freedesktop.org/archives/mesa-dev/2015-February/076098.html</a> >> >> >> > --Jason >> > >> > On Thu, Feb 19, 2015 at 1:53 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> >> > wrote: >> > >> >> >> >> >> >> On Thu, Feb 19, 2015 at 1:25 PM, Francisco Jerez <<a href="mailto:currojerez@riseup.net">currojerez@riseup.net</a> >> > >> >> wrote: >> >> >> >>> Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> writes: >> >>> >> >>> > On Thu, Feb 19, 2015 at 12:13 PM, Francisco Jerez < >> >>> <a href="mailto:currojerez@riseup.net">currojerez@riseup.net</a>> >> >>> > wrote: >> >>> > >> >>> >> Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> writes: >> >>> >> >> >>> >> > On Fri, Feb 6, 2015 at 4:01 PM, Francisco Jerez < >> >>> <a href="mailto:currojerez@riseup.net">currojerez@riseup.net</a>> >> >>> >> > wrote: >> >>> >> > >> >>> >> >> Hey Matt, >> >>> >> >> >> >>> >> >> Matt Turner <<a href="mailto:mattst88@gmail.com">mattst88@gmail.com</a>> writes: >> >>> >> >> >> >>> >> >> > On Fri, Feb 6, 2015 at 6:42 AM, Francisco Jerez < >> >>> >> <a href="mailto:currojerez@riseup.net">currojerez@riseup.net</a>> >> >>> >> >> wrote: >> >>> >> >> >> MRFs cannot be read from anyway so they cannot possibly be a >> >>> valid >> >>> >> >> >> source of LOAD_PAYLOAD. >> >>> >> >> >> --- >> >>> >> >> > >> >>> >> >> > The function only seems to test inst->dst.file == MRF. I don't >> >>> see any >> >>> >> >> > code for handling MRF sources. What am I missing? >> >>> >> >> >> >>> >> >> That test is for "handling" MRF sources -- More precisely, it's >> >>> >> >> collecting the writemask and half flags for MRF writes, which can >> >>> only >> >>> >> >> possibly be useful if we're going to use them later on to read >> >>> something >> >>> >> >> out of an MRF into a payload, which we shouldn't be doing in the >> >>> first >> >>> >> >> place. >> >>> >> >> >> >>> >> >> Aside from simplifying the function somewhat, that allows us to >> >>> drop the >> >>> >> >> 16 register gap reserved for MRFs at register offset zero, what >> will >> >>> >> >> allow us to drop the vgrf_to_reg[] offset calculation completely >> >>> (also >> >>> >> >> in split_virtual_grfs()) in a future patch (not sent for review >> >>> yet). >> >>> >> >> >> >>> >> > >> >>> >> > No, we do read from MRF's sort-of... Send messages have an >> implicit >> >>> >> "read" >> >>> >> > from an MRF. >> >>> >> >> >>> >> Heh, and that's pretty much the only way you "read" from it. >> >>> >> >> >>> >> > This was written precicely so that we could use LOAD_PAYLOAD >> >>> >> > to build MRF payloads. We do on pre-GEN6. >> >>> >> > >> >>> >> I'm aware, but you don't need any of this meta-data to LOAD_PAYLOAD >> >>> >> *into* an MRF, and LOAD_PAYLOAD with an MRF as source should be >> illegal >> >>> >> anyway. >> >>> >> >> >>> > >> >>> > And no one is using it that way. All of the metadata checks you are >> >>> > deleting are checks on the *destination*. >> >>> > >> >>> >> >>> Didn't you write this code yourself? The only use for the collected >> >>> metadata is initializing the instruction flags of the MOVs subsequent >> >>> LOAD_PAYLOAD instructions are lowered to, based on the metadata already >> >>> collected for its source registers, which can never be MRFs, so the >> >>> metadata you collect from MRF writes is never actually used. >> >>> >> >> >> >> Right... I misred something initially. Yes, we should never be tracking >> >> MRF's as a source of a LOAD_PAYLOAD. I'll give it a better look a bit >> >> later, but it looks better. >> >> >> </div></div></blockquote></div> </div></div>