[llvm-dev] Disabling LLVM_ATTRIBUTE_ALWAYS_INLINE for development?

[Davide Italiano via llvm-dev]

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