Ananthakrishna Sowda (asowda) via llvm-dev
2019-Feb-08 21:03 UTC
[llvm-dev] RFC: [DebugInfo] Improving Debug Information in LLVM to Recover Optimized-out Function Parameters
On 2/8/19, 7:15 AM, "paul.robinson at sony.com" <paul.robinson at
sony.com> wrote:
It's fantastic to see this happening! It'll be really nice to start
seeing some DWARF 5 feature work (as opposed to the infrastructure
stuff, which has its own benefits but doesn't help the end-user's
debugging experience). A couple of questions below.
--paulr
Thanks for the comments and feedback. I have added my replies inline.
-Ananth
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Ananthakrishna Sowda (asowda) via llvm-dev
> Sent: Thursday, February 07, 2019 9:32 PM
> To: Adrian Prantl
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] RFC: [DebugInfo] Improving Debug Information in
> LLVM to Recover Optimized-out Function Parameters
>
> Thank you for your interest and comments! Please see my responses
inline.
>
> On 2/7/19, 3:17 PM, "aprantl at apple.com on behalf of Adrian
Prantl"
> <aprantl at apple.com> wrote:
>
>
>
> > On Feb 7, 2019, at 1:49 PM, Ananthakrishna Sowda (asowda) via
llvm-
> dev <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> > Following is a proposal to improve location coverage for
Function
> parameters in LLVM. The patches for review will be posted soon.
> >
> > RFC: [DebugInfo] Improving Debug Information in LLVM to
Recover
> Optimized-out Function Parameters
> >
> > Ananthakrishna Sowda(Cisco), asowda at cisco.com
> > Nikola Prica (RT-RK/Cisco), nprica at rtrk.com
> > Djordje Todorovic(RT-RK/Cisco), djtodorovic at rtrk.com
> > Ivan Baev (Cisco), ibaev at cisco.com
> >
> >
> > Overview of the problem
> > Software release products are compiled with optimization level
–O2
> and higher. Such products might produce a core-file in case of a
failure.
> Support engineers usually begin debug analysis by looking at the
backtrace
> from a core-file. Unfortunately, many parameters in backtraces are
> reported as optimized out due to variety of reasons. This makes
triaging
> the issue and assigning ownership harder due to missing information. It
is
> harder for the product team to understand the cause of the failure. In
> summary, we are describing a well-known serviceability problem for
> optimized production code.
> >
> > Proposal for solution
> > Function parameters have a natural fall-back location which is
> parent frame. Debuggers can easily go up a frame in call-chain and
> evaluate any expression. Expert developers can find what values
parameters
> had at function entry point by examining disassembly of caller frame at
> that particular function call. With additional call-site information
> produced by compiler, debugger can fully automate this technique. DWARF
5
> specification has new tags and attributes to describe call-site
parameter
> information [1][2]. it is already implemented in GCC and GDB since
> 2011[3]. We propose implementing this feature in LLVM to enhance the
> debugging of optimized code experience of LLVM users.
> >
> > Prior mention
> > An initial version of our work was presented as a poster
during LLVM
> Developer Meeting, in San Jose, 2018. The feature is now fully
implemented
> in internal Clang/LLVM 4.0 version.
> > We presented a talk on our work at FOSDEM 2019[4].
> >
>
> Thank you for posting this. This looks very interesting! Since your
> proposal has a lot of different components, Sema support,
> DW_AT_call_site_parameter support, DW_OP_entry_value support, it will
> probably be best to split them out into separate reviews, but it's
also
> good to discuss the proposal in its entirety first. I have a bunch of
> questions to make sure I fully understand what you are doing.
>
> Sure, we will post several patches, each as a logical unit.
>
> > Implementation notes in Clang and LLVM
> > On the callee side the only information that we need is
whether a
> parameter is never modified in the function. If true then we can use
> parameter’s entry value when we lose track of parameter’s location. As
a
> natural way of handling this problem we used Clang’s Sema and its
> constness check to embed this information in variable’s declaration
which
> is later used for DILocalVariable construction.
Do you think we could build on this to offer entry_value location list
entries, if the value is lost (e.g., register is reused) before the
parameter
is modified within the callee?
We are doing that already in our implementation. Such values are reported with
@entry suffix to make it clear. Please look at arguments in frame #3 in the
backtrace example in the RFC.
