[PATCH] LowerDbgDeclare - get rid of redundant dbg.values when an alloca survives optimization
Adrian Prantl
aprantl at apple.com
Sun Mar 30 11:50:23 PDT 2014
Hi Eric and David,
Here is the promised second part, which is actually independent from part 1. This improves the quality of debug info for optimized code a lot, particularly in the case where variables are passed by reference. Also, it solves the mystery of the lost DBG_VALUEs discovered earlier in this thread.
Debug info for optimized code: Support variables that are on the stack and
described by DBG_VALUEs during their lifetime.
Previously, when a variable was at a FrameIndex for any part of its
lifetime, this would shadow all other DBG_VALUEs and only a single
fbreg location would be emitted, which in fact is only valid for a small
range and not the entire lexical scope of the variable. The included
dbg-value-const-byref testcase demonstrates this.
This patch fixes this by
Local.cpp
- emitting dbg.value intrinsics for allocas that are passed by reference
- dropping all dbg.declares (they are now fully lowered to dbg.values)
SelectionDAG
- renamed constructors for SDDbgValue for better readability.
- fix UserValue::match() to handle indirect values correctly
- not inserting an MMI table entries for dbg.values that describe allocas.
- lowering dbg.values that describe allocas into *indirect* DBG_VALUEs.
CodeGenPrepare
- leaving dbg.values for an alloca were they are (see comment)
DwarfDebug
- drive-by fix for Merge() not handling constant values correctly, testcase
included.
Other
- regenerated/updated instcombine-intrinsics testcase and included source
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cheers,
adrian
On Mar 27, 2014, at 1:54 PM, Adrian Prantl <aprantl at apple.com> wrote:
> Hi Eric, David,
>
> time to revive age-old threads! I looked at this again, and here is the first in a series of patches that aim at improving debugging for optimized code.
>
> This one eliminates redundant variables being emitted when LowerDbgDeclare is hoisting a dbg.declare into a different lexical block as it happens in the included testcase.
>
> Next up: dealing with variables that are located at a FrameIndex and described DBG_VALUEs in different parts of the same function.
>
> -- adrian
>
> <0001-Debug-info-Allow-collectVariableInfoFromMMITable-to-.patch>
>
>
> On Oct 29, 2013, at 1:23 PM, Eric Christopher <echristo at gmail.com> wrote:
>
>>
>>
>>
>> On Tue, Oct 29, 2013 at 1:10 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>
>>
>> On Tue, Oct 29, 2013 at 1:06 PM, Adrian Prantl <aprantl at apple.com> wrote:
>>
>> On Oct 29, 2013, at 11:51, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>>
>>>
>>>
>>> On Tue, Oct 29, 2013 at 11:41 AM, Adrian Prantl <aprantl at apple.com> wrote:
>>> Hi Eric and David,
>>>
>>> On Oct 21, 2013, at 17:07, Eric Christopher <echristo at gmail.com> wrote:
>>> Cool deal. I've got some ideas on how to refactor/rewrite some of our
>>>> loc handling anyhow, but it's going to be a few days before it's
>>>> ready.
>>>
>>>
>>> I had some time to look into this some more and I learned a couple of interesting things:
>>> - The ##DEBUG_VALUE: comments in the asm output are really just comments do not necessarily show up in the DWARF. This is maybe not surprising, but what was surprising to me are the circumstances under which a DBG_VALUE makes it into the location list of a local variable:
>>> - dbg.declare and dbg.value intrinsics are mutually exclusive; the presence of a dbg.declare will shadow any dbg.values that refer to the same variable. As soon as there is an entry in the MMI map (FunctionLowering::set() in FunctionLowering.cpp:134), the dbg.values will be ignored by DwarfDebug::collectVariableInfo (DwarfDebug.cpp::1366).
>>> - Simply disabling that check will result in two separate DIEs for the same variable.
>>>
>>> This means that while my patch was correct based on how DwarfDebug currently behaves, that behavior is broken and I should instead allow for dbg.values and dbg.declares to be coalesced into a single location list.
>>>
>>> Before I dive into this I’d be curious to hear more about your ideas for location handling that you mentioned in your last mail, so we don’t accidentally evolve the code into two different directions.
>>>
>>>
>>> I also found a second, related issue that I’d like to tackle.
>>>
>>> This example:
>>>
>>>> foo(int map)
>>>> {
>>>> lookup(&map);
>>>> if (!verify(map)) { }
>>>> }
>>>>
>>>
>>> results in the following DWARF:
>>>
>>>> 0x00000027: TAG_subprogram [2] *
>>>> AT_name( "foo” )
>>>> AT_low_pc( 0x0000000000000000 )
>>>> AT_high_pc( 0x0000000000000026 )
>>>> ...
>>>> 0x00000046: TAG_formal_parameter [3]
>>>> AT_name( "map” )
>>>> AT_type( {0x00000075} ( int ) )
>>>> AT_location( fbreg -4 )
>>>>
>>>> 0x00000054: TAG_lexical_block [4] *
>>>> AT_low_pc( 0x0000000000000016 )
>>>> AT_high_pc( 0x0000000000000020 )
>>>>
>>>> 0x00000065: TAG_formal_parameter [3]
>>>> AT_name( "map" )
>>>> AT_type( {0x00000075} ( int ) )
>>>> AT_location( fbreg -4 )
>>>
>>> I would like teach DWARFDebug::constructScopeDIE() to recognize if a variable DIE describes a “subset" of a DIE in one of its parent lexical blocks.
>>>
>>> To be clear, this just looks buggy - we shouldn't have two formal parameters here... there is only one formal parameter to 'foo'. If we create two DIEs that's a bug, we shouldn't do that.
>>>
>>
>> That would be a fairly trivial fix in DWARFDebug::createScopeChildrenDIE(). I’m wondering: Should there ever be more than one DIE describing a variable in different lexical scopes or should there be only ever a single DIE with multiple entries in the DW_AT_location list?
>>
>> The latter, I'm fairly sure.
>>
>> Yes.
>>
>> -eric
>
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