[PATCH] D26769: [IR] Remove the DIExpression field from DIGlobalVariable.
Adrian Prantl via llvm-commits
llvm-commits at lists.llvm.org
Fri Nov 18 12:10:50 PST 2016
> On Nov 18, 2016, at 11:45 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Fri, Nov 18, 2016 at 11:35 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> On Nov 18, 2016, at 11:09 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>
>>
>>
>> On Fri, Nov 18, 2016 at 10:53 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>> On Nov 18, 2016, at 10:36 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>>
>>>
>>>
>>> On Fri, Nov 18, 2016 at 10:23 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>>> On Nov 18, 2016, at 10:10 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>>>
>>>>
>>>>
>>>> On Fri, Nov 18, 2016 at 10:00 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>>>> On Nov 18, 2016, at 9:51 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>>>>
>>>>> Would it be reasonable/useful to add another level of indirection while we're here - that would describe which part of the variable is being described by this location:
>>>>>
>>>>> DIGlobalVariableExpr(value: DW_OP_const 42) -> DIGlobalVariableFragment(offset: 32 bits) -> DIGlobalVariable(name: foo)
>>>>>
>>>>> (just guessing/pretending there's a DW_OP_const for constant values - I forget how it all works, but purely for demonstration purposes)
>>>>>
>>>>> That way code manipulating DIGlobalVariableExprs wouldn't need to have explicit knowledge of which part of the variable is being described here. (I suppose that could end up with us creating a bottom-up expression tree in metadata, which probably isn't ideal (too heavyweight))
>>>>
>>>> The nice thing about the Dwarf expressions being a stack-based post-fix-notated programming language is that it you can compose functions by simply concatenating them. The typical kind of manipulations that we do (for example, adding an "add offset", "dereference" operation) can done by simply putting the new expression in front of the old expression. I don't think it can get much simpler than that :-)
>>>>
>>>> Wouldn't the piece/bit_piece need to go on top level of the expression tree? I don't think you can simply concatenate things on top of it...
>>>
>>> The expression tree of a stack-based postfix language degenerates to a list. The works in the language can consume a different number of stack arguments, but under the assumption that one well-formed DIExpression takes and produces exactly one value (which is true for all non-constant expressions we support), it is safe to concatenate the expressions.
>>>
>>> But what does this mean to the DWARF consumer? All the examples in the spec seem to show DW_OP_(bit_)piece at the top level of the expression (once the expression stack is evaluated, it may have several DW_OP_(bit_)piece remaining on the stack - and they're all read, oldest to newest (bottom to top of the stack) as descriptions of portions of the variable)
>>
>> I don't think I understand what you're saying here. Can you give me a concrete example maybe?
>>
>> Taking the example from the DWARF4 spec:
>>
>> DW_OP_reg3 DW_OP_piece 4 DW_OP_reg10 DW_OP_piece 2
>>
>> If we evaluate the stack, at the end of that evaluation the stack has two elements:
>>
>> DW_OP_piece(2) { DW_OP_reg10 }
>> DW_OP_piece(4) { DW_OP_reg3 }
>>
>> And the DWARF consumer should (according to the description in the spec) read this as describing a variable who's bytes [0, 4) are in reg3 and bytes [4, 6) are in reg10.
>
> Yes that makes sense.
>
> Actually - I'm going to revisit that claim. I think, maybe, DW_OP_piece doesn't appear on the stack after it's evaluated. It has a side effect of producing part of a location description. I don't think it has any value - that another DW_OP could consume. (& I think it's necessary to think of it that way to handle this part of the spec:
>
> "If the location description is empty, the offset doesn’t matter and the DW_OP_bit_piece operation describes a piece consisting of the given number of bits whose values are undefined. "
Yeah I see the DW_OP_(bit)piece operation as more of a side-channel operation that can be used to concatenate several independently evaluated expressions.
>
> So this is how you create holes. If, say, the two bytes of reg10 needed to be the high 2 bytes of the object, not the 3rd nibble:
>
> DW_OP_reg3 DW_OP_piece 4 DW_OP_piece 2 DW_OP_reg10 DW_OP_piece 2
>
> I believe the hole is added that way - and the stack is empty when the second DW_OP_piece is evaluated to create that hole.
Yes, at least for this there exists an example. And I think with a lone DW_OP_bit_piece x 0 it is even possible to create a non-byte-divisible hole.
>
>
>>
>>
>>>
>>>
>>>>
>>>> Oh, also, DW_OP_bit_piece is ordered, judging by the examples in the spec:
>>>>
>>>> DW_OP_reg3 DW_OP_piece 4 DW_OP_reg10 DW_OP_piece 2
>>>> A variable whose first four bytes reside in register 3 and whose next two bytes reside in register 10.
