[llvm-dev] [Debuginfo] Changing llvm.dbg.value and DBG_VALUE to support multiple location operands

Wed Sep 2 07:01:27 PDT 2020

> I'm not sure this will work as stated here. Indirectness is (mostly) orthogonal to DW_OP_stack_value. DW_OP_stack_value denotes that we reconstructed the value of the variable, but it doesn't exist in the program ("The DW_OP_stack_value operation specifies that the object does not exist in memory but its value is nonetheless known"), for example, a constant value. I think we want something like DW_OP_deref instead, at least for r-values. For l-values (=variables a debugger could write to) we would need to have a discriminator that declares the DBG_VALUE as a memory location (cf. DWARF5 chapter 2.6).

This is a tricky one. Right now, DIExpressions sort-of mimic DWARF, but with differences that aren't always immediately clear. The reason why I chose DW_OP_stack_value for the direct-value-case instead of using DW_OP_deref for the indirect-value-case is that it is more like actual DWARF: a DWARF expression is either empty, a register, a memory address, or an implicit location. The new representation handles each of these faithfully to DWARF, except for being unable to distinguish between the register and memory case for a single register argument. In DWARF, the difference is that a register location uses `DW_OP_reg N`, while any reference to a register's value in any other type of location uses `DW_OP_breg N`. We cannot specify these in LLVM since we only generate these operators at the end; previously, this was the job of the indirectness flag.

Rather than reintroducing a flag just for this purpose however, I instead propose that we treat this as a special (albeit common) case: we can use `DW_OP_LLVM_arg 0, DW_OP_stack_value` with a single register operand. We already reduce the DIExpression to specialized DWARF operators at the point of DWARF emission, for example: `<Register N>, DW_OP_plus_uconst 5` becomes `DW_OP_bregN RSP+5`. If in any case where a stack value expression consists of only a single register it is valid to convert it to a register location, then this should be a valid transformation; I can't think of any cases where it wouldn't be, since if we get a variable's value directly from a single register then it necessarily exists at that location. The only exception would be where, for one reason or another, we want DWARF to believe that the location is implicit and thus cannot be written to; if such a case exists then it might be suitable grounds to change the behaviour here.

This does have the potential to cause confusion to a reader unfamiliar with this behaviour, but for a reader examining debug info in enough detail that the removal of DW_OP_stack_value raises an eyebrows, I think simply noting the behaviour in code comments and the documentation would be sufficient.
________________________________
From: Adrian Prantl <aprantl at apple.com>
Sent: 27 August 2020 17:42
To: Tozer, Stephen <stephen.tozer at sony.com>
Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [Debuginfo] Changing llvm.dbg.value and DBG_VALUE to support multiple location operands

> On Aug 25, 2020, at 11:09 AM, Tozer, Stephen via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Currently there is a series of patches undergoing review[0] that seek to enable the use of multiple IR/MIR values when describing a source variable's location. The current plan for the MIR is to add a new instruction, DBG_VALUE_LIST, that supports this functionality by having a variable number of operands. It may be better however to simply replace the existing DBG_VALUE behaviour entirely instead, and so I'm looking for any comments on this change before pushing ahead with it.

Thank you for writing this up! I think this is generally a good idea.

>
> There are a few differences between the MIR instructions:
>
> Old: DBG_VALUE %x, $noreg, !DILocalVariable("x"), !DIExpression()
> New: DBG_VALUE !DILocalVariable("x"), !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_stack_value), %x
>
> 1) The "location" operand is moved to the end, as the instruction is now variadic such that every operand after the DIExpression is a location operand.

Sounds good.

> 2) The second operand which currently represents "Indirectness" has been removed entirely, because this is now explicitly specified in the DIExpression (see 4).

Sounds good, too. We may need to pay a cost for rewriting more DIExpressions, but I don't see a way to make this work with multiple operands otherwise.

