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.
-
Devang
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