[llvm-dev] [DebugInfo][RFC] Enabling "instruction referencing" variable locations for x86_64

Thu Nov 11 11:48:40 PST 2021

Really broadly, I think Jeremy has done a lot of good work here, and if he
thinks this is ready to enable for x86, we should go forward with it.

Previously, I have expressed concerns about this design direction because
instructions do not necessarily remain in source order. However, I think
the benefit of the increased variable availability probably outweighs the
cost of variable values that don't correspond to the current source
location, and we should go forward with this.

Thanks for working on this, Jeremy!

On Wed, Nov 10, 2021 at 11:49 AM Jeremy Morse via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi llvm-dev@,
>
> tl;dr: Is it okay to enable value tracking (aka "instruction
> referencing") to preserve debug-info variable locations, by default,
> for x86_64?
>
> The high-level summary of instruction referencing is that, after
> instruction selection, instead of describing variable locations as a
> virtual register, we indirectly refer to the MachineInstr operand
> where the variables value is defined. Once compilation is done, some
> standard SSA analysis is used to determine the locations that values
> are in, and what locations contain variable values. The benefit is
> improved variable location coverage because we no longer have to act
> conservatively during register allocation. More in the original RFC
> [0].
>
> We've (Sony) tested this internally and it works well; Caroline kindly
> tested it [1] on some Google benchmarks and it seemed to work well
> there too. It's currently in-tree and can be enabled by passing
> -Xclang -fexperimental-debug-variable-locations to clang, or
> -experimental-debug-variable-locations in LLVM options.
>
> Probably the biggest caveat right now is that I'd like to only enable
> for x86_64: this isn't due to any design limitation, it's just where
> I've been doing all the testing, and it's largely untested on most
> other architectures. I've run a stage2 cross compile to aarch64 for
> clang as it's another popular arch, but that's all. There's a
> migration guide in [2] of how other architectures can benefit: I would
> suggest that other architectures opt into instruction referencing when
> they're ready.
>
> Here's a list of "remaining work" that are things I consider
> incomplete about the new implementation:
>  * A bunch of tests need to be updated to check for the correct
> outputs in this new mode, see D113194 [3].
>  * Variadic variable locations aren't implemented -- this is probably
> a week or two of work.
>  * I've got unit tests for maybe 85% of the important parts of the
> "new" LiveDebugValues, but there are some gaps.
>  * It can be non-trivial for humans to interpret variable locations
> [4] in MIR, some printing improvements are needed.
>
> There are some downsides that can be discussed too: most significantly
> that it's slower on CTMark [5]. The relevant configs there are
> NewPM-ReleaseLTO-g and LegacyPM-ReleaseLTO-g, which are the only
> configs that get optimisations and debug-info. Strictly speaking, this
> slowdown is unavoidable because there's more information and more
> accurate information being produced (one CTMark binary is almost 20%
> larger due to extra debug-info), but it's still unfortunate. There are
> some optimisations that can still be applied, and with instruction
> referencing we can trivially compress all sequences of debug
> instructions into a single instruction. Reids experiments [6] on
> debug-instruction contribution to compile times (applied to IR not
> MIR) suggest that could be a performance win. I should be able to
> prototype this sometime soon.
>
> Other downsides:
>  * Support for i686 FP registers is nonexistant; due to it being a
> stack, it's extra effort to track, which I haven't bothered to do yet.
> Normal DBG_VALUEs don't do particularly well either.
>  * In the original RFC I pointed out that we can define a debug
> use-before-def as a missing location at the point of any instruction
> not dominated by the def, and as a normal location at any instruction
> dominated by the def. Turning on instruction referencing by default
> means this interpretation is implicitly accepted.
>
> In my opinion it's mature enough to turn on by default (for x86_64),
> ideally in good time for LLVM14's branch date, and I'm confident that
> the remaining work can be done by the branch date. What do other
> people think?
>
> [0] https://lists.llvm.org/pipermail/llvm-dev/2020-February/139440.html
> [1] https://lists.llvm.org/pipermail/llvm-dev/2021-September/152457.html
> [2] https://reviews.llvm.org/D113586
> [3] https://reviews.llvm.org/D113194
> [4] https://reviews.llvm.org/D111317#3057563
> [5]
> http://llvm-compile-time-tracker.com/compare.php?from=8a2f40b403745fe7ba305efa86f95f3568cbd0a6&to=7436a1baa8692b4ed5ff577c1a17ed1b1e916961&stat=instructions
> [6] https://lists.llvm.org/pipermail/llvm-dev/2018-October/127140.html
>
> --
> Thanks,
> Jeremy
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