llvm dev - Feb 2020 - Enabling debug entry value production by default

Yep, TAG_call_site_parameter and its children shouldn't require any extra
relocations. Thanks!

vedant
> On Feb 7, 2020, at 2:01 PM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> For that sort of small growth, if it doesn't add more relocations (I
think the call sites need them (but they're already emitted/that's not
what we're discussing enabling here), but call value descriptions
don't), sounds good to me :)
> 
> On Fri, Feb 7, 2020 at 12:04 PM Vedant Kumar <vedant_kumar at apple.com
<mailto:vedant_kumar at apple.com>> wrote:
> Hi all,
> 
> I think we've reached a state where we're ready to enable debug
entry value production by default for the x86_64, ARM, and AArch64 targets. For
context, this is a debug info feature that allows debuggers to recover the value
of unmodified optimized-out parameters by 'going up' a stack frame and
interpreting spilled values, constants, etc. to work out what was passed to the
callee. There's a nice write-up about the impact of this feature in [3]. A
big thank you to all of contributors who've helped us get here (I've
tried to CC everyone, sincere apologies if I've accidentally left you out).
> 
> I believe Djordje and I have shown that there isn't an outsized binary
size or compile time impact of enabling this feature. The aggregate size of
DWARF in a stage2 x86_64 RelWithDebInfo clang build grows by under 0.2%, and we
didn't find a significant compile time impact. We've shared CTMark
results (both compile-time & DWARF section size numbers), as well as a size
analysis of stage2 builds of {llc, clang, lldb} in [1].
> 
> Djordje has also shared a nice location statistics comparison chart in [1],
showing increased variable availability with entry values enabled.
> 
> For my part, I instrumented lldb's DWARFExpression::Evaluate method to
log instances in which a variable location is unavailable. I then used a script
[4] to single-step through a stage2 RelWithDebInfo+entryvals clang binary as it
was compiling sqlite3.c, printing local variables at each stop. Without entry
values, lldb reported "variable not available" 883,918 times. With
entry values lldb reported "variable not available" 687,899 times (a
22% drop).
> 
> Does anyone have concerns about enabling this feature?
> 
> thanks,
> vedant
> 
> [1] [DebugInfo] Enable the debug entry values feature by default
<https://reviews.llvm.org/D73534>
> [2] [CallSiteInfo] Fix the assertions regarding updating the CallSiteInfo
<https://reviews.llvm.org/D73700>, [CallSiteInfo] Use the
isCandidateForCallSiteEntry() when update the cs info
<https://reviews.llvm.org/D74122>[3]
https://robert.ocallahan.org/2018/11/comparing-quality-of-debug-information.html
<https://robert.ocallahan.org/2018/11/comparing-quality-of-debug-information.html>
> [4] https://gist.github.com/vedantk/7602b20a9e1d44c42d32dcca36591cc0
<https://gist.github.com/vedantk/7602b20a9e1d44c42d32dcca36591cc0>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200207/1ab64869/attachment.html>

I've asked someone to check with our (Sony) debugger people, likely hear
back early next week.  I'd say proceed with having it default on/off based
on tuning as you described, and if my guys want to see it turned on, we can fix
it later.  I have to say, a quick skim through D73534 made me think it's not
controlled that way right now, is that still a TODO?
Thanks for all the work to support this feature!
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Vedant
Kumar via llvm-dev
Sent: Friday, February 7, 2020 2:04 PM
To: David Blaikie <dblaikie at gmail.com>
Cc: llvm-dev at lists.llvm.org; nikola.prica at rt-rk.com; ibaev at cisco.com;
asowda at cisco.com
Subject: Re: [llvm-dev] Enabling debug entry value production by default

Yep, TAG_call_site_parameter and its children shouldn't require any extra
relocations. Thanks!

vedant


On Feb 7, 2020, at 2:01 PM, David Blaikie <dblaikie at
gmail.com<mailto:dblaikie at gmail.com>> wrote:

For that sort of small growth, if it doesn't add more relocations (I think
the call sites need them (but they're already emitted/that's not what
we're discussing enabling here), but call value descriptions don't),
sounds good to me :)

On Fri, Feb 7, 2020 at 12:04 PM Vedant Kumar <vedant_kumar at
apple.com<mailto:vedant_kumar at apple.com>> wrote:
Hi all,

I think we've reached a state where we're ready to enable debug entry
value production by default for the x86_64, ARM, and AArch64 targets. For
context, this is a debug info feature that allows debuggers to recover the value
of unmodified optimized-out parameters by 'going up' a stack frame and
interpreting spilled values, constants, etc. to work out what was passed to the
callee. There's a nice write-up about the impact of this feature in [3]. A
big thank you to all of contributors who've helped us get here (I've
tried to CC everyone, sincere apologies if I've accidentally left you out).

I believe Djordje and I have shown that there isn't an outsized binary size
or compile time impact of enabling this feature. The aggregate size of DWARF in
a stage2 x86_64 RelWithDebInfo clang build grows by under 0.2%, and we
didn't find a significant compile time impact. We've shared CTMark
results (both compile-time & DWARF section size numbers), as well as a size
analysis of stage2 builds of {llc, clang, lldb} in [1].

Djordje has also shared a nice location statistics comparison chart in [1],
showing increased variable availability with entry values enabled.

For my part, I instrumented lldb's DWARFExpression::Evaluate method to log
instances in which a variable location is unavailable. I then used a script [4]
to single-step through a stage2 RelWithDebInfo+entryvals clang binary as it was
compiling sqlite3.c, printing local variables at each stop. Without entry
values, lldb reported "variable not available" 883,918 times. With
entry values lldb reported "variable not available" 687,899 times (a
22% drop).

