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[LiveDebugValues] Emit parameter's entry value
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Authored by djtodoro on Feb 11 2019, 3:50 AM.

Details

Summary

Job of this pass to broadcast DBG_VALUES to the all successive blocks where its preserved location is valid. As a natural way it should emit replacements for clobbered parameters location.

Authors: @asowda, @NikolaPrica, @djtodoro, @ivanbaev

Diff Detail

Event Timeline

djtodoro created this revision.Feb 11 2019, 3:50 AM
djtodoro added a project: debug-info.
djtodoro added a subscriber: petarj.
wolfgangp added inline comments.Feb 11 2019, 6:16 PM
lib/CodeGen/LiveDebugValues.cpp
298

I would prefer either the name "ParameterEntryVals" or "ParamEntyrVals", throughout the file, instead of "ParametrEntryVals".

310

This type is used in several locations. Perhaps we could have a using declaration
for it and use a more compact name?

821

This is invariant in the for-loop, no? Perhaps it should be pulled ahead of it?

945

I think this comment needs perhaps some rephrasing. Something like:
Enter DEBUG_VALUE instructions that track parameters into ParameterEntryVals. When we encounter a variable that we have already entered, we assume that we have found all parameter
entry locations and stop.

947

some spelling errors: wheter -> whether, vraible->variable, it->is

Suggested wording:
We check whether MI is inlined in order to deduce whether the variable that it tracks
comes from a different function. If that is the case we can't track its entry value.

@wolfgangp Thanks a lot for your comments! It has been addressed.

djtodoro updated this revision to Diff 186452.Feb 12 2019, 6:40 AM
djtodoro updated this revision to Diff 186613.Feb 13 2019, 2:45 AM
  • Rename: VariableNotChanged ===> ArgumentNotModified
  • Refactor a test case
dstenb added a subscriber: dstenb.Feb 13 2019, 1:19 PM
dstenb added inline comments.
lib/CodeGen/LiveDebugValues.cpp
435–437

Should this use Expr->isEntryValue()? However, the implementation of that function in D58041 only checks if the first element is the operation:

bool isEntryValue() const {
    return getNumElements() > 0 && getElement(0) == dwarf::DW_OP_GNU_entry_value;
}

Is that correct, or should it look through all elements like is done here? Or is there some reason for why they should be disjunct? If so, could you add a comment here?

NikolaPrica added inline comments.Feb 14 2019, 3:05 AM
lib/CodeGen/LiveDebugValues.cpp
435–437

You are right! It should be isEntryValue()! Thanks!

djtodoro updated this revision to Diff 187190.Feb 17 2019, 10:41 PM
  • Use isEntryValue() from DIExpression
dstenb added inline comments.Feb 18 2019, 5:06 AM
lib/CodeGen/LiveDebugValues.cpp
435–437

Okay, thanks for the clarification. So DW_OP_GNU_entry_value will only be located in the first element then?

djtodoro marked an inline comment as done.Feb 19 2019, 12:14 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
435–437

In the current design, yes.
DIExpression will always have entry value DWARF operand as first element.

djtodoro updated this revision to Diff 195197.Apr 15 2019, 8:50 AM
djtodoro edited the summary of this revision. (Show Details)

-Rebase
-Follow-up with the new approach

aprantl added inline comments.Apr 15 2019, 8:54 PM
include/llvm/IR/DebugInfoMetadata.h
2529 ↗(On Diff #195197)

The more flags we add these bool parameters get less and less safe, since the order isn't typesafe. We should probably think about also using an unsigned Flag parameter that is WithDeref | WithEntryValue.

2600 ↗(On Diff #195197)

In LLVM IR I'd prefer using the DWARF 5 DW_OP_entry_value (Or a new LLVM one if we change the semantics). We can still emit the GNU variant, if requested.

