Brian Gesiak via llvm-dev
2018-Dec-15 18:32 UTC
[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?
Hello all! I find that using lldb to debug LLVM libraries can be super frustrating, because a lot of LLVM classes, like the constructor for StringRef, are marked LLVM_ATTRIBUTE_ALWAYS_INLINE. So when I attempt to have lldb evaluate an expression that implicitly instantiates a StringRef, I get 'error: Couldn't lookup symbols: __ZN4llvm9StringRefC1EPKc'. As an example, most recently this happened to me when playing around with llvm::AttributeSet, having attached lldb to an opt built with -DCMAKE_BUILD_TYPE="Debug": (lldb) e $AS.hasAttribute("myattr") error: Couldn't lookup symbols: __ZN4llvm9StringRefC1EPKc Despite having built in a "Debug" configuration, LLVM_ATTRIBUTE_ALWAYS_INLINE makes it very difficult to debug LLVM. How do you all deal with or work around this problem? Is there a good way to do so? If not, would anyone object if I sent up a patch to introduce a CMake variable or something to conditionally disable LLVM_ATTRIBUTE_ALWAYS_INLINE? I'd like to be able to turn it off, but I'm not sure if there's some good reason it needs to stay on even for debug builds? - Brian Gesiak
Vedant Kumar via llvm-dev
2018-Dec-15 20:02 UTC
[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?
Hi,> On Dec 15, 2018, at 10:32 AM, Brian Gesiak via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > Hello all! > > I find that using lldb to debug LLVM libraries can be super > frustrating, because a lot of LLVM classes, like the constructor for > StringRef, are marked LLVM_ATTRIBUTE_ALWAYS_INLINE. So when I attempt > to have lldb evaluate an expression that implicitly instantiates a > StringRef, I get 'error: Couldn't lookup symbols: > __ZN4llvm9StringRefC1EPKc'. > > As an example, most recently this happened to me when playing around > with llvm::AttributeSet, having attached lldb to an opt built with > -DCMAKE_BUILD_TYPE="Debug": > > (lldb) e $AS.hasAttribute("myattr") > error: Couldn't lookup symbols: > __ZN4llvm9StringRefC1EPKc > > Despite having built in a "Debug" configuration, > LLVM_ATTRIBUTE_ALWAYS_INLINE makes it very difficult to debug LLVM. > > How do you all deal with or work around this problem?I don't think there are any great workarounds. The data formatters in utils/lldbDataFormatters.py can help a bit.> Is there a good > way to do so? If not, would anyone object if I sent up a patch to > introduce a CMake variable or something to conditionally disable > LLVM_ATTRIBUTE_ALWAYS_INLINE? I'd like to be able to turn it off, but > I'm not sure if there's some good reason it needs to stay on even for > debug builds?IIRC the motivation for using always_inline in debug builds was to make binaries acceptably fast. IMHO this isn't the right tradeoff because it also makes binaries frustratingly hard to debug. Some might object that with clang modules, it should be no trouble for a debugger to evaluate these kinds of expressions. I've CC'd Raphael who can speak to this much better than I can. My understanding is that modules may help, but lldb (and presumably gdb as well?) are not going to learn how to parse templated code out of C++ modules, instantiate them, and JIT-compile them for you really soon. That seems like an ongoing, longer-term project. I'd like to see us move away from using always_inline in core ADT headers to make debugging easier in the near future. A note about slow Debug binaries. In my experience it's really never necessary have a separate Debug build tree. It takes up a slot of time/space and I've never found it useful. It's usually much more convenient to have a single RelAsserts build tree. If you need to debug code in some .cpp file, you can just: $ touch path/to/the/file.cpp $ export CCC_OVERRIDE_OPTIONS="+-g O0 +-UNDEBUG" $ ninja opt/clang/etc The build system will do an incremental recompile of the binary you want to debug, with the *single file* (or set of files) you care about recompiled with asserts, at -O0, with debug info. The resulting binary is pretty much just as fast as a RelAsserts binary and is perfectly debuggable, modulo always_inline functions ;) vedant> > - Brian Gesiak > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Zachary Turner via llvm-dev
2018-Dec-15 20:08 UTC
[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?
