lldb not stopping on my breakpoint

Question

I have built the Clang program from sources with full debugging information (the default build type for Clang IIUC). I check that debug information is available in the executable by noting that there are compile units in the module:

$ lldb /opt/bin/clang++
(lldb) script lldb.target.module['/opt/bin/clang++'].GetNumCompileUnits()
1341

I have instrumented a file in the Clang source tree, lib/Sema/SemaExpr.cpp with a printf statement in the Sema::DiagnoseAssignmentResult method (which is at line 10853 in my copy). I know this method gets called on my test file test.cc, but I can't get the debugger to stop on breakpoints for this method! I have tried setting the breakpoints two ways,

$ lldb /opt/bin/clang++
(lldb) breakpoint set -m DiagnoseAssignmentResult
Breakpoint 2: where = clang++`clang::Sema::DiagnoseAssignmentResult(clang::Sema::AssignConvertType, clang::SourceLocation, clang::QualType, clang::QualType, clang::Expr*, clang::Sema::AssignmentAction, bool*) + 87 at SemaExpr.cpp:10858, address = 0x0000000100ab9947
(lldb) process launch -- ./test.cc
<< message from my printf statement >>
... then clang++ runs to completion and exits, no breakpoint hit ...
(lldb)

I note that lldb did find the correct place in the source code, but didn't stop when it passed through the method. I also tried setting the breakpoint by specifiying the file and line number,

(lldb) breakpoint set -f SemaExpr.cpp -l 10853
Breakpoint 3: where = clang++`clang::Sema::DiagnoseAssignmentResult(clang::Sema::AssignConvertType, clang::SourceLocation, clang::QualType, clang::QualType, clang::Expr*, clang::Sema::AssignmentAction, bool*) + 87 at SemaExpr.cpp:10858, address = 0x0000000100ab9947

Again it "worked", but does not stop. Am I doing something fundamentally wrong here? How can I get the breakpoint to trigger?

Answer 1

You are debugging the clang driver, which is not what actually does the parsing. Instead, clang spawns off another process to do the compilation, then ld if linking is needed, etc. lldb wasn't stopping at your breakpoints because that code was actually getting run by a child process. The confusing bit here is that clang actually uses the same binary for the driver and the parser, so the breakpoints took, just not in the version of clang that was going to invoke that code.

The way to debug the compilation part of clang is first to run it like this:

$ clang++ -### <all your other arguments>

Note the weird -### argument. That tells clang not to do the compilation but to emit the command line that it will run to do the compilation. It will look something like:

/usr/bin/clang" "-cc1" ...

So that is the command line that you want to use in lldb to debug clang as a compiler rather than clang as a compiler driver...