Thank you very much for tracking this down. +1 for making UniqueCStringMap speak ConstString -- i think it just makes sense given that it already has "unique" in the name.
ConstString already has a GetStringRef accessor. Also adding a conversion operator may be a good idea, although it probably won't help in all situations (you'll still have to write StringRef(X).drop_front() etc. if you want to do stringref operations on the string) pl On 20 April 2017 at 01:46, Zachary Turner <ztur...@google.com> wrote: > It doesn't entirely defeat the purpose, it's just not *as good* as making > the interfaces take ConstStrings. StringRef already has a lot of safety > and usability improvements over raw char pointers, and those improvements > don't disappear just because you aren't using ConstString. Although I > agree that if you can make it work where the interface only accepts and > returns ConstStrings, and make conversion from ConstString to StringRef > more seamless, that would be an even better improvement. > > On Wed, Apr 19, 2017 at 5:33 PM Scott Smith <scott.sm...@purestorage.com> > wrote: > >> If I just assume the pointers are from ConstString, then doesn't that >> defeat the purpose of making the interface safer? Why not use an actual >> ConstString and provide conversion operators from ConstString to >> StringRef? Seems we should be able to rely on the type system to get us >> safety and performance. >> >> I'll try putting something together tomorrow. >> >> On Wed, Apr 19, 2017 at 4:48 PM, Zachary Turner <ztur...@google.com> >> wrote: >> >>> The change was made to make the interface safer and allow propagation of >>> StringRef through other layers. The previous code was already taking a >>> const char *, and so it was working under the assumption that the const >>> char* passed in came from a ConstString. As such, continuing to make that >>> same assumption seems completely reasonable. >>> >>> So perhaps you can just change the operator to compare the pointers, as >>> was being done before. >>> >>> On Wed, Apr 19, 2017 at 4:24 PM Scott Smith <scott.sm...@purestorage.com> >>> wrote: >>> >>>> It looks like it was this change: >>>> >>>> commit 45fb8d00309586c3f7027f66f9f8a0b56bf1cc4a >>>> Author: Zachary Turner <ztur...@google.com> >>>> Date: Thu Oct 6 21:22:44 2016 +0000 >>>> >>>> Convert UniqueCStringMap to use StringRef. >>>> >>>> git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@283494 >>>> 91177308-0d34-0410-b5e6-96231b3b80d8 >>>> >>>> >>>> I'm guessing it's because the old code assumed const string, which >>>> meant that uniqueness comparisons could be done by simply comparing the >>>> pointer. Now it needs to use an actual string comparison routine. This >>>> code: >>>> >>>> bool operator<(const Entry &rhs) const { return cstring < >>>> rhs.cstring; } >>>> >>>> didn't actually change in the revision, but cstring went from 'const >>>> char *' to 'StringRef'. If you know for sure that all the StringRefs come >>>> from ConstString, then it'd be easy enough to change the comparison, but I >>>> don't know how you guarantee that. >>>> >>>> I assume the change was made to allow proper memory cleanup when the >>>> symbols are discarded? >>>> >>>> On Thu, Apr 13, 2017 at 5:37 AM, Pavel Labath <lab...@google.com> >>>> wrote: >>>> >>>>> Bisecting the performance regression would be extremely valuable. If >>>>> you want to do that, it would be very appreciated. >>>>> >>>>> On 12 April 2017 at 20:39, Scott Smith via lldb-dev < >>>>> lldb-dev@lists.llvm.org> wrote: >>>>> >>>>>> For my app I think it's largely parsing debug symbols tables for >>>>>> shared libraries. My main performance improvement was to increase the >>>>>> parallelism of parsing that information. >>>>>> >>>>>> Funny, gdb/gold has a similar accelerator table (created when you >>>>>> link with -gdb-index). I assume lldb doesn't know how to parse it. >>>>>> >>>>>> I'll work on bisecting the change. >>>>>> >>>>>> On Wed, Apr 12, 2017 at 12:26 PM, Jason Molenda <ja...