[LLVMdev] Expressiveness of column numbers in dwarf using clang 3.0?

[trash-stuff at gmx.de]

Hi all,

I am processing DWARF line and column information in (x86 and ARM) 
executables in order to produce a mapping from the machine instructions 
back to the original source code (C/C++). Using the line numbers is 
quite straightforward ("libdwarf" [1] is doing the work me.) But when 
comparing the column numbers (extracted from the DWARF line table) with 
the corresponding source code locations, it becomes clear that they are 
not very "useful".

Consider the following small example (C++):

      1: #include <iostream>
      2: #include <ctime>
      3: #include <cstdlib>
      4: using namespace std;
      5: int main() {
      6:    int j = 0; cin >> j; long sum = (j < 0 ? -5 : 4) + rand();
      7:    for(int i = 0; i < j; i++) { sum += j*j-2; cout << (sum / 2)
    << endl; }
      8:    srand(time(NULL));
      9:    double d = rand() / 10.341; int t = (int)d+j*sum;
    10:    cout << sum << d << t << j;
    11:    return (0);
    12: }

Compiling this with "clang++ Main.cpp -g -O3 -o column" result in the 
following location information within the generated executable:

    $ dwarfdump -l column

    .debug_line: line number info for a single cu
    Source lines (from CU-DIE at .debug_info offset 11):
    <source file>     [line,column] <pc>    //<new stmt or basic block
    .../locale_facets.h:  [868, 2]    0x80488f0  // new statement
                    [...]
    .../Main.cpp: [  8, 2]    0x804896f  // new statement
    .../Main.cpp: [  9,28]    0x8048983  // new statement
    .../ostream:   [165, 9]    0x8048990  // new statement
    .../Main.cpp: [  9,28]    0x80489a0  // new statement
    .../ostream: [209, 9]    0x80489ac  // new statement
    .../Main.cpp: [  9,28]    0x80489b5  // new statement
    .../ostream: [209, 9]    0x80489bb  // new statement
                    [...]
    .../basic_ios.h:      [ 48, 2]    0x8048a23  // new statement // end
    of text sequence

Now, have a look at source code line 9. The extracted debug info above 
says that we've 3 "instruction sets" (beginning at 0x8048983, 0x80489a0 
and 0x80489b5 respectively) which correspond to line 9. But all of them 
are labeled with column number 28! According to my understanding, this 
does not contribute any further information to support my task (= 
mapping assembler code back to the source lines or even to statements 
within a line). Did i miss anything?

Furthermore, I would like to use clang as a cross-compiler for ARM (as 
mentioned above). Is there any "native" or "default" way to achieve 
that? I have already successfully cross-compiled for ARM using 
arm-elf-gcc/g++ and newlib. But, for example, compiling with

    clang++ -march=armv7-a -mfloat-abi=soft -ccc-host-triple arm-elf
    -integrated-as -g Main.cpp -o a.out

results in the following error message:

    Main.cpp:1:10: fatal error: 'iostream' file not found
    #include <iostream>
              ^
    1 error generated.

Obviously, clang++ cannot find the C++ standard header "iostream". As 
far as I know, I have to tell clang++ to use the newlib headers and libs 
but I don't know how to do that...

I would be grateful for any help/hints!

Best regards
   Adrian

PS: I am using LLVM/clang 3.0, SVN rev. 131589.

[1] http://wiki.dwarfstd.org/index.php?title=Libdwarf_And_Dwarfdump

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