> On 2015 Mar 30, at 10:11, Eric Christopher <echristo at gmail.com> wrote: > > > > On Mon, Mar 30, 2015 at 9:52 AM Eric Christopher <echristo at gmail.com> wrote: > From PR18808 I said a few things and that I was going to redirect to the mailing list for further discussion. So here we are, go. > > 1) Whether or not to allow changing of target-cpu/target-feature/triple at link time code generation. > > - Not convinced here of the facility to do so. Could just recompile the individual bitcode files to get what you want, but there are some users that are trying to ship bitcode (as crazy as that sounds).IMO, it's cleanest of the target-cpu/target-feature/etc. are set at compile time. That's where users are accustomed to specifying codegen options already, and besides: the frontend needs to know the backend in order to conform to the ABI, set macros, emit calls to target-specific intrinsics, etc. I'll send a review of r233227 in a moment to that effect ;).> 2) How to pass other sorts of options to the backend for code generation > > - -ffoo options -fno-foo options. I.e. -fno-inline, etc. I think this is really pretty important from the user POV. It affects things at a more global level.This is easy to solve for -fno-inline in particular: we should just add a function attribute (`noinline`?) that the inliner should treat as a synonym for `optnone`. Any functions that come from translation units compiled with `-fno-inline` get ignored by the inliner; functions from other translation units participate fully. But in terms of setting up the LTO pass pipeline, some level of user customization makes sense. I'm not really sure how much is useful. We have a start at that with Peter's recent commits to add -O0/-O1/-O2 (not that anyone thought too carefully about what's happening at those optimization levels).> 3) The llvm developer debugging story > > - It's useful for llvm developers to be able to more accurately debug a set of IR using bisection or being able to turn off code generation options. Should this be done at the command level (i.e. infrastructure that clang and llc etc could even share), or should it be done at an llvm IR rewriting level? Don't know. I kind of want a rewriter, but I'm not wedded to any particular answer.I think some sort of rewriter makes sense. Long-term I'd still like to encode whether an option is overridable in a sane way (via a default attribute sets or something), but I haven't had time yet to go back to my original proposal and refine it :(.> > That said I was actually envisioning something like: > > clang -emit-llvm foo.c -o foo.bc > ... > > clang -O3 -flto all.bc -arch x86_64h -o haswell_slice > clang -O3 -flto all.bc -arch x86_64 -o x86_64_slice > > for the same set of bitcode files. But given the front end language restrictions on doing anything actually interesting there it's not too much of a constraint.Many of the differences between architectures CPUs affect preprocesser definitions, right? Link-time is too late for the frontend to emit Haswell-specific intrinsics, for example. That said, it would be cool if this worked.> Another usage is the (admittedly one I don't think we want to support) halide one that I discovered this week: > > clang foo.c -emit-llvm foo.bc > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 > ...Whereas this is just insane :0.> > I've since convinced them to use the pnacl sort of thing for more target independent code generation at the moment. It's a use case that could be thought about more though - especially as pnacl does the exact same sort of thing, just with a different triple for actual link time code generation, it looks more like: > > clang -target le64-unknown-unknown -emit-llvm foo.c -o foo.bc > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 > > Just to add some more actual use cases in the discussion. > > -eric
> > > On Mon, Mar 30, 2015 at 9:52 AM Eric Christopher <echristo at gmail.com> > wrote: > > From PR18808 I said a few things and that I was going to redirect to the > mailing list for further discussion. So here we are, go. > > > > 1) Whether or not to allow changing of target-cpu/target-feature/triple > at link time code generation. > > > > - Not convinced here of the facility to do so. Could just recompile the > individual bitcode files to get what you want, but there are some users > that are trying to ship bitcode (as crazy as that sounds). > > IMO, it's cleanest of the target-cpu/target-feature/etc. are set at > compile time. That's where users are accustomed to specifying codegen > options already, and besides: the frontend needs to know the backend in > order to conform to the ABI, set macros, emit calls to target-specific > intrinsics, etc. >Cleanest yes, most familiar yes, but doesn't fit the usecase of PNaCl/Emscripten/Renderscript/Halide/... as Eric was mentioning. These indeed need to figure out proper ABI, macros, intrinsics, but the existence of these is a pretty good proof that something can be done (I'm not saying it's clean or pretty!).