>>>>
>>>> So how do we know which DIGlobalVariableExpr goes first? Or if there are holes between exprs?
>>>>
>>>> (am I making sense? not sure - I can try to explain differently, or perhaps just not understanding)
>>>
>>> Please let me know if I'm missing the point. I think there may be some confusion about how multi-piece values are represented in LLVM Metadata.
>>>
>>> One DIExpression only ever represents one DW_OP_piece (with the DW_OP_piece always being the very last element of the expression). To represent a variable with multiple pieces, we use two DIExpression. For example (extracted from the split-global.ll testcase):
>>>
>>> ; struct { int x,y; } Point;
>>> @point.x = global i32 1, align 4, !dbg !12
>>> @point.y = global i32 2, align 4, !dbg !13
>>>
>>> !0 = distinct !DIGlobalVariable(name: "point", scope: !1, file: !2, line: 1, type: !5, isLocal: false, isDefinition: true)
>>> !12 = !DIGlobalVariableExpression(var: !0, expr: !DIExpression(DW_OP_bit_piece, 0, 32))
>>> !13 = !DIGlobalVariableExpression(var: !0, expr: !DIExpression(DW_OP_bit_piece, 32, 32))
>>>
>>> I think this DWARF is wrong/doesn't make sense. DW_OP_bit_piece's two operands refer to the offset within the previous value on the stack (in this case, the previous/initial value on the stack is implied as the location of the respective globals, @point.x and @point.y). So !13 is invalid DWARF - it says "this piece starts 32 bits into a 32 bit region and is 32 bits long" - which is entirely outside the region.
>>
>> This is not yet DWARF, this is our internal representation in LLVM IR. A (non-constant)* DIExpression has an implicit first element that is the location of whatever is referring to the DIExpression. It could be a GlobalVariable, or a machine value tied to the DIExpression by means of a DBG_VALUE instruction.
>>
>> When we generate a DWARF expression from this we generate the implicit element first, which will turn into a DW_OP_(b)reg/addr/..., followed by the contents of the DIExpression
>>
>> Yep yep - the wrongness I was referring to wasn't in that implicit first argument, etc. Will describe more further down.
>>
>>
>> *) constants are a little more special and for the sake of this discussion I'd like to ignore their them because they complicate our current model. I have a plan for generalizing our expression model to support an arbitrary number of locations (including zero), but let's leave that for a separate thread.
>>
>>>
>>> "The DW_OP_bit_piece operation takes two operands. The first is an unsigned LEB128 number that gives the size in bits of the piece. The second is an unsigned LEB128 number that gives the offset in bits from the location defined by the preceding DWARF location description. "
>>>
>>> The DWARF expression to describe the location of that variable split in two would be:
>>>
>>> DW_OP_addr <addr of @point.x> DW_OP_piece 4 DW_OP_addr <addr of @point.y> DW_OP_piece 4
>>>
>>> Or, if you wanted to use OP_bit_piece, they would each be "DW_OP_bit_piece 32 0".
>>>
>>>
>>>
>>>
>>> The DWARF backend collects all the DIGlobalVariableExpressions and their respective GlobalVariables and translates this into a single location:
>>>
>>> DW_AT_location ([0x0000000000000000], piece 0x00000004, [0x0000000000000004], bit-piece 32 32)
>>> @point.x, !12, @point.y, !13.
>>>
>>> I believe this DWARF describes a location consisting of the range ([0x0000, 0x0004), [0x0008, 0x00012)) which is probably not intended.
>>>
>>
>> Oh, sorry! The human-readable output of Darwin dwarfdump is very misleading here: [0x123] is indeed a shorthand for DW_OP_addr 0x123. See also the corresponding CHECK in in split-global.ll in the review.
>>
>> That wasn't my source of confusion - I assumed that was the representation we were talking about.
>>
>> So, after evaluating this location expression, the stack would contain:
>>
>> DW_OP_bit_piece (32, 32) { DW_OP_addr 0x0004 }
>> DW_OP_piece (4) { DW_OP_addr 0x0000 }
>>
>> And I believe this describes a variable who's bytes [0, 4) start at address 0x0000 and who's bytes [4, 8*) start at address (0x0008**). I don't think this is what you intended.
>>
>> * this is 4 bytes because the first operand to DW_OP_bit_piece is 4 bytes: "The first is an unsigned LEB128 number that gives the size in bits of the piece. "
>> * this is 0x0008 because it's 0x0004 bytes + 4 bytes because the second operand to DW_OP_bit_piece is 4 bytes and: "The second is an unsigned LEB128 number that gives the offset in bits from the location defined by the preceding DWARF location description. "
>>
>>
>>>
>>> The example in 2.6.1.3 <http://2.6.1.3/>:
>>>
>>> DW_OP_reg3 DW_OP_piece 4 DW_OP_reg10 DW_OP_piece 2
>>> A variable whose first four bytes reside in register 3 and whose next two bytes reside in register 10.