According to the DWARF spec an entry value can appear anywhere in the dwarf expression, right? If we only accept one as the first element, we also need a verifier that enforces this.

lib/CodeGen/AsmPrinter/DwarfExpression.cpp
333 ↗(On Diff #195197)

This should be DW_OP_entry_value if DWARF 5 is requested (or a perhaps GDB tuning)

lib/CodeGen/LiveDebugValues.cpp
446

Would it be possible to split this part into a separate patch?

djtodoro marked 4 inline comments as done.Apr 15 2019, 11:59 PM
djtodoro added inline comments.
include/llvm/IR/DebugInfoMetadata.h
2529 ↗(On Diff #195197)

I agree with this.

2600 ↗(On Diff #195197)

In LLVM IR I'd prefer using the DWARF 5 DW_OP_entry_value (Or a new LLVM one if we change the semantics). We can still emit the GNU variant, if requested.

We will make it.

According to the DWARF spec an entry value can appear anywhere in the dwarf expression, right?

Yes, but for now we support just basic entry values expressions.

If we only accept one as the first element, we also need a verifier that enforces this.

Sure.

lib/CodeGen/AsmPrinter/DwarfExpression.cpp
333 ↗(On Diff #195197)

Sure.

lib/CodeGen/LiveDebugValues.cpp
446

IIUC, splitting introduction and production of entry_values makes sense.

djtodoro updated this revision to Diff 195722.Apr 18 2019, 6:14 AM

-Use DW_OP_entry_value from DWARF 5
-Split up introduction and production of entry values

ormris removed a subscriber: ormris.Apr 18 2019, 9:27 AM
aprantl added inline comments.Apr 22 2019, 10:10 AM
lib/CodeGen/LiveDebugValues.cpp
210–211

Can you convert this into early-exit form?

if (Var != Other.Var)
  return Var < Other.Var;
...
430

The combination of TODO and also makes these sentences confusing to me.

452

factor out VarLocIDs[ID] (.Var.getVar()) for readability?

453
if (! ...)
  continue
466

auto &foo = **DMI;

480

Transfers.push_back({&MI, EntryValDbgMI});

djtodoro marked 5 inline comments as done.Apr 24 2019, 4:42 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
210–211

Sure.

452

Sure.

djtodoro updated this revision to Diff 196431.Apr 24 2019, 5:39 AM

-Addressing comments

aprantl added inline comments.Apr 24 2019, 1:14 PM
lib/CodeGen/LiveDebugValues.cpp
212

Now we're mixing both styles ;-)

if (Var != Other.Var)
  return Var < Other.Var;
if (Other.Kind != Kind)
  return Other.Kind < Kind;
return Loc.Hash < Other.Loc.Hash;
457

I'm having trouble making sense of DMI (debug machine instruction?) is there a better name?

956

Could this be a helper variable or function with a descriptive name that explains what this condition checks for?

djtodoro marked 5 inline comments as done.Apr 25 2019, 8:36 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
212

Sorry. :)

457

We can try.

956

Sure.

djtodoro updated this revision to Diff 196642.Apr 25 2019, 8:40 AM
djtodoro marked 2 inline comments as done.

-Refactor

I cherry-picked the patches on top of r359425, and tried out some examples. Doing that, I encountered a case where entry values are not inserted with this patch.

When compiling the following code:

int global;
int foo(int p, int q, int r) {
  global = p + 1;
  asm __volatile ("" : : : "edi", "esi", "edx");
  return 123;
}

using the following clang invocation:

clang -O1 -g -Xclang -femit-param-entry-values -mllvm -stop-after=livedebugvalues

gives the following MIR:

body:             |
  bb.0.entry:
    liveins: $edi

    DBG_VALUE $edi, $noreg, !15, !DIExpression(), debug-location !18
    DBG_VALUE $edi, $noreg, !15, !DIExpression(), debug-location !18
    DBG_VALUE $esi, $noreg, !16, !DIExpression(), debug-location !19
    DBG_VALUE $edx, $noreg, !17, !DIExpression(), debug-location !20
    renamable $edi = nsw ADD32ri8 killed renamable $edi, 1, implicit-def dead $eflags, debug-location !21
    DBG_VALUE $edi, $noreg, !15, !DIExpression(DW_OP_entry_value, 1), debug-location !18
    MOV32mr $rip, 1, $noreg, @global, $noreg, killed renamable $edi, debug-location !22 :: (store 4 into @global, !tbaa !23)
    INLINEASM &"", 1, 12, implicit-def dead early-clobber $edi, 12, implicit-def dead early-clobber $esi, 12, implicit-def dead early-clobber $edx, 12, implicit-def dead early-clobber $df, 12, implicit-def dead early-clobber $fpsw, 12, implicit-def dead early-clobber $eflags, !28, debug-location !27
    $eax = MOV32ri 123, debug-location !29
    RETQ killed $eax, debug-location !29