On Sat, Dec 15, 2018 at 12:02 PM Vedant Kumar via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi, > > > On Dec 15, 2018, at 10:32 AM, Brian Gesiak via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > > > Hello all! > > > > I find that using lldb to debug LLVM libraries can be super > > frustrating, because a lot of LLVM classes, like the constructor for > > StringRef, are marked LLVM_ATTRIBUTE_ALWAYS_INLINE. So when I attempt > > to have lldb evaluate an expression that implicitly instantiates a > > StringRef, I get 'error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc'. > > > > As an example, most recently this happened to me when playing around > > with llvm::AttributeSet, having attached lldb to an opt built with > > -DCMAKE_BUILD_TYPE="Debug": > > > > (lldb) e $AS.hasAttribute("myattr") > > error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc > > > > Despite having built in a "Debug" configuration, > > LLVM_ATTRIBUTE_ALWAYS_INLINE makes it very difficult to debug LLVM. > > > > How do you all deal with or work around this problem? > > I don't think there are any great workarounds. The data formatters in > utils/lldbDataFormatters.py can help a bit. > > > > Is there a good > > way to do so? If not, would anyone object if I sent up a patch to > > introduce a CMake variable or something to conditionally disable > > LLVM_ATTRIBUTE_ALWAYS_INLINE? I'd like to be able to turn it off, but > > I'm not sure if there's some good reason it needs to stay on even for > > debug builds? > > IIRC the motivation for using always_inline in debug builds was to make > binaries acceptably fast. IMHO this isn't the right tradeoff because it > also makes binaries frustratingly hard to debug. > > Some might object that with clang modules, it should be no trouble for a > debugger to evaluate these kinds of expressions. I've CC'd Raphael who can > speak to this much better than I can. My understanding is that modules may > help, but lldb (and presumably gdb as well?) are not going to learn how to > parse templated code out of C++ modules, instantiate them, and JIT-compile > them for you really soon. That seems like an ongoing, longer-term project. > > I'd like to see us move away from using always_inline in core ADT headers > to make debugging easier in the near future.I just had a random thought. Could we introduce a flag that would cause the compiler to emit an out of line definition even for always inline funcions? We could enable this flag in debug builds.> > A note about slow Debug binaries. In my experience it's really never > necessary have a separate Debug build tree. It takes up a slot of > time/space and I've never found it useful. It's usually much more > convenient to have a single RelAsserts build tree. If you need to debug > code in some .cpp file, you can just: > > $ touch path/to/the/file.cpp > $ export CCC_OVERRIDE_OPTIONS="+-g O0 +-UNDEBUG" > $ ninja opt/clang/etc > > The build system will do an incremental recompile of the binary you want > to debug, with the *single file* (or set of files) you care about > recompiled with asserts, at -O0, with debug info. The resulting binary is > pretty much just as fast as a RelAsserts binary and is perfectly > debuggable, modulo always_inline functions ;)I’m the opposite, I debug so much and across so many translation units that doing that every single time i wanted to debug something different would be prohibitively time consuming. So i find RelWithAsserts unnecessary, and almost exclusively use debug builds for everything. The last time I built release was several months ago.>>-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181215/bc0d3355/attachment.html>
Kristina Brooks via llvm-dev
2018-Dec-15 20:16 UTC
[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?
My understanding of `always_inline` is that the attribute is meant to be used where not inlining a function would cause a (incorrect) change in semantics of the function (anything that uses certain builtins or inline assembly getting values of certain common registers like FP/SP/PC). Could it be worth adding another macro for the attribute that is less ambiguous in terms of whether forced inlining is a case of optimization, to distinguish it from uses (I'm not actually sure if there are any) where it may actually inadvertently change semantics? On Sat, Dec 15, 2018 at 8:02 PM Vedant Kumar via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi, > > > On Dec 15, 2018, at 10:32 AM, Brian Gesiak via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > > > Hello all! > > > > I find that using lldb to debug LLVM libraries can be super > > frustrating, because a lot of LLVM classes, like the constructor for > > StringRef, are marked LLVM_ATTRIBUTE_ALWAYS_INLINE. So when I attempt > > to have lldb evaluate an expression that implicitly instantiates a > > StringRef, I get 'error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc'. > > > > As an example, most recently this happened to me when playing around > > with llvm::AttributeSet, having attached lldb to an opt built with > > -DCMAKE_BUILD_TYPE="Debug": > > > > (lldb) e $AS.hasAttribute("myattr") > > error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc > > > > Despite having built in a "Debug" configuration, > > LLVM_ATTRIBUTE_ALWAYS_INLINE makes it very difficult to debug LLVM. > > > > How do you all deal with or work around this problem? > > I don't think there are any great workarounds. The data formatters in > utils/lldbDataFormatters.py can help a bit. > > > > Is there a good > > way to do so? If not, would anyone object if I sent up a patch to > > introduce a CMake variable or something to conditionally disable > > LLVM_ATTRIBUTE_ALWAYS_INLINE? I'd like to be able to turn it off, but > > I'm not sure if there's some good reason it needs to stay on even for > > debug builds? > > IIRC the motivation for using always_inline in debug builds was to make > binaries acceptably fast. IMHO this isn't the right tradeoff because it > also makes binaries frustratingly hard to debug. > > Some might object that with clang modules, it should be no trouble for a > debugger to evaluate these kinds of expressions. I've CC'd Raphael who can > speak to this much better than I can. My understanding is that modules may > help, but lldb (and presumably gdb as well?) are not going to learn how to > parse templated code out of C++ modules, instantiate them, and JIT-compile > them for you really soon. That seems like an ongoing, longer-term project. > > I'd like to see us move away from using always_inline in core ADT headers > to make debugging easier in the near future. > > A note about slow Debug binaries. In my experience it's really never > necessary have a separate Debug build tree. It takes up a slot of > time/space and I've never found it useful. It's usually much more > convenient to have a single RelAsserts build tree. If you need to debug > code in some .cpp file, you can just: > > $ touch path/to/the/file.cpp > $ export CCC_OVERRIDE_OPTIONS="+-g O0 +-UNDEBUG" > $ ninja opt/clang/etc > > The build system will do an incremental recompile of the binary you want > to debug, with the *single file* (or set of files) you care about > recompiled with asserts, at -O0, with debug info. The resulting binary is > pretty much just as fast as a RelAsserts binary and is perfectly > debuggable, modulo always_inline functions ;) > > vedant > > > > > > - Brian Gesiak > > _______________________________________________ > > 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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181215/7c9d5e55/attachment.html>
Davide Italiano via llvm-dev
2018-Dec-16 02:12 UTC
[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?