@molenda.com> >>>>>> wrote: >>>>>> >>>>>>> I don't know exactly when the 3.9 / 4.0 branches were cut, and what >>>>>>> was done between those two points, but in general we don't expect/want >>>>>>> to >>>>>>> see performance regressions like that. I'm more familiar with the perf >>>>>>> characteristics on macos, Linux is different in some important regards, >>>>>>> so >>>>>>> I can only speak in general terms here. >>>>>>> >>>>>>> In your example, you're measuring three things, assuming you have >>>>>>> debug information for MY_PROGRAM. The first is "Do the initial read of >>>>>>> the >>>>>>> main binary and its debug information". The second is "Find all symbol >>>>>>> names 'main'". The third is "Scan a newly loaded solib's symbols" >>>>>>> (assuming you don't have debug information from solibs from /usr/lib >>>>>>> etc). >>>>>>> Technically there's some additional stuff here -- launching the process, >>>>>>> detecting solibs as they're loaded, looking up the symbol context when >>>>>>> we >>>>>>> hit the breakpoint, backtracing a frame or two, etc, but that stuff is >>>>>>> rarely where you'll see perf issues on a local debug session. >>>>>>> >>>>>>> Which of these is likely to be important will depend on your >>>>>>> MY_PROGRAM. If you have a 'int main(){}', it's not going to be dwarf >>>>>>> parsing. If your binary only pulls in three solib's by the time it is >>>>>>> running, it's not going to be new module scanning. A popular place to >>>>>>> spend >>>>>>> startup time is in C++ name demangling if you have a lot of solibs with >>>>>>> C++ >>>>>>> symbols. >>>>>>> >>>>>>> >>>>>>> On Darwin systems, we have a nonstandard accelerator table in our >>>>>>> DWARF emitted by clang that lldb reads. The "apple_types", >>>>>>> "apple_names" >>>>>>> etc tables. So when we need to find a symbol named "main", for Modules >>>>>>> that have a SymbolFile, we can look in the accelerator table. If that >>>>>>> SymbolFile has a 'main', the accelerator table gives us a reference into >>>>>>> the DWARF for the definition, and we can consume the DWARF lazily. We >>>>>>> should never need to do a full scan over the DWARF, that's considered a >>>>>>> failure. >>>>>>> >>>>>>> (in fact, I'm working on a branch of the llvm.org sources from >>>>>>> mid-October and I suspect Darwin lldb is often consuming a LOT more >>>>>>> dwarf >>>>>>> than it should be when I'm debugging, I need to figure out what is >>>>>>> causing >>>>>>> that, it's a big problem.) >>>>>>> >>>>>>> >>>>>>> In general, I've been wanting to add a new "perf counters" >>>>>>> infrastructure & testsuite to lldb, but haven't had time. One thing I >>>>>>> work >>>>>>> on a lot is debugging over a bluetooth connection; it turns out that BT >>>>>>> is >>>>>>> very slow, and any extra packets we send between lldb and debugserver >>>>>>> are >>>>>>> very costly. The communication is so fast over a local host, or over a >>>>>>> usb >>>>>>> cable, that it's easy for regressions to sneak in without anyone >>>>>>> noticing. >>>>>>> So the original idea was hey, we can have something that counts packets >>>>>>> for >>>>>>> distinct operations. Like, this "next" command should take no more >>>>>>> than 40 >>>>>>> packets, that kind of thing. And it could be expanded -- "b main should >>>>>>> fully parse the DWARF for only 1 symbol", or "p *this should only look >>>>>>> up 5 >>>>>>> types", etc. >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> > On Apr 12, 2017, at 11:26 AM, Scott Smith via lldb-dev < >>>>>>> lldb-dev@lists.llvm.org> wrote: >>>>>>> > >>>>>>> > I worked on some performance improvements for lldb 3.9, and was >>>>>>> about to forward port them so I can submit them for inclusion, but I >>>>>>> realized there has been a major performance drop from 3.9 to 4.0. I am >>>>>>> using the official builds on an Ubuntu 16.04 machine with 16 cores / 32 >>>>>>> hyperthreads. >>>>>>> > >>>>>>> > Running: time lldb-4.0 -b -o 'b main' -o 'run' MY_PROGRAM > >>>>>>> /dev/null >>>>>>> > >>>>>>> > With 3.9, I get: >>>>>>> > real 0m31.782s >>>>>>> > user 0m50.024s >>>>>>> > sys 0m4.348s >>>>>>> > >>>>>>> > With 4.0, I get: >>>>>>> > real 0m51.652s >>>>>>> > user 1m19.780s >>>>>>> > sys 0m10.388s >>>>>>> > >>>>>>> > (with my changes + 3.9, I got real down to 4.8 seconds! But I'm >>>>>>> not convinced you'll like all the changes.) >>>>>>> > >>>>>>> > Is this expected? I get roughly the same results when compiling >>>>>>> llvm+lldb from source. >>>>>>> > >>>>>>> > I guess I can spend some time trying to bisect what happened. 5.0 >>>>>>> looks to be another 8% slower. >>>>>>> > >>>>>>> > _______________________________________________ >>>>>>> > lldb-dev mailing list >>>>>>> > lldb-dev@lists.llvm.org >>>>>>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev >>>>>>> >>>>>>> >>>>>> >>>>>> _______________________________________________ >>>>>> lldb-dev mailing list >>>>>> lldb-dev@lists.llvm.org >>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev >>>>>> >>>>>> >>>>> >>>> >>
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