As seen, there is an entry value location inserted for p after $edi is clobbered by the ADD32ri8 for p + q, but there are no entry value locations inserted for q and r after the inline assembly that clobbers their respective register. Is this perhaps due to some known limitation, or should this be considered a bug? Or have I overlooked something when testing this?

If I comment out the line that assigns the value to global, then LiveDebugValues emits entry value locations after the inline assembly for all three parameters.

dstenb added a comment.May 6 2019, 8:46 AM

I cherry-picked the patches on top of r359425, and tried out some examples. Doing that, I encountered a case where entry values are not inserted with this patch.

I had a look at this. LiveDebugValues::ExtendRanges() stops adding debug value instructions to ParamEntryVals as soon as it encounters a debug value for a variable that has already been seen, as pointed out by the "Insert DEBUG_VALUE instructions that track parameters into ParamEntryVals." block comment.

In this test case we are left with redundant DEBUG_VALUE instructions for p after LiveDebugVariables:

# *** IR Dump After Virtual Register Rewriter ***:                                                                                               
# Machine code for function foo: NoPHIs, TracksLiveness, NoVRegs, NoGlobalPredicates                                                             
Function Live Ins: $edi                                                                                                                          
                                                                                                                                                 
0B  bb.0.entry:                                                                                                                                  
      liveins: $edi                                                                                                                              
      DBG_VALUE $edi, $noreg, !"p", !DIExpression(), debug-location !18; kommentar.c:2:13 line no:2                                              
      DBG_VALUE $edi, $noreg, !"p", !DIExpression(), debug-location !18; kommentar.c:2:13 line no:2
      DBG_VALUE $esi, $noreg, !"q", !DIExpression(), debug-location !19; kommentar.c:2:20 line no:2                                              
      DBG_VALUE $edx, $noreg, !"r", !DIExpression(), debug-location !20; kommentar.c:2:27 line no:2

so that seems to be why entry values are not emitted for q and r, unless I have misunderstood something.

I don't know what it would take to stop LiveDebugVariables from emitting the redundant debug values in the first place. If it is not trivial, or there might perhaps be other cases where we can land in redundant entries, then perhaps this patch should be able to handle those entries?

@dstenb Thanks a lot for this test case! We should handle this as well.

djtodoro updated this revision to Diff 199220.May 13 2019, 1:50 AM

-Handle more cases within LiveDebugValues::ExtendRanges()
-Add one more test
-Rebase

aprantl added inline comments.May 13 2019, 11:49 AM
lib/CodeGen/LiveDebugValues.cpp
215

FYI, I recently learned a neat trick:

return std::tie(Var, Kind, Hash) < std::tie(Other.Var, Other.Kind, Other.Hash);

IIUC, this is equivalent to the above code?

219

How was 6 derived (I'm asking because a non-power-of 2 always looks very deliberate).

298

If you don't want the vector size to leak into the function, you can use a SmallVectorImpl<MachineInstr *>

431

K -> Kind

598

IfAvailable ... can this return a nullptr?

935

I think the middle condition is redundant? Or do we expect SP to be 0?

952

return std::any_of()

djtodoro marked 6 inline comments as done.May 16 2019, 5:12 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
215

This looks better. Thanks :)

219

It should be 8.

598

Yes, thanks.

djtodoro updated this revision to Diff 199794.May 16 2019, 5:14 AM

-Address suggestions
-Rename ParamEntryVals into DebugEntryVals
-Avoid entry values on a frame reg and fragments (found those cases with better verifier)

aprantl added inline comments.May 16 2019, 3:41 PM
lib/CodeGen/LiveDebugValues.cpp
220

DebugParamSet is fine.