On Sat, Dec 15, 2018 at 12:02 PM Vedant Kumar via llvm-dev <llvm-dev at lists.llvm.org> wrote:> > Hi, > > > On Dec 15, 2018, at 10:32 AM, Brian Gesiak via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > > > Hello all! > > > > I find that using lldb to debug LLVM libraries can be super > > frustrating, because a lot of LLVM classes, like the constructor for > > StringRef, are marked LLVM_ATTRIBUTE_ALWAYS_INLINE. So when I attempt > > to have lldb evaluate an expression that implicitly instantiates a > > StringRef, I get 'error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc'. > > > > As an example, most recently this happened to me when playing around > > with llvm::AttributeSet, having attached lldb to an opt built with > > -DCMAKE_BUILD_TYPE="Debug": > > > > (lldb) e $AS.hasAttribute("myattr") > > error: Couldn't lookup symbols: > > __ZN4llvm9StringRefC1EPKc > > > > Despite having built in a "Debug" configuration, > > LLVM_ATTRIBUTE_ALWAYS_INLINE makes it very difficult to debug LLVM. > > > > How do you all deal with or work around this problem? > > I don't think there are any great workarounds. The data formatters in utils/lldbDataFormatters.py can help a bit. > > > > Is there a good > > way to do so? If not, would anyone object if I sent up a patch to > > introduce a CMake variable or something to conditionally disable > > LLVM_ATTRIBUTE_ALWAYS_INLINE? I'd like to be able to turn it off, but > > I'm not sure if there's some good reason it needs to stay on even for > > debug builds? > > IIRC the motivation for using always_inline in debug builds was to make binaries acceptably fast. IMHO this isn't the right tradeoff because it also makes binaries frustratingly hard to debug. > > Some might object that with clang modules, it should be no trouble for a debugger to evaluate these kinds of expressions. I've CC'd Raphael who can speak to this much better than I can. My understanding is that modules may help, but lldb (and presumably gdb as well?) are not going to learn how to parse templated code out of C++ modules, instantiate them, and JIT-compile them for you really soon. That seems like an ongoing, longer-term project. > > I'd like to see us move away from using always_inline in core ADT headers to make debugging easier in the near future. >+1 to everything you said. I happened to remember this wasn't always the case for StringRef, so I did some archeology and found: [llvm] r247253 - [ADT] Apply a large hammer to StringRef functions: attribute always_inline. I'm afraid I missed the commit at the time but there are few remarks I would like to make: 1) The motivation for this change, at least from what I understand, is trying to optimize `check-llvm`. As somebody who spends a fair amount of time tapping fingers while waiting for the tests to run, I do believe this is a laudable goal, but I'm uncertain of the results. In fact, it seems it only saves 1 second (according to the commit message). 2) I can't speak for anybody else, but I'm not entirely sure a 48-cores box is a common setup (I might be completely wrong on this one, but I have [and had] access to much more modest hardware). In other words, I'm not entirely sure the setup that drove the change is representative of the "typical" llvm developer (for some definition of "typical"). 3) I think the tradeoff here is just not great. I personally devote much more time debugging llvm (and related subprojects) than running the testsuite. YMMV. Chandler, is there anything that can be done here to make the experience less frustrating (i.e. revising the decision or reverting)? The original commit pointed out this was a workaround, but the status quo is that it makes debugging llvm itself much harder. Of course, happy to help. -- Davide
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