[llvm-dev] Enabling debug entry value production by default

[David Blaikie via llvm-dev]

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> 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
>
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