[llvm-dev] Debugging clang with debugger breakpoints ?

Mon Nov 4 13:14:58 PST 2019

On Mon, Nov 4, 2019 at 1:10 PM Joan Lluch <joan.lluch at icloud.com> wrote:

> Hi David,
>
> Thank you for your patience but I still don’t get it:  I don’t see how
> that is a “command”, as it’s just a list of strings that state command
> options.
>
> I know how to use the debugger, this is what I attempt to debug:
>
>  clang --target=msp430 -emit-llvm -c -S -Oz main.c
>

OK, so wherever you have that ^  you should replace it with this \/

clang "-cc1" "-triple" "msp430" "-emit-llvm" "-disable-free"
"-main-file-name" "main.c" "-mrelocation-model" "static" "-mthread-model"
"posix" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases"
"-nostdsysteminc" "-dwarf-column-info" "-debugger-tuning=gdb"
"-target-linker-version" "305" "-momit-leaf-frame-pointer"
"-coverage-notes-file"
"/Users/joan/Documents-Local/Relay/RelayNou/main.gcno" "-resource-dir"
"/Users/joan/LLVM-9/llvm-project/build/Debug/lib/clang/9.0.1"
"-internal-isystem"
"/Users/joan/LLVM-9/llvm-project/build/Debug/bin/../msp430/include" "-Oz"
"-fdebug-compilation-dir" "/Users/joan/Documents-Local/Relay/RelayNou"
"-ferror-limit" "19" "-fmessage-length" "224" "-fobjc-runtime=gcc"
"-fdiagnostics-show-option" "-fcolor-diagnostics" "-vectorize-slp"
"-faddrsig" "-o" "main.ll" "-x" "c" "main.c"

It's just a ton of command line arguments, but it's not fundamentally
different from the command you were already debugging - it's clang with
some command line arguments.

This clang command (the one with the first argument of -cc1) will not
invoke a child process - this is the command that the clang command you ran
would run as its child process. So it's stripping off that layer so you can
debug the underlying command/child process.

