> > A big downside of the current LSR algorithm is it's slow. I had initially > hoped that some of the heuristics would protect it better, but the problem > is > more complex than I had expected. I haven't done any measurements, > but it's likely that OSR is faster, which may interest some people > regardless > of how the output compares. >A few years ago, I implemented OSR (with some slight modifications) in GCC, though it was never committed to mainline (it's on a branch somewhere) It was significantly faster than ivopts (which does what you guys are using LSR for), and found more cases than ivopts did, I just never integrated the same target dependent stuff so it never made it into the mainline. Note that as written in the paper, OSR is pretty target independent because of the order of processing. It expects to do it's processing on each SCC as it completes the SCC, so it doesn't gather all the possible things it could do before doing them, and then decide what is best. It is also possible for a "do everything" OSR to completely blow up register pressure if there are a number of conditional iv updates + operations based on them in the loop, since it will have to generate a new variable for each of these cases that will end up live over the entire loop. So i think you may see good things if you took the OSR code and used it as a basis for LSR. There is one thing both the original paper, the original MSCP implementation did (too bad the links to this point to ftp.cs.rice.edu, which no longer works, the web files were a great implementation resource) , and my GCC implementation did, which is LFTR (Linear Function Test Replacement). LFTR after OSR can help reduce register pressure since it enables eliminating the IV's that no longer serve any useful purpose. I don't see any implementation in this code. --Dan -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20101114/09479798/attachment.html>
On 11/13/10 11:05 PM, Daniel Berlin wrote:> > A big downside of the current LSR algorithm is it's slow. I had > initially > hoped that some of the heuristics would protect it better, but the > problem is > more complex than I had expected. I haven't done any measurements, > but it's likely that OSR is faster, which may interest some people > regardless > of how the output compares. > > > There is one thing both the original paper, the original MSCP > implementation did (too bad the links to this point to ftp.cs.rice.edu > <http://ftp.cs.rice.edu>, which no longer works, the web files were a > great implementation resource) , and my GCC implementation did, which > is LFTR (Linear Function Test Replacement). LFTR after OSR can help > reduce register pressure since it enables eliminating the IV's that no > longer serve any useful purpose. I don't see any implementation in > this code. > > --DanDan, LFTR (Linear Function Test Replacement) was mentioned in the original paper. I considered including LFTR with OSR, but decided to get OSR to trunk first and then add LFTR (-lftr) as a separate pass later. The LLVM development documentation suggests that new work be committed piecemeal over time. LLVM does have an optimization pass, -instcombine, which will delete unused induction variables. I recommend that -instcombine be run after OSR. It is my understanding that LFTR would attempt to remove induction variables whose only use is to participate in an end-loop-of-loop test condition. thanks for your comments, Brian West -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20101115/a1fb2e9b/attachment.html>
On Mon, Nov 15, 2010 at 10:38 AM, Brian West <bnwest at rice.edu> wrote:> On 11/13/10 11:05 PM, Daniel Berlin wrote: > > A big downside of the current LSR algorithm is it's slow. I had >> initially >> hoped that some of the heuristics would protect it better, but the problem >> is >> more complex than I had expected. I haven't done any measurements, >> but it's likely that OSR is faster, which may interest some people >> regardless >> of how the output compares. >> > > There is one thing both the original paper, the original MSCP > implementation did (too bad the links to this point to ftp.cs.rice.edu, > which no longer works, the web files were a great implementation resource) , > and my GCC implementation did, which is LFTR (Linear Function Test > Replacement). LFTR after OSR can help reduce register pressure since it > enables eliminating the IV's that no longer serve any useful purpose. I > don't see any implementation in this code. > > --Dan > > Dan, > > LFTR (Linear Function Test Replacement) was mentioned in the original > paper. I considered including LFTR with OSR, but decided to get OSR to > trunk first and then add LFTR (-lftr) as a separate pass later. >I'm not sure why you'd add it as a separate pass, it is about 80-150 lines of code, and adding it as as a separate pass requires you to do things like induction variable detection + etc all over again. See http://gcc.gnu.org/ml/gcc-patches/2007-01/msg01035.html, record_edge, apply_lftr_edge, follow_lftr_edge and perform_lftr The LLVM development documentation suggests that new work be committed> piecemeal over time. > > Sure, but that doesn't mean you should commit something that is going> LLVM does have an optimization pass, -instcombine, which will delete unused > induction variables. >LFTR does not delete unused IV's directly, it does reductions to transform the IV into something else. instcombine could only do this if it knew the sequence of reductions we applied to strength reduce the IV in the first place, or if it computed equivalence of ivs itself. Both of these are not cheap operations. I recommend that -instcombine be run after OSR.>In general, there is a careful balance between leaving the IR in a state that requires expensive cleanup, and doing some of that cleanup yourself where it's cheap and easy. If you were to simply fall on the extreme of running cleanup passes after every optimization, your compiler would be much slower.> It is my understanding that LFTR would attempt to remove induction > variables whose only use is to participate in an end-loop-of-loop test > condition. >Well, no, any IV whose only use is a comparison and a linear function of an existing IV. This is mostly end of loop test conditions, but you'd be surprised where else this pops up. Logging or progress tracking, for example, where you do things like if (i % 100 == 0) { printf("."); } etc Anyway, it looks like the consensus so far is that you need to produce some compile time and benchmark numbers showing OSR is worth it as it's own pass as opposed to replacing/augmenting the LSR implementation that exists now with it. --Dan -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20101116/43822996/attachment.html>
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