>>>
>>> The pieces are considered to fill the variable left to right.
>>
>> Yes.
>>
>>>
>>> & the wording for op_bit_piece says that the offset refers to the location previous in the stack - not about where that fragment goes in the resulting variable.
>>
>> I don't follow. The previous value on the stack is the DW_OP_addr.
>>
>> Yes - so the offset is an offset of that address. It says "look 4 bytes past the address" not "put this 4 bytes into the outer variable description".
>>
>> To break down the language further:
>>
>> "Each composition operation is immediately preceded by a simple location description which describes the location where part of the resultant value is contained." - so the "preceeding location description" is the thing on the stack before the DW_OP_(bit_)piece is evaluated. The DW_OP_addr in your examples and DW_OP_regX in the spec example.
>>
>> "The second is an unsigned LEB128 number that gives the offset in bits from the location defined by the preceding DWARF location description." - this is saying the offset is an offset within that preceeding location description, specifically:
>>
>> "If the location is a memory address, the DW_OP_bit_piece operation describes a sequence of bits relative to the location whose address is on the top of the DWARF stack using the bit numbering and direction conventions that are appropriate to the current language on the target system. "
>
> Huh. Thanks for pointing this out!
>
> Sure thing - glad we were able to figure it out! :)
>
> So far I always thought of DW_OP_(bit_)piece as interchangeable,
>
> Not sure I understand this statement ^ - they are pretty much interchangeable... but, yes, perhaps in a different way.
>
> with DW_OP_piece being a more efficient encoding and the bit_offset being the offset in the source variable rather than in the location.
>
> *nod* sorry for the confuison, wish I coudl've distilled that down better sooner.
>
>
>>
>> So what DW_OP_bit_piece(32, 32) says is "there's 32 bits of stuff, 32 bits from where the previous expression on the stack points to" - or, in your example, it describes the range [0x0008, 0x0012).
>>
>> I believe you were trying to describe a variable that consisted of ([0x0000, 0x0004), [0x0004, 0x0008)) - is that correct?
>>
>> I believe the correct DWARF description for that would be: DW_OP_addr(0x0000) DW_OP_piece(4) DW_OP_addr(0x0004) DW_OP_piece(4)
>
> I think you're right. It's really unfortunate that there are no examples of the correct usage of DW_OP_bit_piece in Appendix D.
>
> Indeed - an example would be dandy (possibly even one of all the different ways DW_OP_bit_piece can behave depending on the value preceeding it on the stack (including the absence of a value))
>
> Looks like I will need to fix both LLVM's and LLDB's interpretation of DW_OP_bit_piece.
>
> Ah! :/ that's going to be tricky, I imagine. (how LLDB will know which way to interpret bit_piece, etc)
I actually have a very narrow time window right now where this transition will be really easy (at least for Darwin). We're about to switch clang to emit DWARF 4 on Darwin (that change for this is already in trunk) and we could use the old interpretation if (Darwin && Dwarf < 4).
>
> With that cleared up I now also get the concern about using DW_OP_bit_piece in DIExpression. Yes, we should at least replace this with an LLVM-specific operator that has the semantics we need (which is an offset into the source variable). I still think it can/should be part of the DIExpression though. Straw man proposal: {LLVM_OP_var_piece, size, ofs}. I'll create a separate review for that.
>
> I still have the original concern that DW_OP_piece must appear at the top of the stack (it doesn't produce a value that can be consumed by any other operation) - and thus any transformation that wants to touch a DIExpression would need to walk through the OP_piece (or LLVM_OP_var_piece) to get to the value to manipulate.
I don't quite understand this concern. Couldn't any transformation simply ignore the LLVM_OP_piece at the end of the DIExpression since it has no effect?
> It breaks the orthogonality of concerns - a transformation that moves a value from a register to memory doesn't need to know how that piece of data is being referred to (whether it's a sub-portion of a variable, etc), just that it moved. (perhaps that's not a good example - what's a good example where we want to manipulate an existing location description instead of rewriting it from scratch?
This is not a good example because we need to keep locations out of the DIExpressions. Locations are always part of the proper IR and DIExpressions are metadata (and thus can't/shouldn't point back to IR). They are only tied together by dbg.value instrinsics or !dbg attachments that are part of the instruction stream.
