Debug information under windows

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#1

Hello,

Recently I found binaries produced with LLVM impossible to debug under Windows. This was probably related to the following bug: http://llvm.org/bugs/show_bug.cgi?id=13636

Asm generated from .ll files revealed that some offsets to debug information were incorrect: they were absolute instead of relative to their sections.

Following patch seemed to have repaired the problem, so I'm just leaving it here. Hopefully this will help someone.

Regards,
Daniel

diff -r 547972237a05 -r 6ed9378f04a3 lib/MC/MCAsmInfoCOFF.cpp
--- a/lib/MC/MCAsmInfoCOFF.cpp Wed Oct 10 20:39:45 2012 +0200
+++ b/lib/MC/MCAsmInfoCOFF.cpp Thu Oct 18 12:41:27 2012 +0200
@@ -38,6 +38,10 @@
    HasMicrosoftFastStdCallMangling = true;

    SupportsDataRegions = false;

#2

Can you add some tests?

#3

Hi,

I'm having some trouble applying this patch. Which svn version is it against? Because from what I can tell, the Dwarf... are not defined in this class or any of it's base classes in HEAD.

regards,
Emil

#4

I was manually running generated binaries under gdb and using objdump to analyze their debug symbols. I do not have a script for that.

Basically what needs to be checked is whether or not offsets to .debug_<something> sections (e.g. DW_FORM_strp, DW_AT_ranges) are valid.

W dniu 2012-10-26 22:46, Rafael Espíndola pisze:

#5

W dniu 2012-10-26 16:55, Daniel Kłobuszewski pisze:

Hello,

Recently I found binaries produced with LLVM impossible to debug under
Windows. This was probably related to the following bug:
http://llvm.org/bugs/show_bug.cgi?id=13636

Asm generated from .ll files revealed that some offsets to debug
information were incorrect: they were absolute instead of relative to
their sections.

Following patch seemed to have repaired the problem, so I'm just leaving
it here. Hopefully this will help someone.

Regards,
Daniel

diff -r 547972237a05 -r 6ed9378f04a3 lib/MC/MCAsmInfoCOFF.cpp
--- a/lib/MC/MCAsmInfoCOFF.cpp Wed Oct 10 20:39:45 2012 +0200
+++ b/lib/MC/MCAsmInfoCOFF.cpp Thu Oct 18 12:41:27 2012 +0200
@@ -38,6 +38,10 @@
    HasMicrosoftFastStdCallMangling = true;

    SupportsDataRegions = false;
+
+ DwarfUsesLabelOffsetForRanges = false;
+ DwarfRequiresRelocationForSectionOffset = false;
+ DwarfUsesRelocationsForStringPool = false;
  }

  void MCAsmInfoMicrosoft::anchor() { }

Hello again,

"fix" from my previous message was helpful only for one compilation unit, but was working incorrectly in case when more CUs were linked together.

Since the bug I've linked seems to still be unresolved, I'm sending my solution to it. Basically all that needed to be done was to emit .secrel32 instructions in few places instead of sections offsets in label difference form.

Here is how I've been building executable for tests (under MinGW console):

llc test.bc -filetype=asm -march=x86 -x86-asm-syntax=att -mtriple=i686-pc-mingw32 -disable-fp-elim
llc test_main.bc -filetype=asm -march=x86 -x86-asm-syntax=att -mtriple=i686-pc-mingw32 -disable-fp-elim
gcc -g -c test.s -o test.o
gcc -g -c test_main.s -o test_main.o
gcc -g test_main.o test.o -o test

Someone more familiar with LLVM code than me should probably review my proposed patch. My source revision was 3.1 LLVM release.

Regards,
Daniel

llvm_patch.diff (3.11 KB)

#6

For reference, I have attached a patch I created some time ago to solve this issue. My implementation differs in that I created a new type to represent the section offset instead of special casing DIEDelta. When it was originally committed it broke one of the build bots so it was uncommitted. I didn’t have the resources to investigate the issue so I have been maintaining the patch on my own build.

windebug.patch (7.15 KB)