[llvm-commits] [llvm] r59011 - in /llvm/trunk: include/llvm/Transforms/Utils/DbgInfoUtils.h lib/Transforms/Utils/DbgInfoUtils.cpp

[Devang Patel]

On Nov 17, 2008, at 11:19 AM, Chris Lattner wrote:

>
> On Nov 17, 2008, at 9:58 AM, Devang Patel wrote:
>
>>
>> On Nov 15, 2008, at 10:41 PM, Chris Lattner wrote:
>>
>>>
>>> On Nov 10, 2008, at 4:53 PM, Devang Patel wrote:
>>>
>>>> Author: dpatel
>>>> Date: Mon Nov 10 18:53:02 2008
>>>> New Revision: 59011
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=59011&view=rev
>>>> Log:
>>>> Add utility routines to remove dead debug info.
>>>
>>> Hi Devang,
>>>
>>> Instead of making this a "minipass" which is called by other  
>>> passes, why not just make instcombine (and only instcombine) do  
>>> this?  Instcombine runs many times in a typical compilation, and  
>>> it can do this very cheaply (cost proportional to the number of  
>>> debug intrinsics, instead of scanning entire functions even when  
>>> there are none).
>>
>> However we want
>> 	$ opt -adce foo.bc -o foo.optimzed.bc
>> to preserve valid debug info.
>
> How wouldn't it?

Well, this routine will be used by adce and any other pass that  
removes dead instructions. If we just make instcombine responsible for  
this cleanup then we are requiring that instcombine is always  
scheduled after such passes. Is that OK ?
>
>
>
>>>> +    // If a llvm.dbg.stoppoint is placed just before an  
>>>> unconditional
>>>> +    // branch then remove the llvm.dbg.stoppoint intrinsic.
>>>> +    else if (BranchInst *UC = dyn_cast<BranchInst>(Next)) {
>>>> +      if (UC->isUnconditional()
>>>
>>> Why?  This doesn't seem right.
>>
>> If it is a unconditional branch then sooner or later the cfg will  
>> be simplified and this may get in a way for a pass doing simple  
>> clean.
>
> I don't understand.  You're saying that the intrinsic will prevent  
> simplifycfg from doing a transformation?  That seems like a problem  
> that should be fixed in the simplifycfg pass.

Yes, simplifycfg pass should be fixed (if that is the case, I need to  
verify it thoroughly). But many passes do their own little cleanups.  
On the other side, unconditional branches are usually removed by the  
optimizer, which means associated debug info is also gone.
>
>
> -Chris

-
Devang