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

[Seb]

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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