> I suppose even rewriting it from scratch the OP_piece would get in the way - "rewrite it, but preserve the OP_piece" seems awkward when this entity is just meant to represent this specific location & shouldn't care about whetehr it's a fragment of something larger)
At the moment we only do three transformations:
1. Split one variable (piece) up into multiple pieces. We have API in DIExpression to make this simple.
2. Put a DW_OP_deref in front of an expression.
3. Put a DW_OP_plus DW_OP_constu <ofs> DW_OP_deref in front of an expression.
For these examples I don't think it's particularly awkward. In the future we might want to generate more complex expressions for loop induction variables, but that would likely be creating fresh DIExpressions rather than modifying existing ones.
I would like to revisit this if this becomes and issue in the future. At this point I don't think it's necessary to create something special for pieces (other than perhaps renaming it to avoid semantics confusion).
-- adrian
>
>
> thanks!!
>
> Thank you for sticking with me - I know it was a bit confusing for both of us!
>
> - David
>
>
> -- adrian
>
>>
>> - Dave
>>
>>
>>>
>>> "The DW_OP_bit_piece operation takes two operands. The first is an unsigned LEB128 number that gives the size in bits of the piece. The second is an unsigned LEB128 number that gives the offset in bits from the location defined by the preceding DWARF location description. "
>>
>> What may also contribute to confusion is that DWARF expression operands' operands are different from DWARF expression stack values. Operands (such as the two numbers following DW_OP_bit_piece) are part of the DWARF expression itself and not pushed onto the stack.
>>
>> -- adrian
>>
>>>
>>>
>>> does that make sense?
>>>
>>> -- adrian
>>>
>>>>
>>>
>>>>
>>>> --- adrian
>>>>
>>>>>
>>>>> Perhaps keeping the bit_piece parts out of the DWARF expression bytes and in a separate field of the DIGlobalVariableExpr?
>>>>>
>>>>> DIGlobalVariableExpr(offset: 32, value: DW_OP_const 42)
>>>>>
>>>>> Then at least when a variable moves around (does that happen? maybe? - I suppose something like asan, smooshing everything into a single alloca would move something around without splitting it up) the expression can be updated relatively simply without unpacking through a DW_OP_bit_piece, etc?
>>>>>
>>>>> Maybe this doesn't matter - Ih aven't looked at much of the expression/location handling code.
>>>>>
>>>>> On Fri, Nov 18, 2016 at 9:32 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>>>>> aprantl removed rL LLVM as the repository for this revision.
>>>>> aprantl updated this revision to Diff 78544.
>>>>> aprantl added a comment.
>>>>>
>>>>> Improved type-safety by adding a DIGlobalVarExpr pointer union.
>>>>>
>>>>>
>>>>> https://reviews.llvm.org/D26769 <https://reviews.llvm.org/D26769>
>>>>>
>>>>> Files:
>>>>> include/llvm/Bitcode/LLVMBitCodes.h
>>>>> include/llvm/IR/DIBuilder.h
>>>>> include/llvm/IR/DebugInfo.h
>>>>> include/llvm/IR/DebugInfoMetadata.h
>>>>> include/llvm/IR/GlobalVariable.h
>>>>> include/llvm/IR/Metadata.def
>>>>> lib/Analysis/ModuleDebugInfoPrinter.cpp
>>>>> lib/AsmParser/LLParser.cpp
>>>>> lib/Bitcode/Reader/BitcodeReader.cpp
>>>>> lib/Bitcode/Writer/BitcodeWriter.cpp
>>>>> lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
>>>>> lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
>>>>> lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
>>>>> lib/CodeGen/AsmPrinter/DwarfDebug.cpp
>>>>> lib/IR/AsmWriter.cpp
>>>>> lib/IR/DIBuilder.cpp
>>>>> lib/IR/DebugInfo.cpp
>>>>> lib/IR/DebugInfoMetadata.cpp
>>>>> lib/IR/LLVMContextImpl.h
>>>>> lib/IR/Metadata.cpp
>>>>> lib/IR/Verifier.cpp
>>>>> lib/Transforms/IPO/StripSymbols.cpp
>>>>> lib/Transforms/Instrumentation/AddressSanitizer.cpp
>>>>> test/Assembler/diglobalvariable.ll
>>>>> test/Assembler/diglobalvariableexpression.ll
>>>>> test/Bitcode/DIGlobalVariableExpr.ll
>>>>> test/Bitcode/DIGlobalVariableExpr.ll.bc
>>>>> test/Bitcode/diglobalvariable-3.8.ll
>>>>> test/Bitcode/diglobalvariable-3.8.ll.bc
>>>>> test/DebugInfo/X86/multiple-at-const-val.ll
>>>>> test/DebugInfo/X86/pr12831.ll
>>>>> test/DebugInfo/X86/split-global.ll
>>>>> test/DebugInfo/X86/stack-value-dwarf4.ll
>>>>> test/DebugInfo/X86/unattached-global.ll
>>>>> test/Transforms/GlobalMerge/debug-info.ll
>>>>> unittests/IR/MetadataTest.cpp
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