>
> By looking at whether an argument is modified in the function, you
can
> identify variables that can be described with an entry value location
and
> that entry value would be valid throughout the function. Are you using
> this information in the function body to identify whether to emit an
entry
> value location, or are you using this information at the call site to
> identify call sites for which call site parameters would be beneficial
(or
> both)?
>
> Is emitting an entry value location in the function body an
either-or
> thing or do you also emit plain old locations if you have them
available
> in the location list together with the entry values?
>
> In the function, I assume you don't know whether all call sites
will
> have call site parameters. How do you decide whether to emit entry
value
> locations?
>
> Entry value location is added to same conventional location list. We
emit
> them when we see holes in the coverage, looking at the whole function
> scope. It is used by the debugger when there is no conventional
location
> for a program range. It is reported by the debugger as
“<optimized-
> out>, @entry = <value at call site>”.
> Unmodified argument is a sub-set for which debugger can report @entry
> value is same as actual value. So, it will not be reported as
optimized-
> out.
> We generate entry value location whenever conventional does not cover
one
> hundred percent. We generate call-site information when the call-site
> parameter values can be evaluated by unwinding to the parent frame.
Only
> debugger can tell if look-up of an entry value finds matching call site
> and call site parameter.
Is the idea to use DW_OP_entry_value in a default-location list entry?
I think we do not emit those currently, but it seems like an ideal match
for an unmodified formal parameter.
Yes, it is the idea. Even for lost values ( reported as optimized-out), showing
@entry value has some benefit to the debug user.
>
>
> > For call-site information, new DINode metadata DICallSite and
> DICallSiteParam are defined and these are emitted by the Clang
frontend.
> The metadata is associated to the call or invoke IR instruction. Here
is
> an example:
> >
> > %call5 = call i32 @fed_em_strncmp(i8* %arraydecay, i8*
%arraydecay1,
> i64 5), !dbg !114, !call_site !101
> > …
> > !99 = !DICallSiteParam(argno: 1, variable: !91, expr:
> !DIExpression())
> > !100 = !DICallSiteParam(argno: 2, variable: !95, expr:
> !DIExpression())
> > !101 = !DICallSite(scope: !87, file: !3, parameters: !102,
line: 40,
> calledSubprogram: !13)
> > !102 = !{!99, !100, !103}
> > !103 = !DICallSiteParam(argno: 3, expr:
!DIExpression(DW_OP_lit5)
> > For tracking call sites and call site parameters in backend
two new
> pseudo instructions, DBG_CALLSITE and DBG_CALLSITEPARAM, are
introduced.
> See the MIR code bellow:
> >
> > DBG_CALLSITE 0, %noreg, <!19>; dbg:strncmp.c:40:47
> > * DBG_CALLSITEPARAM %RDX, <0x727fee0> >
!DIExpression(DW_OP_lit5), 5, %noreg ; dbg:strncmp.c:40:47
> > * DBG_CALLSITEPARAM %RSI, "str2"
<0x71a6dd0> > !DIExpression(), %RBX, %noreg ; dbg:strncmp.c:40:47
> > * DBG_CALLSITEPARAM %RDI, "str1"
<0x71a6dd0> > !DIExpression(), %, %RSP, 4 ; dbg:strncmp.c:40:47
>
> I'll refrain from bike-shedding the actual implementation;
let's save
> this for phabricator, but conceptually, this makes sense to me. If I
> understand correctly, you are identifying at the call site parameters
that
> are in locations that can be restored by unwinding the function call,
such
> as constants, stack slots, and callee/r-saved registers.
> Can you explain why you need to identify them at the IR level?
Could
> you do it just in MIR, too, or is there some information missing in
MIR?
>
> What happens to your DICallSiteParam when a function call gets
> inlined?
>
> Your understanding about call site parameter is right!
> When a function call gets inlined, call site information is
eliminated.
> We kept to the LLVM guidelines of introducing DI metadata in the front
end
> and carrying it through IR and MIR. I don’t think there is missing
> information in MIR, though.
>
> >
> > There is a challenge in ISel phase to produce them. Algorithm
that
> collects information about call site parameters iterates over call
> sequence chained nodes returned from target specific call lowering
> interface. Goal of the algorithm is to recognize SDNodes that represent
> instructions which will load function arguments into registers that
> transfer them into another function call frame. There is a question
> whether this is effectively implemented as a general matching algorithm
or
> it should be lowered to target specific level. DBG_CALLSITE pseudo
> instruction will need to be revisited since information whether a call
is
> tail call or not could be extracted differently but for sake of
simplicity
> we chose this.
