Pieb, Wolfgang via llvm-dev
2015-Sep-21 18:16 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
Hello! At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its liverange to the end of the function. So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler, is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away. At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value. The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level. Code emission would simply ignore the new opcode. Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection throughout the entire function body. One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be eventually support for a -Og mode (optimize for debugging), that mode could enable it. Any comments or alternative ideas are appreciated. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150921/c4e79331/attachment.html>
Adrian Prantl via llvm-dev
2015-Sep-21 23:26 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
> On Sep 21, 2015, at 11:16 AM, Pieb, Wolfgang via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > Hello! > > At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during > debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its > liverange to the end of the function. > > So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler, > is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away. > > At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are > proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value. > The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level. > Code emission would simply ignore the new opcode. > > Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option > (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection > throughout the entire function body.> > One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be > eventually support for a -Og mode (optimize for debugging), that mode could enable it. > > Any comments or alternative ideas are appreciated.The clang frontend creates all variables in stack slots and relies on LLVM to lower them into registers if applicable. Would the new intrinsic refer to the stack slot, would it describe a load of the value? What happens if the function is inlined, will the new intrinsic prevent other optimizations? — adrian> _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Joseph Tremoulet via llvm-dev
2015-Sep-22 01:32 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
We faced a similar issue in LLILC, where our runtime requires us to report certain GC pointers as live (and have their values preserved) throughout certain methods. We considered some sort of fake_use construct, but we need these things to be live even in code that doesn't reach return (e.g. paths that end in a noreturn call and `unreachable`), and weren't keen on adding them to all such paths (and making sure optimizations can't expose new such paths as they discover that code is unreachable), nor did we put much thought into how that might interfere with tailcall generation/modeling. So, at least for now, we're calling the @llvm.localescape intrinsic on these addresses in the function entry block, with the expectation that doing so will make them appear live wherever we need them to be (which specifically is GC safe-points, which for us all involve calls to external code). Your situation sounds different both in that you want this for debugging whereas we need it for correctness (so maybe the paths that lead to unreachable or infinite loops aren't as interesting for you) and in that you may care about extending liveness past the last call (unlike we who just need to cover the GC safe-points), but FYI that's what we've done in a somewhat similar situation. We'd be happy to have something to use that's explicitly for lifetime extension, so long as it covers the paths which don't reach return. Thanks -Joseph From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Pieb, Wolfgang via llvm-dev Sent: Monday, September 21, 2015 2:17 PM To: llvm-dev at lists.llvm.org Subject: [llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode Hello! At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its liverange to the end of the function. So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler, is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away. At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value. The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level. Code emission would simply ignore the new opcode. Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection throughout the entire function body. One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be eventually support for a -Og mode (optimize for debugging), that mode could enable it. Any comments or alternative ideas are appreciated. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150922/e767c836/attachment.html>
Smith, Kevin B via llvm-dev
2015-Sep-22 02:12 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
Why extend the live-range? If it isn't already in memory (and for many architectures, it is already in memory), put the this pointer into memory, and change the debug information so that the location-expression of the this parameter is marked to be that memory? That has two nice properties: 1 - If you do this early (in the function IR, not necessarily in the pass ordering), it really only costs a single store, and doesn't otherwise really affect register allocation. Mark the memory itself in such a way that it cannot be deleted. 2 - This is only required for architectures/calling conventions where the this ptr isn't already in memory, and where this extra debugging is required/desired. Kevin Smith From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Pieb, Wolfgang via llvm-dev Sent: Monday, September 21, 2015 11:17 AM To: llvm-dev at lists.llvm.org Subject: [llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode Hello! At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its liverange to the end of the function. So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler, is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away. At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value. The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level. Code emission would simply ignore the new opcode. Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection throughout the entire function body. One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be eventually support for a -Og mode (optimize for debugging), that mode could enable it. Any comments or alternative ideas are appreciated. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150922/1fb1fae4/attachment.html>
