Re: [Python-Dev] constant folding of -0
On Thu, Mar 10, 2011 at 2:17 AM, Eugene Toder elto...@gmail.com wrote: Indeed, see http://bugs.python.org/issue11244 Yes, I've noticed that too. However, if I'm not missing something, your patches do not address folding of -0. Hmm, it seems that way. Could you post a comment on the tracker issue about that, please? I'm not sure why the original changeset went in, but I agree it looks as though it's no longer necessary. Certainly there should be enough -0.0 versus 0.0 tests around to detect any issues, so if all the tests pass with the extra PyObject_IsTrue check disabled then there probably isn't a problem. Mark ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] constant folding of -0
I've posted a patch. Eugene On Thu, Mar 10, 2011 at 3:30 PM, Mark Dickinson dicki...@gmail.com wrote: On Thu, Mar 10, 2011 at 2:17 AM, Eugene Toder elto...@gmail.com wrote: Indeed, see http://bugs.python.org/issue11244 Yes, I've noticed that too. However, if I'm not missing something, your patches do not address folding of -0. Hmm, it seems that way. Could you post a comment on the tracker issue about that, please? I'm not sure why the original changeset went in, but I agree it looks as though it's no longer necessary. Certainly there should be enough -0.0 versus 0.0 tests around to detect any issues, so if all the tests pass with the extra PyObject_IsTrue check disabled then there probably isn't a problem. Mark ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] constant folding of -0
On Thu, 10 Mar 2011 02:17:34 + (UTC) Eugene Toder elto...@gmail.com wrote: Indeed, see http://bugs.python.org/issue11244 Yes, I've noticed that too. However, if I'm not missing something, your patches do not address folding of -0. Btw, there's an alternative approach to allow recursive constant folding. Instead of keeping a stack of last constants, you can keep a pointer to the start of the last (LOAD_CONSTs + NOPs) run and the number of LOAD_CONSTs in that run (called lastlc in the current version). When you want Nth constant from the end, you start from that pointer and skip lastlc-N constants. You also make a function to get next constant from that point. This approach has worse time complexity for searching in a long run of LOAD_CONSTs, Yes, the stack basically acts as a cache to avoid computing all this again and again. however, there are upsides: - very long runs of constants are rare in real code True, but they might appear in generated code. - it uses less memory and doesn't have arbitrary limits on the size of the run Neither does the latest patch. - it's book-keeping overhead is smaller, so when you don't have long runs of constants (common case, I believe), it should be faster The book-keeping overhead should be quite small really, it's a simple autogrowing array with O(1) access and amortized append time. What's left is the overhead of the initial malloc() (and final free()). - I think it's simpler to implement Feel free to propose an alternate patch, but I'm not sure that it would be significantly simpler (and a stack is a well-known data structure). Also, please present some benchmarks if you do. Regards Antoine. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] constant folding of -0
Well, that was just a though. You're right that long runs of constants can appear, and it's better to avoid pathological behaviour in such cases. Your second path looks good. Eugene On Thu, Mar 10, 2011 at 6:30 PM, Antoine Pitrou solip...@pitrou.net wrote: On Thu, 10 Mar 2011 02:17:34 + (UTC) Eugene Toder elto...@gmail.com wrote: Indeed, see http://bugs.python.org/issue11244 Yes, I've noticed that too. However, if I'm not missing something, your patches do not address folding of -0. Btw, there's an alternative approach to allow recursive constant folding. Instead of keeping a stack of last constants, you can keep a pointer to the start of the last (LOAD_CONSTs + NOPs) run and the number of LOAD_CONSTs in that run (called lastlc in the current version). When you want Nth constant from the end, you start from that pointer and skip lastlc-N constants. You also make a function to get next constant from that point. This approach has worse time complexity for searching in a long run of LOAD_CONSTs, Yes, the stack basically acts as a cache to avoid computing all this again and again. however, there are upsides: - very long runs of constants are rare in real code True, but they might appear in generated code. - it uses less memory and doesn't have arbitrary limits on the size of the run Neither does the latest patch. - it's book-keeping overhead is smaller, so when you don't have long runs of constants (common case, I believe), it should be faster The book-keeping overhead should be quite small really, it's a simple autogrowing array with O(1) access and amortized append time. What's left is the overhead of the initial malloc() (and final free()). - I think it's simpler to implement Feel free to propose an alternate patch, but I'm not sure that it would be significantly simpler (and a stack is a well-known data structure). Also, please present some benchmarks if you do. Regards Antoine. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] constant folding of -0
Hello, I've noticed since version 3.2 python doesn't fold -0: Indeed, see http://bugs.python.org/issue11244 Regards Antoine. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] constant folding of -0
Indeed, see http://bugs.python.org/issue11244 Yes, I've noticed that too. However, if I'm not missing something, your patches do not address folding of -0. Btw, there's an alternative approach to allow recursive constant folding. Instead of keeping a stack of last constants, you can keep a pointer to the start of the last (LOAD_CONSTs + NOPs) run and the number of LOAD_CONSTs in that run (called lastlc in the current version). When you want Nth constant from the end, you start from that pointer and skip lastlc-N constants. You also make a function to get next constant from that point. This approach has worse time complexity for searching in a long run of LOAD_CONSTs, however, there are upsides: - very long runs of constants are rare in real code - it uses less memory and doesn't have arbitrary limits on the size of the run - it's book-keeping overhead is smaller, so when you don't have long runs of constants (common case, I believe), it should be faster - I think it's simpler to implement (There's also an optimization -- if (current_position - start_of_run) / 3 == lastlc there are no NOPs in the run and you can get constants by simple indexing). Eugene ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
