Effectively Debugging Clang

Clang Frontend
#1

Hello,

I am having a more general question related to debugging clang.

I start gdb (in MacOS) as follows;

ggdb clang++
…
…
(ggdb) run -cc1 -std=c++11 -stdlib=libc++ -I/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1 test1.cpp

In most of the time during debugging session I am unable to evaluate and print values of a certain variable. Sometimes my debugger freezes for long time, sometime i get a segmentation faults.

For example if I execute following on a QualType variable I get a segmentation fault.

(gdb) p FnRetType.getAsString()
line 1: 39759 Segmentation fault: 11 ggdb /Users/thejaka/development/crest/pxc/llvm/350/Debug+Asserts/bin/clang++

So my question is, what are the techniques you use to effectively debug clang ? and how can I overcome crashes like above ?

Your feedback is greatly appreciated.

Thank you
Regards
Thejaka Amila

#2

You can use Xcode (by generating Xcode project with CMake) or use lldb
instead of gdb. Kind of weakly-related note; lldb has vim integration
as well.

#3

Probably not helpful: I’m always booting up a Windows machine and use VS if I really need to debug clang…

#4

Hello,

I am having a more general question related to debugging clang.

I start gdb (in MacOS) as follows;

>ggdb clang++
....
....
(ggdb) run -cc1 -std=c++11 -stdlib=libc++
-I/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1
test1.cpp

In most of the time during debugging session I am unable to evaluate and
print values of a certain variable. Sometimes my debugger freezes for long
time, sometime i get a segmentation faults.

For example if I execute following on a QualType variable I get a
segmentation fault.

(gdb) p FnRetType.getAsString()
line 1: 39759 Segmentation fault: 11 ggdb
/Users/thejaka/development/crest/pxc/llvm/350/Debug+Asserts/bin/clang++

So my question is, what are the techniques you use to effectively debug
clang ? and how can I overcome crashes like above ?

I debug Clang and LLVM on Linux using gdb all the time. Which version of
gdb are you using?

#5

Hi David,

I am using 7.7.1.

Thanks
-Thejaka

#6

Hi,

What compiler do you use to build clang? Older versions of GDB (7.2,
7.4) had problems understanding objects (or the debug info) created by
GCC 4.8 and higher (typical symptom: none of the local variables were
visible to the debugger).

GDB 7.7 (on Linux) seems to be better, but I switched to the latest
gdb 7.8 anyway, built from sources.

Csaba

#7

Hi,

What compiler do you use to build clang?

Well.. I am using Clang to build clang. Specific version is as follows;

[~]>clang -v

Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)

Target: x86_64-apple-darwin13.4.0

Thread model: posix
Thanks
-Thejaka

#8

Oh, so it's clang all the way down :slight_smile:

Anyway, you could try building GDB 7.8 from source and using that.
I never used lldb so I don't know how it would behave.

Csaba