>
> The debugger works fine, but only on the main thread. However breakpoints
> do not work with the code that was invoked as a child process with
> posix_spawn
>
> The call to posix_spawn happens in file “Program.inc” on the ‘Execute'
> function. The actual call is this
>
> Err = posix_spawn(&PID, Program.str().c_str(), FileActions,
>                         /*attrp*/ nullptr, const_cast<char **>(Argv),
>                         const_cast<char **>(Envp));
>
> the variable Program.str() contains
> "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/clang” at this point,
> which I got with a breakpoint set at that point.
>
> HOWEVER, any code that is run under that child process is not seen by the
> debugger.
>
> I hope my issue is clearer now.
>
> Thanks anyway.
>
> John
>
>
> On 4 Nov 2019, at 21:43, David Blaikie <dblaikie at gmail.com> wrote:
>
> that command:
>
> "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/clang" "-cc1" "-triple"
> "msp430" "-emit-llvm" "-disable-free" "-main-file-name" "main.c"
> "-mrelocation-model" "static" "-mthread-model" "posix" "-fmath-errno"
> "-masm-verbose" "-mconstructor-aliases" "-nostdsysteminc"
> "-dwarf-column-info" "-debugger-tuning=gdb" "-target-linker-version" "305"
> "-momit-leaf-frame-pointer" "-coverage-notes-file"
> "/Users/joan/Documents-Local/Relay/RelayNou/main.gcno" "-resource-dir"
> "/Users/joan/LLVM-9/llvm-project/build/Debug/lib/clang/9.0.1"
> "-internal-isystem"
> "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/../msp430/include" "-Oz"
> "-fdebug-compilation-dir" "/Users/joan/Documents-Local/Relay/RelayNou"
> "-ferror-limit" "19" "-fmessage-length" "224" "-fobjc-runtime=gcc"
> "-fdiagnostics-show-option" "-fcolor-diagnostics" "-vectorize-slp"
> "-faddrsig" "-o" "main.ll" "-x" "c" "main.c"
>
> Or wherever you had: -emit-llvm -S -g main.c (or something like that)
> You now have:  "-cc1" "-triple" "msp430" "-emit-llvm" "-disable-free"
> "-main-file-name" "main.c" "-mrelocation-model" "static" "-mthread-model"
> "posix" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases"
> "-nostdsysteminc" "-dwarf-column-info" "-debugger-tuning=gdb"
> "-target-linker-version" "305" "-momit-leaf-frame-pointer"
> "-coverage-notes-file"
> "/Users/joan/Documents-Local/Relay/RelayNou/main.gcno" "-resource-dir"
> "/Users/joan/LLVM-9/llvm-project/build/Debug/lib/clang/9.0.1"
> "-internal-isystem"
> "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/../msp430/include" "-Oz"
> "-fdebug-compilation-dir" "/Users/joan/Documents-Local/Relay/RelayNou"
> "-ferror-limit" "19" "-fmessage-length" "224" "-fobjc-runtime=gcc"
> "-fdiagnostics-show-option" "-fcolor-diagnostics" "-vectorize-slp"
> "-faddrsig" "-o" "main.ll" "-x" "c" "main.c"
>
>
> On Mon, Nov 4, 2019 at 12:29 PM Joan Lluch <joan.lluch at icloud.com> wrote:
>
>> Hi David,
>>
>> Thank you for your help. Please, can you elaborate on this?. The command
>> line that I get with -### starts with this:
>>
>> clang version 9.0.1 (https://github.com/llvm/llvm-project.git
>> 6e38ee067b8fa08792f551fb565bbb8ada4864b1)
>> Target: msp430
>> Thread model: posix
>> InstalledDir: /Users/joan/LLVM-9/llvm-project/build/Debug/bin
>>  "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/clang" "-cc1" "-triple"
>> "msp430" "-emit-llvm" "-disable-free" "-main-file-name" "main.c"
>> "-mrelocation-model" "static" "-mthread-model" "posix" "-fmath-errno"
>> "-masm-verbose" "-mconstructor-aliases" "-nostdsysteminc"
>> "-dwarf-column-info" "-debugger-tuning=gdb" "-target-linker-version" "305"
>> "-momit-leaf-frame-pointer" "-coverage-notes-file"
>> "/Users/joan/Documents-Local/Relay/RelayNou/main.gcno" "-resource-dir"
>> "/Users/joan/LLVM-9/llvm-project/build/Debug/lib/clang/9.0.1"
>> "-internal-isystem"
>> "/Users/joan/LLVM-9/llvm-project/build/Debug/bin/../msp430/include" "-Oz"
>> "-fdebug-compilation-dir" "/Users/joan/Documents-Local/Relay/RelayNou"
>> "-ferror-limit" "19" "-fmessage-length" "224" "-fobjc-runtime=gcc"
>> "-fdiagnostics-show-option" "-fcolor-diagnostics" "-vectorize-slp"
>> "-faddrsig" "-o" "main.ll" "-x" "c" "main.c"
>>
>> So what should I set on the debugger?
>>
>> Thanks
>>
>> John
>>
>>
>>
>>
>>
>>
>> On 4 Nov 2019, at 21:03, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> Run clang with -### to get the underlying command line (the one that has
>> the first argument "-cc1") and then run that command under the debugger
>> instead.
>>
>> On Mon, Nov 4, 2019 at 12:02 PM Joan Lluch via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi All,
>>>
>>> I want to understand some parts of ‘clang' by setting debug breakpoints.
>>>
>>> I have successfully done so with “llc” but I found that debugger
>>> breakpoints do not work for ‘clang’
>>>
>>> The apparent cause is that the clang code is run as a child process
>>> which is created in the ‘Execute' function of ‘Program.inc'. The debugger
>>> stops fine at breakpoints set on the main thread, but breakpoints do not
>>> work for any code that is executed as the child process. I am compiling
>>> with Xcode in case this makes a difference.
>>>
>>> Thanks in advance for any help.
>>>
>>> John
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>
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