Does anyone have concerns about enabling this feature?

thanks,
vedant

[1] [DebugInfo] Enable the debug entry values feature by
default<https://reviews.llvm.org/D73534>
[2] [CallSiteInfo] Fix the assertions regarding updating the
CallSiteInfo<https://reviews.llvm.org/D73700>, [CallSiteInfo] Use the
isCandidateForCallSiteEntry() when update the cs
info<https://reviews.llvm.org/D74122>
[3]
https://robert.ocallahan.org/2018/11/comparing-quality-of-debug-information.html
[4] https://gist.github.com/vedantk/7602b20a9e1d44c42d32dcca36591cc0

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200207/448e72d6/attachment.html>

The actual DWARF emission for call site parameters is gated inside of
DwarfDebug::constructCallSiteEntryDIEs by `tuneForGDB() || tuneForLLDB()`.

However, we are creating+updating CallSiteInfo (basically, in-memory only
bookkeeping used by the backend to keep track of call sites) even when the
debugger tuning is set to the Sony debugger. If this creates problems, feel free
to file a bug and I'll address it. However, there's an upside to
maintaining all this CallSiteInfo even if TAG_call_site_parameters won't be
emitted. The backend asserts whenever a MachineInstr carrying CallSiteInfo is
destroyed without the call information being updated. This provides useful test
coverage.

vedant
> On Feb 7, 2020, at 2:50 PM, Robinson, Paul <paul.robinson at
sony.com> wrote:
> 
> I’ve asked someone to check with our (Sony) debugger people, likely hear
back early next week.  I’d say proceed with having it default on/off based on
tuning as you described, and if my guys want to see it turned on, we can fix it
later.  I have to say, a quick skim through D73534 made me think it’s not
controlled that way right now, is that still a TODO?
> Thanks for all the work to support this feature!
> --paulr
>  
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Vedant Kumar
via llvm-dev
> Sent: Friday, February 7, 2020 2:04 PM
> To: David Blaikie <dblaikie at gmail.com <mailto:dblaikie at
gmail.com>>
> Cc: llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>;
nikola.prica at rt-rk.com <mailto:nikola.prica at rt-rk.com>; ibaev at
cisco.com <mailto:ibaev at cisco.com>; asowda at cisco.com
<mailto:asowda at cisco.com>
> Subject: Re: [llvm-dev] Enabling debug entry value production by default
>  
> Yep, TAG_call_site_parameter and its children shouldn't require any
extra relocations. Thanks!
>  
> vedant
> 
> 
> On Feb 7, 2020, at 2:01 PM, David Blaikie <dblaikie at gmail.com
<mailto:dblaikie at gmail.com>> wrote:
>  
> For that sort of small growth, if it doesn't add more relocations (I
think the call sites need them (but they're already emitted/that's not
what we're discussing enabling here), but call value descriptions
don't), sounds good to me :)
>  
> On Fri, Feb 7, 2020 at 12:04 PM Vedant Kumar <vedant_kumar at apple.com
<mailto:vedant_kumar at apple.com>> wrote:
> Hi all,
>  
> I think we've reached a state where we're ready to enable debug
entry value production by default for the x86_64, ARM, and AArch64 targets. For
context, this is a debug info feature that allows debuggers to recover the value
of unmodified optimized-out parameters by 'going up' a stack frame and
interpreting spilled values, constants, etc. to work out what was passed to the
callee. There's a nice write-up about the impact of this feature in [3]. A
big thank you to all of contributors who've helped us get here (I've
tried to CC everyone, sincere apologies if I've accidentally left you out).
>  
> I believe Djordje and I have shown that there isn't an outsized binary
size or compile time impact of enabling this feature. The aggregate size of
DWARF in a stage2 x86_64 RelWithDebInfo clang build grows by under 0.2%, and we
didn't find a significant compile time impact. We've shared CTMark
results (both compile-time & DWARF section size numbers), as well as a size
analysis of stage2 builds of {llc, clang, lldb} in [1].
>  
> Djordje has also shared a nice location statistics comparison chart in [1],
showing increased variable availability with entry values enabled.
>  
> For my part, I instrumented lldb's DWARFExpression::Evaluate method to
log instances in which a variable location is unavailable. I then used a script
[4] to single-step through a stage2 RelWithDebInfo+entryvals clang binary as it
was compiling sqlite3.c, printing local variables at each stop. Without entry
values, lldb reported "variable not available" 883,918 times. With
entry values lldb reported "variable not available" 687,899 times (a
22% drop).
>  
> Does anyone have concerns about enabling this feature?
>  
> thanks,
> vedant
>  
> [1] [DebugInfo] Enable the debug entry values feature by default
<https://reviews.llvm.org/D73534>
> [2] [CallSiteInfo] Fix the assertions regarding updating the CallSiteInfo
<https://reviews.llvm.org/D73700>, [CallSiteInfo] Use the
isCandidateForCallSiteEntry() when update the cs info
<https://reviews.llvm.org/D74122>
> [3]
https://robert.ocallahan.org/2018/11/comparing-quality-of-debug-information.html
<https://robert.ocallahan.org/2018/11/comparing-quality-of-debug-information.html>
> [4] https://gist.github.com/vedantk/7602b20a9e1d44c42d32dcca36591cc0
<https://gist.github.com/vedantk/7602b20a9e1d44c42d32dcca36591cc0>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200207/be604b7b/attachment.html>