Pieb, Wolfgang via llvm-dev
2015-Sep-22 16:51 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
> -----Original Message----- > From: aprantl at apple.com [mailto:aprantl at apple.com] > Sent: Monday, September 21, 2015 4:27 PM > To: Pieb, Wolfgang > Cc: llvm-dev at lists.llvm.org > Subject: Re: [llvm-dev] extending liveness of 'this' pointer via > FAKE_USE opcode > > > > On Sep 21, 2015, at 11:16 AM, Pieb, Wolfgang via llvm-dev <llvm- > dev at lists.llvm.org> wrote: > > > > Hello! > > > > At Sony we've seen some serious customer interest in having the > 'this' > > pointer visible throughout an entire function during debugging. > > However, optimizations may eliminate it after its last use, so we've > been looking for a way to artificially extend its liverange to the end > of the function. > > > > So far, the most compelling way we can think of, and one we have used > > successfully in the past in at least one other compiler, is to create > a 'fake use' of the 'this' pointer at the end of the function, > compelling the rest of the compiler to not optimize it away. > > > > At the moment there doesn't seem to be a good way to create such a > > fake use in LLVM (please enlighten us if you know of one), so we are > proposing to introduce a new intrinsic (e.g. llvm.fake_use), which > would take a single value argument, representing a use of that value. > > The intrinsic would be lowered to a new invariant TargetOpcode (e.g. > FAKE_USE), which serves the same purpose at the MI level. > > Code emission would simply ignore the new opcode. > > > > Frontends could use the intrinsic to extend liveranges of variables > as > > desired. As a first use case, clang would accept a new option (e.g. > > -fkeep-this-ptr) which would cause a fake use of 'this' to be > inserted at the end of a function, making it available for inspection > throughout the entire function body. > > > > > One important note is that since such an option would affect code > > generation, it cannot be automatically enabled by -g. However, should > there be eventually support for a -Og mode (optimize for debugging), > that mode could enable it. > > > > Any comments or alternative ideas are appreciated. > > The clang frontend creates all variables in stack slots and relies on > LLVM to lower them into registers if applicable. Would the new > intrinsic refer to the stack slot, would it describe a load of the > value?The intent is to represent a 'use' as if it were passed to a function as an argument, so a load would precede the invocation of the intrinsic.> > What happens if the function is inlined, will the new intrinsic prevent > other optimizations?Only indirectly as a result of extending its argument's lifetime. Obviously we'd have to make sure inline heuristics are not influenced by the presence of the intrinsic. Register pressure may increase which may influence scheduling etc. but there shouldn’t be any explicit suppression of particular optimizations.> > — adrian > > > _______________________________________________ > > LLVM Developers mailing list > > llvm-dev at lists.llvm.org > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Pieb, Wolfgang via llvm-dev
2015-Sep-22 19:16 UTC
[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode
We thought about putting the 'this' pointer into memory but that would mean that even small member functions would acquire a stack frame (on architectures where leaf routines can get away without one), which may degrade performance considerably. You could apply some heuristics and determine when a store is unnecessary, but inlining may complicate things. A fake_use operation would be inlined like any other instruction. We performed some internal evaluations of location coverage (at -O1 on x86), and about half of the member functions already had 100% location coverage for the this pointer, mostly because they are small. For these methods extending the live range to the end of the function would probably make no difference in code generation anyway. For the longer routines, a fake-use at the end of the function would probably cause a spill early on, similar to the effect of an explicit store. Another consideration is that we'd like to extend the concept to other variables as well. This 'this' pointer Is invariant, but other parameters and locals are generally not, so we believe extending the live range would be a more general solution. -- wolfgang From: Smith, Kevin B [mailto:kevin.b.smith at intel.com] Sent: Monday, September 21, 2015 7:12 PM To: Pieb, Wolfgang Cc: llvm-dev at lists.llvm.org Subject: RE: extending liveness of 'this' pointer via FAKE_USE opcode Why extend the live-range? If it isn't already in memory (and for many architectures, it is already in memory), put the this pointer into memory, and change the debug information so that the location-expression of the this parameter is marked to be that memory? That has two nice properties: 1 - If you do this early (in the function IR, not necessarily in the pass ordering), it really only costs a single store, and doesn't otherwise really affect register allocation. Mark the memory itself in such a way that it cannot be deleted. 2 - This is only required for architectures/calling conventions where the this ptr isn't already in memory, and where this extra debugging is required/desired. Kevin Smith From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Pieb, Wolfgang via llvm-dev Sent: Monday, September 21, 2015 11:17 AM To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org> Subject: [llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode Hello! At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its liverange to the end of the function. So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler, is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away. At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value. The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level. Code emission would simply ignore the new opcode. Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection throughout the entire function body. One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be eventually support for a -Og mode (optimize for debugging), that mode could enable it. Any comments or alternative ideas are appreciated. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150922/0abd5c7c/attachment.html>