llvm dev - Aug 2011 - [LLVMdev] Debugging information problem: code being reordered / debug point jumping around

I'm seeing some undesirable behavior where, when generating debugging
information, sometimes the point in the debugger will jump forwards and
backwards during single stepping, even though the output from our frontend (this
is the ispc compiler), is emitting LLVM instructions with a strictly
forward-moving/increasing set of source locations.  I'm wondering if
we're doing something wrong here, or if this points to a bug somewhere in
LLVM.

I've attached a case that demonstrates this.  Specifically, the instructions
in the LLVM assembly language for the foo() function have the following metadata
items associated with them, in the following order: 5519, 5520, 5522, 5523,
5524, 5525, 5526, 5527, 5528, 5529, 5530, 5531, 5532, 5533, 5534, 5521.  These
correspond to the source file locations: [4,22], [5,5], [5,11], [5,22], [5,23],
[6,14], [6,22], [6,24], [6,26], [8,11], [8,15], [9,9], [9,10], [9,11], [10,12],
[10,12].  ([line, column]).

However, if I run "llc -O0 debug.ll -o debug.s" with top of tree, the
set of source locations indicated in comments jumps from [5,23] to [9,10], and
then back to [6,22].  (And if I single-step through this code in gdb, I see the
corresponding jump around.)

I'd be happy to hear any comments or suggestions!

Thanks,
-matt

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On Jul 31, 2011, at 10:49 PM, Matt Pharr wrote:
> I'm seeing some undesirable behavior where, when generating debugging
information, sometimes the point in the debugger will jump forwards and
backwards during single stepping, even though the output from our frontend (this
is the ispc compiler), is emitting LLVM instructions with a strictly
forward-moving/increasing set of source locations.  I'm wondering if
we're doing something wrong here, or if this points to a bug somewhere in
LLVM.
> 
> I've attached a case that demonstrates this.  Specifically, the
instructions in the LLVM assembly language for the foo() function have the
following metadata items associated with them, in the following order: 5519,
5520, 5522, 5523, 5524, 5525, 5526, 5527, 5528, 5529, 5530, 5531, 5532, 5533,
5534, 5521.  These correspond to the source file locations: [4,22], [5,5],
[5,11], [5,22], [5,23], [6,14], [6,22], [6,24], [6,26], [8,11], [8,15], [9,9],
[9,10], [9,11], [10,12], [10,12].  ([line, column]).
> 
> However, if I run "llc -O0 debug.ll -o debug.s" with top of tree,
the set of source locations indicated in comments jumps from [5,23] to [9,10],
and then back to [6,22].  (And if I single-step through this code in gdb, I see
the corresponding jump around.)
> 
> I'd be happy to hear any comments or suggestions!
What do you see after instruction select ? 
I'll take a look at this. Meanwhile, please file bugzilla report so that
this does not get lost.
-
Devang

On Aug 1, 2011, at 3:27 PM, Devang Patel wrote:
> What do you see after instruction select ? 
Good question--I just checked.  Things are still in the right order going into
the x86 DAG->DAG instruction select pass, but then are out of order coming
out of it.  So that looks like the culprit...
> I'll take a look at this. Meanwhile, please file bugzilla report so
that this does not get lost.
Thanks.  http://llvm.org/bugs/show_bug.cgi?id=10550

-matt

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