599

if (auto *TPC = getAnalysisIfAvailable<TargetPassConfig>())

djtodoro updated this revision to Diff 200035.May 17 2019, 6:38 AM

-Address suggestions

aprantl added inline comments.May 17 2019, 9:44 AM
lib/CodeGen/LiveDebugValues.cpp
887

Can you land this as an NFC refactoring (no need for a separate phab review?)

968

We need a longer comment here that explains why each of these conditions are necessary and what is being checked for.

969

Why can't fragments be entry values?

aprantl added inline comments.May 17 2019, 9:49 AM
lib/CodeGen/LiveDebugValues.cpp
458

This is going into (low) n^2 territory here. Should we make DebugEntryVals a SmallPtrSet to avoid catastrophic behavior for functions with many arguments?

952

What does "New" mean here?

djtodoro marked 5 inline comments as done.May 20 2019, 6:58 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
458

Mmm, we see.. But we will need another set for caching parameters debug variables, since we consider a parameters DBG_VALUE as a "new" if didn't encountered a debug variable from it.

887

Sure.

952

It means that a parameter is not collected yet inside the DebugEntryVals. We should name it IsParamEntryValueCollected or something like that.

968

Sure. We will add that.

969

Fragments can be entry values as well, but don't support it in this initial set of patches.
We will leave a TODO for that here.

djtodoro updated this revision to Diff 200489.May 21 2019, 6:44 AM

-Use map instead of vector for storing debug entry values
-TODO: Land a separate patch with renaming DMI-->DebugInstr

djtodoro updated this revision to Diff 200957.May 23 2019, 6:28 AM

-Addressing comments
-Avoid indirect dbg_values (in addition to SP and FP locations) in order to avoid memory locations as entry values locations

TODO: Rebase this on top of NFC D62295

djtodoro updated this revision to Diff 201513.May 27 2019, 5:20 AM

-Refactor test cases a bit
-Rebase

aprantl accepted this revision.May 28 2019, 9:29 AM

LGTM with an extra assertion

lib/CodeGen/LiveDebugValues.cpp
168

This looks dangerous.. are entry values somehow implicitly also register values? Iguess besed on the other patches the answer is yes, but that also means that we must assert((!EntryValueKind || RegNo) && "entry values must be register locations")

This revision is now accepted and ready to land.May 28 2019, 9:29 AM
djtodoro updated this revision to Diff 201881.May 29 2019, 5:47 AM

-Addressing suggestions

djtodoro marked an inline comment as done.May 29 2019, 5:52 AM
djtodoro added inline comments.
lib/CodeGen/LiveDebugValues.cpp
168

Yes, you are right! Thanks!

djtodoro updated this revision to Diff 202419.May 31 2019, 5:08 AM

-Rebase tests

dstenb added a comment.EditedJun 5 2019, 5:40 AM

Have you measured what the effect the emission of entry value locations has on the "scope bytes covered" statistics? I guess that it can increase quite a bit (especially later on if we start describing non-parameter variables using parameter entry values)? If so, if the caller(s) do not have call site information, it will still not be able to print the variable. Would it make sense, and would it be possible, to introduce a "scope bytes covered without entry values" statistic, or some other statistic, which helps you assess how large part of the locations that rely on entry values?

@dstenb Thanks for the comment!

Yes, we are constantly measuring debug location statistics. It is possible (and it makes sense) to extend the existing location statistic reporting, within LLVM, by avoiding locations that are entry values (we are actually working on that).

We will share the numbers.

@dstenb Thanks for the comment!

Yes, we are constantly measuring debug location statistics. It is possible (and it makes sense) to extend the existing location statistic reporting, within LLVM, by avoiding locations that are entry values (we are actually working on that).

We will share the numbers.

Okay, thanks!