> > That said I was actually envisioning something like: > > > > clang -emit-llvm foo.c -o foo.bc > > ... > > > > clang -O3 -flto all.bc -arch x86_64h -o haswell_slice > > clang -O3 -flto all.bc -arch x86_64 -o x86_64_slice > > > > for the same set of bitcode files. But given the front end language > restrictions on doing anything actually interesting there it's not too much > of a constraint. > > Many of the differences between architectures CPUs affect preprocesser > definitions, right? Link-time is too late for the frontend to emit > Haswell-specific intrinsics, for example. > > That said, it would be cool if this worked. >Agreed.> > Another usage is the (admittedly one I don't think we want to support) > halide one that I discovered this week: > > > > clang foo.c -emit-llvm foo.bc > > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 > > ... > > Whereas this is just insane :0. >Disagreed: different usecase from above :-) Whoever maintains "portable" things has to figure out how this works, and I think right now it's still the wild west (hence insane may not be too far of a qualification), but I don't think it's an invalid usecase. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150401/645c10dd/attachment.html>
> On 2015 Apr 1, at 17:07, JF Bastien <jfb at google.com> wrote: > > > On Mon, Mar 30, 2015 at 9:52 AM Eric Christopher <echristo at gmail.com> wrote: > > From PR18808 I said a few things and that I was going to redirect to the mailing list for further discussion. So here we are, go. > > > > 1) Whether or not to allow changing of target-cpu/target-feature/triple at link time code generation. > > > > - Not convinced here of the facility to do so. Could just recompile the individual bitcode files to get what you want, but there are some users that are trying to ship bitcode (as crazy as that sounds). > > IMO, it's cleanest of the target-cpu/target-feature/etc. are set at > compile time. That's where users are accustomed to specifying codegen > options already, and besides: the frontend needs to know the backend in > order to conform to the ABI, set macros, emit calls to target-specific > intrinsics, etc. > > Cleanest yes, most familiar yes, but doesn't fit the usecase of PNaCl/Emscripten/Renderscript/Halide/... as Eric was mentioning. These indeed need to figure out proper ABI, macros, intrinsics, but the existence of these is a pretty good proof that something can be done (I'm not saying it's clean or pretty!).But the typical clang user shouldn't suffer just because there are interesting use cases out there. Cleanest and familiar are important. I'd be happy enough with a command-line option to specify "don't encode the target" to support this kind of thing. Although Eric's idea from elsewhere in the thread seems better than adding a driver option: $ clang -target le64-unknown-unknown -emit-llvm foo.c -o foo.bc $ clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 $ clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 In this scenario, I figure the Frontend would recognize `le64` as a special architecture whose target shouldn't get encoded in the IR... or the backend would recognize that it should be overwritten.> > That said I was actually envisioning something like: > > > > clang -emit-llvm foo.c -o foo.bc > > ... > > > > clang -O3 -flto all.bc -arch x86_64h -o haswell_slice > > clang -O3 -flto all.bc -arch x86_64 -o x86_64_slice > > > > for the same set of bitcode files. But given the front end language restrictions on doing anything actually interesting there it's not too much of a constraint. > > Many of the differences between architectures CPUs affect preprocesser > definitions, right? Link-time is too late for the frontend to emit > Haswell-specific intrinsics, for example. > > That said, it would be cool if this worked. > > Agreed. > > > Another usage is the (admittedly one I don't think we want to support) halide one that I discovered this week: > > > > clang foo.c -emit-llvm foo.bc > > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 > > ... > > Whereas this is just insane :0. > > Disagreed: different usecase from above :-) > > Whoever maintains "portable" things has to figure out how this works, and I think right now it's still the wild west (hence insane may not be too far of a qualification), but I don't think it's an invalid usecase.I didn't say invalid :).
