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

[Devang Patel]

On May 31, 2011, at 10:36 AM, trash-stuff at gmx.de wrote:

> On 31.05.2011 19:22, Devang Patel wrote:
>> 
>> 
>> On May 30, 2011, at 11:11 AM, trash-stuff at gmx.de wrote:
>> 
>>> 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?
>> 
>> You are looking at the line table produced at -O3, i.e. after aggressive optimizer had opportunities to optimize code. Try -O0 and see if it helps.
> First of all, thanks for your reply!
> 
> I've already checked that at -O0 but it results in the same information.

You mean, the instructions with given line and column number do not match the source code construct at that location ? 

> (The documentation about "Source Level Debugging with LLVM" says "LLVM debug information always provides information to accurately read the source-level state of the program, regardless of which LLVM optimizations have been run, and without any modification to the optimizations themselves." [1])

It means the instructions with given line and column number matches the source code construct at that line/col number. It does not mean that optimizer/code generator will not reorder instruction. It also does not mean that optimizer/code generator will not emit instruction without line number information. It means, if there is a line number information, it is as accurate as possible to map source construct.

LLVM debug information does not prevent many important optimizations from happening (for example inlining, basic block reordering/merging/cleanup, tail duplication, etc), further reducing the amount of the compiler that eventually is "aware" of debugging information.

-
Devang
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