> > Most of passes handle DBG_CALLSITE and DBG_CALLSITEPARAM
through
> target instruction info interface method isDebugInstr(). This method is
> used to skip processing of pseudo debug instructions. Since these new
> pseudo debug instructions relay on virtual registers and frame objects
we
> need to follow up their substitution through the compilation phases.
There
> were several backend passes that needed special attention: Register
> Coalesce, Inline Spiller, Prologue Epilog Inserter, Split Kit and
Virtual
> Register Rewriter. Virtual Register Rewriter required implementation of
> target specific salvaging interface for “call site parameter
identities” –
> situation following identity copy instructions that leads to
overlapping
> of parameter transferring register location and location that is loaded
> into that register.
> > The last challenge is to extend LiveDebugValues pass to
generate
> additional DBG_VALUE instructions with new kind of debug expression
(with
> ‘DW_OP_entry_value’) for parameters that meet the requirements
described
> in [1].
> > Finally, emitting call-site debug information can be
controlled by
> ‘-debugger-tune’ and ‘-dwarf-version’ LLVM code-generation flags, since
> not all debuggers used in the community consume this DWARF information.
>
> Have you considered to instead insert a very late MIR pass that
does
> some backwards analysis on the machine code to yield potential call
site
> parameters instead of threading it all the way through the compiler? If
> yes, why did you choose this implementation?
>
> If I have not convinced you on this, we are open for suggestion. I will
> ask my colleagues on this project to respond too.
>
>
> >
> > Location coverage improvement
> > The important criteria in debugging-optimized-code is whether
the
> compiler has location information for variables and parameters. We use
> ‘locstats’ utility from elfutils [5] package to guide us in improving
> overall location coverage in final executable. For each non-artificial
> variable or formal parameter - or Debugging Information Entry in DWARF
-
> ‘locstats’ computes what percentage from the code section bytes where
the
> variable is in scope, the variable has a non-empty location
description.
> 100% coverage is not expected for non-global variables and function
> parameters, since value may not be ‘live’ through the entire scope. On
the
> other end, 0% coverage for variables which are used in the code is
> indicative of compiler losing track of values.
> > The second column in Table 1 shows ‘locstats’ report for
gdb-7.11
> compiled for x86-64 with “-g –O2” by Clang 4.0. For example, there are
> 29476 parameters whose coverage is in 91..100% range. The third column
> shows locstats’ report with “-g –O2” and the parameter-entry-value
> feature. There are now 37671 parameters whose coverage is in 91..100%
> range – for a 28% improvement.
> > Because our implementation computes an additional location
list
> entry to parameters whenever possible, and DW_OP_entry_value is valid
> through the entire scope of the parameter, the numbers at 91..100 row
are
> relevant indication of improvement with parameter-entry-value.
> >
> > Coverage Parameters Parameters with emit-param-entry-values
> > (% range) (number/%)) (number/%)
> > 0..10 22682/30% 21342/28%
> > 11..20 3498/4% 2337/3%
> > 21..30 3083/4% 1986/2%
> > 31..40 3050/4% 1862/2%
> > 41..50 2534/3% 1574/2%
> > 51..60 2349/3% 1571/2%
> > 61..70 2184/2% 1649/2%
> > 71..80 2620/3% 2069/2%
> > 81..90 3432/4% 2847/3%
> > 91..100 29476/39% 37671/50%
> > Table 1 Location coverage statistics for function parasmeters
> >
> > Improved backtrace for optimized code in debugger
> > Figure 1 below shows improved backtrace for optimized code
when
> compiled with parameter entry value tracking feature. Please note the
new
> @entry values reported for parameters in backtrace. These parameters
will
> otherwise be reported as <optimized-out>.