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@aprantl This still looks OK after the rebasing? :)

A rebase shouldn't affect the spirit of the change :-)

This revision was automatically updated to reflect the committed changes.
djtodoro updated this revision to Diff 207287.Jul 1 2019, 6:29 AM

-Avoid a single location represented as an entry value (+ add a test for that)
-Allow a debug entry value after a first terminator

djtodoro updated this revision to Diff 207290.Jul 1 2019, 6:38 AM

Nit: Use DWARF 5 in a test case.

djtodoro reopened this revision.Jul 1 2019, 6:44 AM

Due to very latest patches we came up with situations when we need to avoid single location represented as an entry value.
Also, having in mind the debug entry value is special case of the 'DBG_VALUE' instruction, we found it is OK in some situations to generate it after a first terminator.
Is this still OK to go ?

This revision is now accepted and ready to land.Jul 1 2019, 6:44 AM
djtodoro updated this revision to Diff 207294.Jul 1 2019, 7:02 AM

-Clarify in the test the entry value coming from a location list

aprantl added inline comments.Jul 1 2019, 10:09 AM
lib/CodeGen/AsmPrinter/DwarfDebug.cpp
1267 ↗(On Diff #207294)

Can you change this comment from describing what the code does (which is fairly obvious), to why it does it?

thanks!

dstenb added inline comments.Jul 2 2019, 1:14 AM
lib/CodeGen/LiveDebugValues.cpp
959

I'm sorry for the late comment, but in the future I guess this could be extended so that entry_values are emitted for locals that are expressed in terms of parameters, as GCC does?

For example:

int foo(int arg) {
  int local = arg + 3;
  clobber();
  return 0;
}
00000039 0000000000000004 000000000000000d (DW_OP_breg5 (rdi): 3; DW_OP_stack_value)
0000004e 000000000000000d 0000000000000018 (DW_OP_GNU_entry_value: (DW_OP_reg5 (rdi)); DW_OP_plus_uconst: 3; DW_OP_stack_value)
00000066 <End of list>

As far as I can tell, we don't tag non-parameter variables as NotModified, but can Clang later on be extended to mark them as such?

Can you change this comment from describing what the code does (which is fairly obvious), to why it does it?

@aprantl Sure. Thanks for the comment!

I'm sorry for the late comment, but in the future I guess this could be extended so that entry_values are emitted for locals that are expressed in terms of parameters, as GCC does?

@dstenb Thanks for your attention and comments. Sure, that is the plan. This is initial set of patches that covers just non-modified function parameters. Even the initial patch within the GCC was handling just this situation. The ways for improving this (using benefits of the debug entry values) are extending the support for modified parameters (if its modification could be described as an expression over its entry value), supporting local variables, etc.

As far as I can tell, we don't tag non-parameter variables as NotModified, but can Clang later on be extended to mark them as such?

Yes, it can be extended.

In addition, I did not forget about sharing the info about the debug location statistic measuring. :)
The 'llvm-dwarfdump' does calculate the debug location statistics, but maybe we could think of reporting it with more information, since the debug location info is such important debug info. Please find a proposal for having a separate tool that will calculate only the debug location statistics on my github (https://github.com/djolertrk/llvm-locstats) and let me know if it can be useful for us. Potentially, we could add more options, functionalities, etc..

dstenb added a comment.Jul 2 2019, 4:09 AM

Can you change this comment from describing what the code does (which is fairly obvious), to why it does it?

@aprantl Sure. Thanks for the comment!

I'm sorry for the late comment, but in the future I guess this could be extended so that entry_values are emitted for locals that are expressed in terms of parameters, as GCC does?

@dstenb Thanks for your attention and comments. Sure, that is the plan. This is initial set of patches that covers just non-modified function parameters. Even the initial patch within the GCC was handling just this situation. The ways for improving this (using benefits of the debug entry values) are extending the support for modified parameters (if its modification could be described as an expression over its entry value), supporting local variables, etc.

As far as I can tell, we don't tag non-parameter variables as NotModified, but can Clang later on be extended to mark them as such?

Yes, it can be extended.

Okay, thanks for the clarification! Would it perhaps make sense to add a small sentence about that in those TODOs?

Would it perhaps make sense to add a small sentence about that in those TODOs?