On Tue, Mar 31, 2015 at 8:59 PM Duncan P. N. Exon Smith < dexonsmith at apple.com> wrote:> > > On 2015 Mar 30, at 10:11, Eric Christopher <echristo at gmail.com> wrote: > > > > > > > > On Mon, Mar 30, 2015 at 9:52 AM Eric Christopher <echristo at gmail.com> > wrote: > > From PR18808 I said a few things and that I was going to redirect to the > mailing list for further discussion. So here we are, go. > > > > 1) Whether or not to allow changing of target-cpu/target-feature/triple > at link time code generation. > > > > - Not convinced here of the facility to do so. Could just recompile the > individual bitcode files to get what you want, but there are some users > that are trying to ship bitcode (as crazy as that sounds). > > IMO, it's cleanest of the target-cpu/target-feature/etc. are set at > compile time. That's where users are accustomed to specifying codegen > options already, and besides: the frontend needs to know the backend in > order to conform to the ABI, set macros, emit calls to target-specific > intrinsics, etc. > > I'll send a review of r233227 in a moment to that effect ;). > > > 2) How to pass other sorts of options to the backend for code generation > > > > - -ffoo options -fno-foo options. I.e. -fno-inline, etc. I think this is > really pretty important from the user POV. It affects things at a more > global level. > > This is easy to solve for -fno-inline in particular: we should just > add a function attribute (`noinline`?) that the inliner should treat > as a synonym for `optnone`. Any functions that come from translation > units compiled with `-fno-inline` get ignored by the inliner; functions > from other translation units participate fully. > >This is pretty terrible as you allude to here, this is a hack for -fno-inline, but it's also not good for "I'd like to inline at the individual translation unit compile time, but not at the LTO time."> But in terms of setting up the LTO pass pipeline, some level of user > customization makes sense. I'm not really sure how much is useful. We > have a start at that with Peter's recent commits to add -O0/-O1/-O2 > (not that anyone thought too carefully about what's happening at those > optimization levels). >Yeah, I commented pretty heavily on that thread if you'll remember. It's a hackish workaround for the moment, but will work in the short term.> > > 3) The llvm developer debugging story > > > > - It's useful for llvm developers to be able to more accurately debug a > set of IR using bisection or being able to turn off code generation > options. Should this be done at the command level (i.e. infrastructure that > clang and llc etc could even share), or should it be done at an llvm IR > rewriting level? Don't know. I kind of want a rewriter, but I'm not wedded > to any particular answer. > > I think some sort of rewriter makes sense. > > Long-term I'd still like to encode whether an option is overridable in > a sane way (via a default attribute sets or something), but I haven't > had time yet to go back to my original proposal and refine it :(. > >If I had any bright ideas I'd have said something. :)> > > > That said I was actually envisioning something like: > > > > clang -emit-llvm foo.c -o foo.bc > > ... > > > > clang -O3 -flto all.bc -arch x86_64h -o haswell_slice > > clang -O3 -flto all.bc -arch x86_64 -o x86_64_slice > > > > for the same set of bitcode files. But given the front end language > restrictions on doing anything actually interesting there it's not too much > of a constraint. > > Many of the differences between architectures CPUs affect preprocesser > definitions, right? Link-time is too late for the frontend to emit > Haswell-specific intrinsics, for example. > >*nod* But useful for making code generation decisions (vectorization etc).> That said, it would be cool if this worked. > >Yep, which leads us to:> > Another usage is the (admittedly one I don't think we want to support) > halide one that I discovered this week: > > > > clang foo.c -emit-llvm foo.bc > > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 > > ... > > Whereas this is just insane :0. >Sorta...> > > > > I've since convinced them to use the pnacl sort of thing for more target > independent code generation at the moment. It's a use case that could be > thought about more though - especially as pnacl does the exact same sort of > thing, just with a different triple for actual link time code generation, > it looks more like: > > > > clang -target le64-unknown-unknown -emit-llvm foo.c -o foo.bc > > clang -target aarch64-linux-gnu foo.bc -O3 -o foo.aarch64 > > clang -target x86_64-linux-gnu foo.bc -O3 -o foo.x86_64 >This is probably a bit more sane, i.e. a generic situation. PNaCl has been using this exact use case for quite a while now and, IIUC it's also the basis of the new Khronos proposal. It'd be nice to support this sort of thing in some fashion (i.e. make restrictions), but I think at this point telling them their behavior isn't allowed would be a little mean :) -eric -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150402/5df6d7cf/attachment.html>