> >
> > gdb) bt
> > #0 get_next_move_from_list (list=list at entry=0x7fffffffbf88,
> > color=color at entry=1, moves=moves at entry=0x7fffffffbfb0,
> > cutoff=cutoff at entry=100) at engine/owl.c:3032
> > #1 0x000000000042a957 in do_owl_attack (str=<optimized
out>,
> > move=<optimized out>, move at entry=0x7fffffffc334,
wormid=<optimized
> out>, wormid at entry=0x7fffffffc33c, owl=<optimized out>, owl
at entry=0x0,
> > komaster=komaster at entry=0, kom_pos=kom_pos at
entry=0,escape=<optimized
> out>)
> > at engine/owl.c:1306
> > #2 0x000000000042a0d0 in owl_attack (target=target at
entry=148,
> > attack_point=attack_point at entry=0x7fffffffc580,
certain=<optimized
> out>, certain at entry=0xb63048 <dragon+11288>,
> kworm=kworm at entry=0x7fffffffc3c4) at engine/owl.c:1144
> > #3 0x0000000000412c71 in make_dragons (color=<optimized
out>,
> color at entry=1, stop_before_owl=<optimized out>,
stop_before_owl at entry=0,
> save_verbose=<optimized out>, save_verbose at entry=0) at
engine/dragon.c:346
> > #4 0x0000000000417fdc in examine_position (color=color at
entry=1,
> how_much=how_much at entry=99) at engine/genmove.c:152
> > #5 0x00000000004183c6 in do_genmove (move=move at
entry=0x7fffffffd344,
> color=1, color at entry=3, pure_threat_value=<optimized out>,
> > allowed_moves=<optimized out>, allowed_moves at
entry=0x0)
> > at engine/genmove.c:334
> > #6 0x000000000041926d in genmove_conservative
> (i=i at entry=0x7fffffffd36c, j=j at entry=0x7fffffffd368, color=3) at
> engine/genmove.c:255
> > #7 0x00000000004618ae in gtp_gg_genmove (s=<optimized
out>) at
> interface/play_gtp.c:2163
> > #8 0x000000000045b0f8 in gtp_main_loop (commands=<optimized
out>,
> gtp_input=0xb8b100) at interface/gtp.c:126
> > Figure 1: Backtrace with @entry value parameters
>
> Very nice!
>
> >
> > Cost in disk image size increase and compile-time
> > The parameter-entry-value feature is enabled with -g
compilation.
> Due to new DebugInfo metadata generation which adds entries to DWARF
> sections such as .debug_info and .debug_loc, there is expected size
> increase of disk image of the executable built with “-g –O”. For SPEC
CPU
> 2006 benchmark, the average size increase is 15%. However, there is no
> change in sections loaded at runtime such as .text. .data, .bss.
Hence,
> there is no runtime size increase.
> > Compile-time cost increase is 1-3% percent for SPEC CPU 2006.
>
> At this early stage I'm not yet worried by the size increase.
We'll
> probably find some opportunities to fine-tune the heuristics that
decide
> whether a call site parameter / entry value is profitable. We can also
> always provide a tuning option to turn the feature off.
>
>
> > Community up-streaming
> > Since we have implemented this for LLVM-4.0 we are currently
in
> process of porting this implementation on LLVM trunk. We are planning
to
> share this set of patches with LLVM community and seek feedback in
> improving certain parts of our implementation.
>
> Sounds great!
> thanks for sharing this,
> adrian
>
>
> Thanks,
> Ananth
>
> >
> >
> > References
> > [1]Jakub Jelínek and Roland McGrath. DWARF DW_OP_entry_value
> extension proposal. http://dwarfstd.org/ShowIssue.php? issue=100909.1.
> > [2 Jakub Jelínek, Roland McGrath, Jan Kratochvíl, and
Alexandre
> Oliva. DWARF DW_TAG_call_site extension proposal. http://dwarfstd.org/
> ShowIssue.php?issue=100909.2
> > [3] J. Jelinek “Improving debug info for optimized away
parameters”
>
https://gcc.gnu.org/wiki/summit2010?action=AttachFile&do=view&target=jelin
> ek.pdf
> > [4] FOSDEM talk
> http://bofh.nikhef.nl/events/FOSDEM/2019/K.4.201/llvm_debug.webm
> > [5] Elfutils https://sourceware.org/elfutils/
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Nikola Prica via llvm-dev
2019-Feb-12 13:07 UTC
[llvm-dev] RFC: [DebugInfo] Improving Debug Information in LLVM to Recover Optimized-out Function Parameters
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. 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 -> This is the case from ‘VirtRegMap’ pass, but I think it can happen elsewhere. 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. 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. Thanks, Nikola
Adrian Prantl via llvm-dev
2019-Feb-12 17:06 UTC
[llvm-dev] RFC: [DebugInfo] Improving Debug Information in LLVM to Recover Optimized-out Function Parameters
[+ some folks more knowledgable about the Machine layer than me.]> 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?> 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?> 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?> > 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> > > Thanks, > > Nikola
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