> 3) The DIExpression no longer implicitly treats the location operand as the first element of the expression, instead each location must be explicitly referenced in the expression using `DW_OP_LLVM_arg, N` for the Nth location operand.

This is nice and consistent. If we are worried about the extra memory needed we can still come up with a more efficient encoding of the common case, but the DIExpression *interface* should present it like this.

> 4) The DIExpression itself must be explicit about whether it evaluates to the location of a variable or its literal value, by using DW_OP_stack_value in the latter case (instead of relying on the Indirectness flag, which is both confusing and redundant[1]).

I'm not sure this will work as stated here. Indirectness is (mostly) orthogonal to DW_OP_stack_value. DW_OP_stack_value denotes that we reconstructed the value of the variable, but it doesn't exist in the program ("The DW_OP_stack_value operation specifies that the object does not exist in memory but its value is nonetheless known"), for example, a constant value. I think we want something like DW_OP_deref instead, at least for r-values. For l-values (=variables a debugger could write to) we would need to have a discriminator that declares the DBG_VALUE as a memory location (cf. DWARF5 chapter 2.6).

I think this is going in the right direction, we just need to sort out that last point!

thanks,
adrian

>
> I believe this is a strict improvement to the expressiveness and clarity of DBG_VALUE. Although it increases the verbosity of simple expressions, such a change is necessary to remove potential ambiguities in constant debug expressions[2]. We will also be relying on the DIExpression to replace the "Indirectness" flag, since it should now solely determine whether or not a value is indirect; this brings us closer to the final DWARF representation. One potential downside is that using DW_OP_stack_value for a simple single-register DBG_VALUE (as in the example above) would currently lose information, as it would output the DWARF expression `DW_OP_breg0 RSP+0, DW_OP_stack_value` instead of the current output `DW_OP_reg0 RSP`. The former is larger and gives less information, as both expressions evaluate to the same value but only the latter gives a location for the variable that can be modified by a debugger. This can be fixed with some pattern matching in the DwarfExpression class to cover this specific (albeit common) case.
>
> The current approach for the IR is not to add a new instruction, but to add a new metadata node that contains a list of IR value references (wrapped as ValueAsMetadata) and use it as the first argument to dbg.value. There is no syntactic incompatibility between this and the current dbg.value, and therefore it is possible to support both simultaneously, but I believe it would be unnecessarily complicated to maintain two separate forms of dbg.value. There is no immediate plan to change dbg.declare and dbg.addr in the same way: there is some value in the distinction between the intrinsics, the addresses do not use constant values (and so avoid the ambiguity described in [2]), and there are few (possibly no) cases where dbg.addr or dbg.declare intrinsics that use more than one IR value would actually be produced: only salvageDebugInfo can produce multi-value debug intrinsics, and debug address intrinsics usually use a non-salvageable alloca as the location (I am currently unsure as to whether non-alloca address intrinsics can or should be produced anywhere).
>
> Described here are the differences in the IR intrinsics:
>
> Old: @llvm.dbg.value(metadata i32 %x, metadata !DILocalVariable("x"), metadata !DIExpression())
> New: @llvm.dbg.value(metadata !DIValueList(i32 %x), metadata !DILocalVariable("x"), metadata !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_stack_value))
>
> 1) The location operand is changed from a single Value to a list of 0 or more Values.
> 2) The DIExpression is modified in the same manner as in the MIR instruction (see above).
>
> In summary, this is a notice of the intent to introduce these changes in the patch described above. Currently the patches add these modified instructions alongside the existing ones, but a total replacement would be a better outcome. This is not a full RFC but is intended to ensure that this change doesn't catch anyone by surprise and that there are no significant objections.
>
> [0] https://reviews.llvm.org/D82363
> [1] https://bugs.llvm.org/show_bug.cgi?id=41675#c8
> [2] http://lists.llvm.org/pipermail/llvm-dev/2020-February/139441.html
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