>From PR18808 I said a few things and that I was going to redirect to themailing 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). 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. 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. -eric -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150330/380fb7a0/attachment.html>
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). > > 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. > > 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. >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. 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 ... 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150330/873251e5/attachment.html>
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). > > 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. >I assume many of these still could/should be function-specific attributes, no? (it seems like there's a logical interpretation of compiling one object file with -fno-inline and another without it - the same as having __attribute__((noinline)) on the functions of the first object and not on those of the second - this may not apply to other options, but perhaps there's a better example of options we should think about here that don't have a per-function interpretation?)> 3) The llvm developer debugging story >Was this the thing Duncan was talking about/proposing on llvmdev a few weeks ago? Something about command line overridable options, etc? (I can try to find the thread if that doesn't sound familiar)> > - 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. > > -eric > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150330/6cc0dd91/attachment.html>
On Mon, Mar 30, 2015 at 10:50 AM David Blaikie <dblaikie 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). >> >> 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. >> > > I assume many of these still could/should be function-specific attributes, > no? (it seems like there's a logical interpretation of compiling one object > file with -fno-inline and another without it - the same as having > __attribute__((noinline)) on the functions of the first object and not on > those of the second - this may not apply to other options, but perhaps > there's a better example of options we should think about here that don't > have a per-function interpretation?) > >Some, yes, all? Dunno. I was thinking mostly of CGSCC or Module passes in general. Ideally it'll just be options that turn on/off things like that though.> 3) The llvm developer debugging story >> > > Was this the thing Duncan was talking about/proposing on llvmdev a few > weeks ago? Something about command line overridable options, etc? (I can > try to find the thread if that doesn't sound familiar) > >Yep. Just widening it and giving the whole set of things its own thread. -eric> >> - 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. >> >> -eric >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >>-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150330/bb7609a2/attachment.html>
> 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