[LLVMdev] Problem with x86 32-bit debug information ?

[Paul Robinson]

On Wed, Mar 7, 2012 at 6:50 AM, Seb <babslachem at gmail.com> wrote:
> Hi James,
>
> I fully agree with you and understand your statement about -O2.
>
> Now some questions for you:
> Did you try to reproduce experiments described in my previous e-mail ?
> Did you look at debug informations generated for 'n' parameter on x86 32-bit
> & x86 64-bit ?
> I'm working on my own front-end for LLVM and I had difficulties with debug
> information when they are related to x86 32-bits. So far there are two
> options:
> 1) metadata that I generate are incorrect.
> 2) LLVM is not handling in a correct manner those metadata for x86 32-bit
> target.
> I've already posted problem related to metadata that I generate and they are
> in LLVM 2.9 format. I've been adviced to move to most recent format. Before
> starting any move into that direction, I would like to be sure that LLVM
> trunk could solve the problem. Using clang at -O2 -g is giving me some
> indication that it won't solve my problem and that we are failing into
> option (2).
> So to summarize, I would be nice if someone can confirm that debug
> informations generated on this specific case are correct for x86 32-bit and
> that I would have to deal with that.
>
> Thanks
> Best Regards
> Seb
>
> 2012/3/7 James Molloy <james.molloy at arm.com>
>>
>> Hi Seb,
>>
>>
>>
>> I’m going to reiterate – Clang can decide when it wants to optimise away a
>> variable. You asked for that behaviour when you specified –O2. You can’t
>> expect deterministically the same behaviour on both x86 and x86-64 platforms
>> – the procedure call standards are different and different decisions go in
>> to deciding how to optimise.
>>
>>
>>
>> You can’t expect debug information for an optimised build to fully track
>> that of the source because by definition the source is being modified to
>> optimise.
>>
>>
>>
>> Cheers,
>>
>>
>>
>> James
>>
>>
>>
>> From: Seb [mailto:babslachem at gmail.com]
>>
>> Sent: 07 March 2012 13:37
>> To: James Molloy
>> Cc: llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] Problem with x86 32-bit debug information ?
>>
>>
>>
>> Hi James,
>>
>> clang is able to generate correct debug informations for 64-bit target at
>> -O2. My feeling, given some other experiments I've done,  is that debug
>> information generated for x86 32-bit might be broken for parameters as long
>> as they are not 'homed' in the code (local copy to an automatic variable).
>> It seems that when llvm.declare is turned into a llvm.value for parameter
>> there is something incorrect with respect to parameters debug informations
>> that is generated by clang/llvm. I just would like confirmation of this.
>>
>> Thanks for your answer
>> Best Regards
>> Seb
>>
>> 2012/3/7 James Molloy <james.molloy at arm.com>
>>
>> Hi Seb,
>>
>>
>>
>> Clang cannot generate debug information for something that it has
>> optimised away. You should reduce the optimisation level.
>>
>>
>>
>> In general debug information is only really accurate at –O0.
>>
>>
>>
>> Cheers,
>>
>>
>>
>> James
>>
>>
>>
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
>> Behalf Of Seb
>> Sent: 07 March 2012 13:17
>> To: llvmdev at cs.uiuc.edu
>> Subject: [LLVMdev] Problem with x86 32-bit debug information ?
>>
>>
>>
>> Hi all,
>>
>> I'm using trunk version of LLVM/CLANG.
>> When I compile attached files on my 64-bit Ubuntu 10.04 LTS system as
>> follows:
>>
>> clang -O2 -g check.c main.c -o check64
>>
>> When I do gdb check64 and set a breakpoint to the check routine and
>> executes to the breakpoint, I've got:
>>
>> Breakpoint 1, check (result=0x601110, expect=0x601020, n=53) at check.c:7
>> 7    {
>>
>> As you can see I can inspect 'n' value.
>>
>> Now if I compile for x86 32-bit as follows:
>>
>> clang -m32 -O2 -g check.c main.c -o check32
>>
>> When I do gdb check32 and set a breakpoint to the check routine and
>> executes to the breakpoint, I've got:
>>
>> Breakpoint 1, check (result=<value optimized out>,
>>     expect=<value optimized out>, n=0) at check.c:7
>> 7    {
>>
>> As you can see I can NOT inspect 'n' value. Is there a way to inforce even
>> at -O2 clang to generate debug informations so that I can inspect 'n' value
>> ?
>> Or is it a BUG from clang for x86 32-bit ?
>> Thanks for your answers.
>> Best Regards
>> Seb
>>
>>
>
>
>
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I do have to take exception to James Molloy's assessment of the variable "n"
as "optimized away" because the debug info clearly thought it wasn't.  (The
"n=0" shows that the debug info described some kind of location; if "n" was
indeed optimized away, it should have said so.  Either the debug info does
have a bug--giving a location for a nonexistent variable--or something else is
interfering with our expectations.)

It is very easy for people to dismiss problems with optimized-code debugging
with a "well, what did you expect??" kind of attitude.  Actually there are three
broad categories that can apply to any such issue.

(a) The compiler is trying to keep the debug info in sync with the generated
code, but got it wrong.  This is a correctness bug.
(b) The compiler isn't bothering to keep the debug info in sync with the
generated code, even though it reasonably could do so. This is a
quality-of-implementation issue.
(c) Optimization has made the situation too complicated for the debug info to
reasonably keep track of things.  This happens.

There is some slop between the categories, because there are judgement
calls involved in whether something is bad enough to be a bug or is more of
a heuristic that isn't as good as we'd like; equally, there are judgement calls
in what's a "reasonable" degree of effort to keep track of complicated cases.

Blithely tossing every problem into the third category is
inappropriate.  I spent
nontrivial time (on a previous compiler project) tracking down optimized-code
debugging issues, and probably half of the time I could do something easy to
address it.  Sometimes the compiler was attaching the wrong source location
(bug), sometimes it wasn't bothering to keep track at all even though
it would be
easy (quality of implementation).  In a few cases, generating accurate debug
info required extra analysis, and once or twice we went to that extra trouble;
the rest of the time, it didn't seem worthwhile (judgement calls).

Getting down to the specifics of this case, I downloaded the example programs
and tried them as described.  I got the same behavior as Seb described.

As for our friend "n=0", after I hit the breakpoint I tried stepping
once.  At that
point, "print n" showed "53" just as we would want.  While it would be ideal to
see "n=53" at the breakpoint, sometimes debug info and function prologs don't
line up exactly, and stepping sometimes causes things to become more sensible.

So, I think the 32-bit debug info is doing something reasonable, if not exactly
what you would want.

Pogo