@dstenb Sure! Thanks!

djtodoro updated this revision to Diff 207543.Jul 2 2019, 6:27 AM

-Add support for entry values that are complex variables address (that avoids entry value not be safe for a single locations)
-Add a TODO for supporting local variables that are expressed in terms of parameters entry values

Is this OK to go ?

dstenb added a comment.Jul 4 2019, 7:07 AM

The 'llvm-dwarfdump' does calculate the debug location statistics, but maybe we could think of reporting it with more information, since the debug location info is such important debug info. Please find a proposal for having a separate tool that will calculate only the debug location statistics on my github (https://github.com/djolertrk/llvm-locstats) and let me know if it can be useful for us. Potentially, we could add more options, functionalities, etc..

That seems like a useful tool!

Did you consider what should be done with llvm-dwarfdump's "scope bytes covered" statistics?

lib/CodeGen/LiveDebugValues.cpp
168

I think this should be Kind != EntryValueKind?

djtodoro marked an inline comment as done.Jul 4 2019, 7:49 AM

The 'llvm-dwarfdump' does calculate the debug location statistics, but maybe we could think of reporting it with more information, since the debug location info is such important debug info. Please find a proposal for having a separate tool that will calculate only the debug location statistics on my github (https://github.com/djolertrk/llvm-locstats) and let me know if it can be useful for us. Potentially, we could add more options, functionalities, etc..

That seems like a useful tool!

Thanks a lot for taking a look!

Did you consider what should be done with llvm-dwarfdump's "scope bytes covered" statistics?

I think that it sounds reasonable to add an additional field there, something like "non-entry-val scope bytes covered".

lib/CodeGen/LiveDebugValues.cpp
168

Oh, nice catch! Thanks!

djtodoro updated this revision to Diff 208042.Jul 4 2019, 7:51 AM

-Fix the assertion

dstenb added a comment.Jul 5 2019, 1:03 AM

Did you consider what should be done with llvm-dwarfdump's "scope bytes covered" statistics?

I think that it sounds reasonable to add an additional field there, something like "non-entry-val scope bytes covered".

Just to clarify, as the emission of entry values is behind a non-driver flag I'm not opposed to landing these patches without changing llvm-dwarfdump, so please don't let me stand in the way. I think that a "non-entry-val scope bytes covered" field would be helpful, but someone with more say in the debug info area than me should decide that.

Did you consider what should be done with llvm-dwarfdump's "scope bytes covered" statistics?

I think that it sounds reasonable to add an additional field there, something like "non-entry-val scope bytes covered".

Just to clarify, as the emission of entry values is behind a non-driver flag I'm not opposed to landing these patches without changing llvm-dwarfdump, so please don't let me stand in the way. I think that a "non-entry-val scope bytes covered" field would be helpful, but someone with more say in the debug info area than me should decide that.

@dstenb We could land this right now, and the patch for the 'llvm-dwarfdump' stats could be a separate patch. Do you agree?
Also, in the thread we could discuss about the new tool I shared.

djtodoro updated this revision to Diff 208321.Jul 8 2019, 12:07 AM

-Nit in a comment

dstenb added a comment.Jul 8 2019, 3:01 AM

@dstenb We could land this right now, and the patch for the 'llvm-dwarfdump' stats could be a separate patch. Do you agree?

Yes, that sounds good to me!

lib/CodeGen/LiveDebugValues.cpp
168

Also, I think we want to move the assertion out of this if statement? We want the check for all types of operands, not just for those that we have seen are register. (Currently, the RegNo check, with the assertion placed inside that if statement should mean that the assert can never fail.)

djtodoro updated this revision to Diff 208365.Jul 8 2019, 4:26 AM

-Remove redundant code
-Fix the debug entry value assertion from the VarLoc constructor

djtodoro marked an inline comment as done.Jul 8 2019, 4:29 AM

Yes, that sounds good to me!

@dstenb Thanks!

lib/CodeGen/LiveDebugValues.cpp
168

Yes, it will never fail.
Also, the code looks much cleaner right now! Thanks!

This revision was automatically updated to reflect the committed changes.