[llvm-dev] RFC: [DebugInfo] Improving Debug Information in LLVM to Recover Optimized-out Function Parameters

Nikola Prica via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 13 09:09:40 PST 2019

On 12.02.2019. 18:06, Adrian Prantl wrote:
> [+ some folks more knowledgable about the Machine layer than me.]
That would be useful for us too! :)

>> On Feb 12, 2019, at 5:07 AM, Nikola Prica <nikola.prica at rt-rk.com> wrote:
>> Hi,
>> I am one of the authors of this feature. On Phabricator, we agreed to
>> take discussion whether encoding this in IR and threading it through the
>> compiler or performing a late MIR analysis is the better approach.
>> Regarding late MIR pass approach, it would need to go back from call
>> instruction by recognizing parameter's forwarding instructions and
>> interpret them. We could interpret register moves, immediate moves and
>> some of stack object loadings. There would be need to write
>> interpretation of various architectures instructions. We were not
>> positive about completeness of such recognition.
> So you're saying that in late MIR, the target-specific MachineInstructions don't have enough generic meta information to understand where data was copied/loaded from in a target-independent way? Would it be easier in earlier pre-regalloc MIR, or does that have the same problem because the instructions are already target-specific?

It has enough generic meta information for some kind of instructions but
not for all. MachineInstr has bits for isMoveImm, isMoveReg and mayLoad
that can be useful for recognizing some kind of parameter loading
instructions. But we're not quite sure whether it is enough for
recognizing all of them. For example there is no support for recognizing
X86::LEA instructions with such mechanism (and there is significant
number of such parameter loading instructions). This instruction made us
give up with late MIR pass approach because we were not sure about
various architectures complexities. As a result from tacking it from
ISEL phase we were able to get entry values from some LEA instructions.
This is very important for this approach. Currently, for various
architectures, this approach gives us more information about values
loaded into parameter forwarding registers than late MIR pass would give
use (because of lack of special instruction interpretation).

But nevertheless, in the end, we lose some information through the
optimization pipeline, and in order to salvage some information we
implemented target specific machine instruction interpreter. For example
situations like:

%vreg = LEA <smt>
$rdi = COPY %vreg
call foo
 *DBG_CALLSITEPARAM $rdi, , %vreg

==== replace %vreg with $rdi ====

%rdi = LEA <smt>
$rdi = COPY %rdi
call foo
 *DBG_CALLSITEPARAM $rdi, , %rdi

==== delete redudant instruction identities ====

%rdi = LEA <smt>
call foo
 *DBG_CALLSITEPARAM $rdi, , %rdi

In order to salvage this we go backward from call instruction and try to
interpret instruction that loads value to $rdi.

This salvaging part could be used for interpreting in late MIR pass but
it would need extension for other architecture specific instructions.
But with current approach that starts tracking parameter forwarding
instructions from ISEL phase we are able to cover some of them without
such interpretor.

ISEL phase is important for matching DICallSiteParam metadata to
respective DBG_CALLSITEPARAM. It also recognizes COPY instructions that
are part of calling sequence but do not forward any argument (for
example for variadic, C calling convention, functions we have copy to AL
register). If we are able to dispatch such non transferring argument
copy instructions from calling convention and we potentially drop IR
metadata about call site parameters, we might be able to do all
necessary parameter's tracking in some separate MIR pass (not very late,
somewhere after ISEL).

>> However, such analysis
>> might not be complete as current approach. It would not be able to
>> produce ‘DW_OP_entry_values’ in ‘DW_TAG_call_site_value’ expression of
>> call site parameter as a late pass.
>> As example think of callee function that forwards its argument in
>> function call in entry block and never uses that argument again:
>>  %vreg = COPY $rsi;             ->
>>  …. <no use of $rsi nor %vreg>  ->   …<no use of $rsi nor %vreg>
>>  $rsi = COPY $vreg;             ->   call foo
>>  call foo                       ->
> I'm not sure I can follow this example (yet). Is this about creating a call site parameter for an argument of the call to foo, or is it meant to illustrate that the call to foo() makes it hard to write a backwards analysis for inserting a call site parameter for a call site below the call to foo()?
> >> This is the case from ‘VirtRegMap’ pass, but I think it can happen
>> elsewhere.
> As I'm not super familiar with the various MIR passes: Is VirtRegMap the pass inserting the vreg copy from $rsi here? Does it do something else?

Oh, I didn't explain it fully. In virtual register rewriter, for
previous example %vreg gets replaced with $rsi and we get two 'identity
copies' ($rsi = COPY $rsi) that get deleted. Such situation is special
for call at entry block whose argument is callee's argument that is used
only at that place. For example like:

baa(int a) {
  <code that does not use 'a' variable>;

Variable 'a' is dead after 'foo' call and there is no interest in
preserving it in further flow of function. At that point we lose
information about parameter forwarding instruction. No instruction, no
way to interpret it. In order to track such situations we need
DBG_CALLSITEPARAM instructions to track parameter transferring register
through the backend. For situations like this we use DICallSiteParam in
order to find it in previous frame. One late pass wont do the trick.

Call at entry block that implicitly re-forwards argument is special
situation and can possibly be handled with emitting DW_OP_entry_value
for call site parameter value expression.

Also, it is worth mentioning that for situations where we could have
call of 'foo(a)' nested at some machine block, parameter loading
instruction can be in different block than the call of 'foo(a)'. Such
situations would not be handled so easily by late pass.

>>  Recreation of this might be possible when function ‘foo’ is
>> in current compilation module, but we are not sure if it is possible for
>> external modules calls. In order to follow such cases we need
>> DBG_CALLSITEPARAM that can track such situation.
> What information exactly would you need about foo's implementation that you cannot get from just knowing the calling convention?

Having in mind previous example where we have just call of fucntion foo,
we would need to know how many arguments does 'foo' have and through
which registers are they forwarded. I'm not sure how would we get such

>> Since after ISEL phase we have explicit pseudo COPY instructions that
>> forward argument to another function frame, it came naturally to
>> recognize such instructions at this stage. There we can say with 100%
>> certainty that those instruction indeed forward function arguments.
> My main motivation for this discussion is to minimize the added complexity of the feature. For example, if we figure out that we can get by without introducing new IR constructs (that optimization authors would need to be taught about, that would need to be supported in all three instruction selectors) and can get by with only adding new MIR instructions, that would be a win. However, if we can prove that deferring the analysis to a later stage would result in inferior quality then the extra maintenance burden of new IR constructs may be the right tradeoff.
>> Thanks for taking the time to walk me through your thought process.
> -- adrian

In general we use DICallSiteParam as a backup solution (when we lose
track of location loaded int parameter forwarding register) for
representing value at entry point. As a consequence we are able to
produce 'DW_OP_entry_values' in 'DW_AT_call_site_value'(GCC generates
such expressions) which allow us to go 2 or more frames behind. I've
showed one example for calls at entry block that could also produce such
expressions without DICallSiteParam, but that is the only case that I
can think of now. But since it is a backup when something fails it could
at some point be removed once it is no longer needed as backup.
Currently it gives use more information about call site forwarding values.

Thanks for time! It is our common goal to make this right!

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