So what is the status about LiveVariables. Is there a plan to remove it? After searching in old lvm-dev-mails it is mentioned that LiveVariable still exists due to one pass needing it. And a comment in TargetPassConfig.cpp indicates that the pass in question is TwoAddressInstruction: // FIXME: Once TwoAddressInstruction pass no longer uses kill flags, // LiveVariables can be removed completely, and LiveIntervals can be directly // computed. (We still either need to regenerate kill flags after regalloc, or // preferably fix the scavenger to not depend on them). When looking in TwoAddressInstructionPass.cpp it looks like the pass only use/update LV when there is no LIS. And there seems to be an experimental -early-live-intervals option to calculcate LIS before TwoAddressInstructionPass. I also noticed that the -early-live-intervals option was added back in 2012. So I assume that the plans to replace LiveVariables by LiveIntervals in TwoAddressInstructionPass got some history. Has there been any evaluation (lately) from using -early-live-intervals? I got a feeling that the SSA deconstruction passes are overly complicated right now, since for example PHIElimination tries to update both LV and LIS conditionally depending on if the analyses are available or not. But there are no test cases that set -early-live-intervals afaict, so there is a bulk of code that seems to not be regression tested related to LIS in PHIElimination and TwoAddressInstructionPass. Given that no one is still actively working on removing LiveVariables; Maybe it is better to remove -early-live-intervals and the not so well tested code regarding LIS in PHIElimination/TwoAddressInstructionPass. And then we could say that LiveVariables should be used during SSA deconstruction (instead of saying that it is deprecated). /Björn> -----Original Message----- > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Matthias > Braun via llvm-dev > Sent: den 20 september 2018 22:23 > To: Jason Ott <jott002 at ucr.edu> > Cc: llvm-dev at lists.llvm.org > Subject: Re: [llvm-dev] Liveness Analysis > > Assuming you talk about the machine representation: > - The register allocators use the LiveIntervals analysis (there is also > LiveVariables but that is deprecated, please don't use it). > - When register allocation is done we keep the block live-in lists around > and you can use LivePhysReg or LiveRegUnits to recompute the liveness > information in the middle of a basic block when necessary. > - The RDF framework in lib/Target/Hexagon performs a dataflow analysis > post-regalloc to determine values in physical registers. > > - Matthias > > > On Sep 20, 2018, at 11:31 AM, Jason Ott via llvm-dev <llvm- > dev at lists.llvm.org> wrote: > > > > Where is the liveness analysis pass? I have been looking for days but > cannot find anything. > > > > I just want to know transfer variables in and out of basic blocks, that > are calculated using simple data flow equations. > > > > Thanks! > > _______________________________________________ > > LLVM Developers mailing list > > llvm-dev at lists.llvm.org > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> On Sep 26, 2018, at 6:01 AM, Björn Pettersson A <bjorn.a.pettersson at ericsson.com> wrote: > > So what is the status about LiveVariables. Is there a plan to remove it? > > After searching in old lvm-dev-mails it is mentioned that LiveVariable > still exists due to one pass needing it. And a comment in > TargetPassConfig.cpp indicates that the pass in question is > TwoAddressInstruction: > > // FIXME: Once TwoAddressInstruction pass no longer uses kill flags, > // LiveVariables can be removed completely, and LiveIntervals can be directly > // computed. (We still either need to regenerate kill flags after regalloc, or > // preferably fix the scavenger to not depend on them).Yes as far as I know this is the last pass. PHIElimination uses it too today, but it supports both and actually works, while TwoAddress AFAIK is broken with LiveIntervals mostly because of target callbacks I think. I believe the talk about kill flags in the comment to not be true anymore (we regenerate the kill flags anyway in virtregrewrite which comes before the scavenger and I switch the scavenger to not rely on them anymore in nearly all cases).> > > When looking in TwoAddressInstructionPass.cpp it looks like the pass only > use/update LV when there is no LIS. And there seems to be an experimental > -early-live-intervals option to calculcate LIS before > TwoAddressInstructionPass. > > I also noticed that the -early-live-intervals option was added back in 2012. > So I assume that the plans to replace LiveVariables by LiveIntervals > in TwoAddressInstructionPass got some history. > > Has there been any evaluation (lately) from using -early-live-intervals?Unfortunately the work wasn’t finished in 2012 and As far as I know noone else really attempted it seriously since then.> > > I got a feeling that the SSA deconstruction passes are overly complicated > right now, since for example PHIElimination tries to update both LV and > LIS conditionally depending on if the analyses are available or not. > But there are no test cases that set -early-live-intervals afaict, so > there is a bulk of code that seems to not be regression tested related > to LIS in PHIElimination and TwoAddressInstructionPass. > > Given that no one is still actively working on removing LiveVariables; > Maybe it is better to remove -early-live-intervals and the not > so well tested code regarding LIS in PHIElimination/TwoAddressInstructionPass. > And then we could say that LiveVariables should be used during SSA > deconstruction (instead of saying that it is deprecated).Are you planning bigger changes to these passes? I feel like removing the LIS code is admitting defeat and cementing the worse system... On the other hand if it becomes in the way of making changes we could discuss dropping it... - Matthias> > /Björn > > >> -----Original Message----- >> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Matthias >> Braun via llvm-dev >> Sent: den 20 september 2018 22:23 >> To: Jason Ott <jott002 at ucr.edu> >> Cc: llvm-dev at lists.llvm.org >> Subject: Re: [llvm-dev] Liveness Analysis >> >> Assuming you talk about the machine representation: >> - The register allocators use the LiveIntervals analysis (there is also >> LiveVariables but that is deprecated, please don't use it). >> - When register allocation is done we keep the block live-in lists around >> and you can use LivePhysReg or LiveRegUnits to recompute the liveness >> information in the middle of a basic block when necessary. >> - The RDF framework in lib/Target/Hexagon performs a dataflow analysis >> post-regalloc to determine values in physical registers. >> >> - Matthias >> >>> On Sep 20, 2018, at 11:31 AM, Jason Ott via llvm-dev <llvm- >> dev at lists.llvm.org> wrote: >>> >>> Where is the liveness analysis pass? I have been looking for days but >> cannot find anything. >>> >>> I just want to know transfer variables in and out of basic blocks, that >> are calculated using simple data flow equations. >>> >>> Thanks! >>> _______________________________________________ >>> LLVM Developers mailing list >>> llvm-dev at lists.llvm.org >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> -----Original Message----- > From: Matthias Braun <matze at braunis.de> > Sent: den 26 september 2018 19:25 > To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com> > Cc: Jason Ott <jott002 at ucr.edu>; llvm-dev at lists.llvm.org > Subject: Re: [llvm-dev] Liveness Analysis > > > > > On Sep 26, 2018, at 6:01 AM, Björn Pettersson A > <bjorn.a.pettersson at ericsson.com> wrote: > > > > So what is the status about LiveVariables. Is there a plan to remove it? > > > > After searching in old lvm-dev-mails it is mentioned that LiveVariable > > still exists due to one pass needing it. And a comment in > > TargetPassConfig.cpp indicates that the pass in question is > > TwoAddressInstruction: > > > > // FIXME: Once TwoAddressInstruction pass no longer uses kill flags, > > // LiveVariables can be removed completely, and LiveIntervals can be > directly > > // computed. (We still either need to regenerate kill flags after > regalloc, or > > // preferably fix the scavenger to not depend on them). > Yes as far as I know this is the last pass. PHIElimination uses it too > today, but it supports both and actually works, while TwoAddress AFAIK is > broken with LiveIntervals mostly because of target callbacks I think. > I believe the talk about kill flags in the comment to not be true anymore > (we regenerate the kill flags anyway in virtregrewrite which comes before > the scavenger and I switch the scavenger to not rely on them anymore in > nearly all cases).Looks like TwoAddressInstructionPass::rescheduleKillAboveMI is the primary user of liveness info in TwoAddressInstructionPass. Otherwise it mostly seems to be about keeping the LiveVariables up-to-date while doing transformations (unless there is some target that takes decisions in the convertToThreeAddress hook). It could be interesting to analyse how important the rescheduleKillAboveMI transformation is nowadays. A more brutal solution (and really admitting defeat) regarding liveness during SSA deconstruction would be to remove the use of both LV and LIS in TwoAddressInstructionPass, and spend the time on implementing other tranformations to compensate for the loss.> > > > > > > When looking in TwoAddressInstructionPass.cpp it looks like the pass > only > > use/update LV when there is no LIS. And there seems to be an > experimental > > -early-live-intervals option to calculcate LIS before > > TwoAddressInstructionPass. > > > > I also noticed that the -early-live-intervals option was added back in > 2012. > > So I assume that the plans to replace LiveVariables by LiveIntervals > > in TwoAddressInstructionPass got some history. > > > > Has there been any evaluation (lately) from using -early-live-intervals? > Unfortunately the work wasn’t finished in 2012 and As far as I know noone > else really attempted it seriously since then. > > > > > > > I got a feeling that the SSA deconstruction passes are overly > complicated > > right now, since for example PHIElimination tries to update both LV and > > LIS conditionally depending on if the analyses are available or not. > > But there are no test cases that set -early-live-intervals afaict, so > > there is a bulk of code that seems to not be regression tested related > > to LIS in PHIElimination and TwoAddressInstructionPass. > > > > Given that no one is still actively working on removing LiveVariables; > > Maybe it is better to remove -early-live-intervals and the not > > so well tested code regarding LIS in > PHIElimination/TwoAddressInstructionPass. > > And then we could say that LiveVariables should be used during SSA > > deconstruction (instead of saying that it is deprecated). > > Are you planning bigger changes to these passes? I feel like removing the > LIS code is admitting defeat and cementing the worse system... On the > other hand if it becomes in the way of making changes we could discuss > dropping it...Not really planning any bigger changes, but... Lately I've been trouble shooting PHIElimination and ended up writing this PR https://bugs.llvm.org/show_bug.cgi?id=39085 A fix might need to verify that it updates both LiveVariable and LiveIntervals correctly. I've also proposed some improvement/fixes to PHIElimination here: https://reviews.llvm.org/D52558 and here https://reviews.llvm.org/D52553 Those patches use the "deprecated" -livevars pass in the regression tests. Unfortunate if LiveVariables is to be removed (but if there has been no progress for the last six years, the maybe my new tests will work for another six years at least...) I also stumbled upon the LIS related code when looking at https://reviews.llvm.org/D36224 That patch should not have any impact on LV/LIS afaict. But the code related to LIS is there, and it took me some time to understand that it is an "untested code path" unless I use -early-live-intervals. - Björn> > - Matthias > > > > > /Björn > >