<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jun 18, 2020, at 1:58 AM, Djordje <<a href="mailto:djordje.todorovic@syrmia.com" class="">djordje.todorovic@syrmia.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" class="">
<div class=""><p class="">Hi Vedant,</p><p class="">Thanks a lot for your comments!</p><p class="">>It looks like a lot of the new infrastructure introduced <a href="https://github.com/djolertrk/llvm-di-checker/commit/9d26ac2557c584f6cf82ac5535fc47f8bd267a27" class="">here</a> consists of logic copied from the debugify
implementation. Why is introducing a new pair of passes better
than extending the ones we have? The core infrastructure needed to
track location loss for real (non-synthetic) source variables is
is in place already.</p><p class="">Since it is a proposal, I thought it'd easier to understand the
idea if I duplicate things. Ideally, we can make an API that could
be used from both tools. Initially, I made a few patches locally
turning the real debug info into debugify ones, but I realized it
breaks the original idea/design of the debugify & that is why
I decided to make a separate pass(es). This cannot stay as is with
the respect to the implementation, it should be either merged into
debugify file(s) or refactored using the API mentioned above.</p></div></div></blockquote><div>Oh, this wasn’t clear to me. In the future, please describe what is/isn’t part of the proposal. It’d be especially helpful to include some discussion of the pros & cons of the prototype design and its alternatives.</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><p class=""> Another reason for implementing it as a different pass was the
fact the debugify is meant to be used from 'opt' level only, but
if we want to invoke the option from front end level, we need to
merge it into the IR library.</p></div></div></blockquote><div>The debugify passes are now linked by llc via the Transforms library as part of the -mir-debugify implementation. A frontend should be able to use them.</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><p class="">
</p>
<div class="">>Stepping back a bit, I’m also surprised by the
decision to move away from synthetic testing when there’s still so
much low-hanging fruit to pick using that technique. The example
from <a href="https://reviews.llvm.org/D81939" class="">https://reviews.llvm.org/D81939</a> illustrates
this perfectly: in this case it’s not necessary to invent a new
testing technique to uncover the bug, because simply running
`./bin/llvm-lit -Dopt="opt -debugify-each"
test/Transforms/DeadArgElim` finds the same issue.</div><p class="">As I mentioned in the previous mail, I do really think the
debugify technique is great & I use it. But, in order to
detect that variable "x" was optimized-out starting from pass Y, I
only run the di-checker option (that performs analysis only) &
find the variable in the final html report. I think that is very
user friendly concept.</p></div></div></blockquote><div>About the analysis — why not emit a report in -check-debugify when the # of non-undef debug uses of a variable drops to 0? This doesn’t require the -debugify step. The list of local variables is preserved via DISubprogram::retainedNodes.</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class=""><p class=""> At the end, when we detected what was the
spot of loosing the location, we can run debugify on the
pass-directory-tests (but there is a concern the tests does not
cover all the possible cases; and the case found from the high
level could be new to the pass). In addition, the di-checker
detects issues for metadata other than locations (currently, the
preservation map keeps the disubprograms only, but it should keep
other kinds too).</p></div></div></blockquote><div>Note that it’s possible to to increase code coverage by running a -debugify-each pipeline on -g0 IR produced by a frontend.</div><div><br class=""></div><div>Is it common for a pass to drop an entire DISubprogram? I would hope this either never happens or is extremely rare.</div><div><br class=""></div><div>best,</div><div>vedant</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class=""><p class="">
</p><p class="">>In D81939, you discuss finding the new tool useful when
responding to bug reports about optimized-out variables or missing
locations. We sorely do need something better than
-opt-bisect-limit, but why not start with something simple?
-check-debugify already knows how to report when & where a
location is dropped, it would be simple to teach it to emit a
report when a variable is fully optimized-out.</p><p class="">I agree. We can do that and that could be used from both
utilities.<br class="">
</p><p class=""><br class="">
</p><p class="">Best regards,</p><p class="">Djordje</p><p class=""><br class="">
</p>
<div class="moz-cite-prefix">On 17.6.20. 21:14, Vedant Kumar wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:74E217F2-67F5-44FF-BAA1-470DEFB75549@apple.com" class="">
Hey Djordje,
<div class=""><br class="">
</div>
<div class="">It looks like a lot of the new infrastructure
introduced <a href="https://github.com/djolertrk/llvm-di-checker/commit/9d26ac2557c584f6cf82ac5535fc47f8bd267a27" class="" moz-do-not-send="true">here</a> consists of logic
copied from the debugify implementation. Why is introducing a
new pair of passes better than extending the ones we have? The
core infrastructure needed to track location loss for real
(non-synthetic) source variables is is in place already.</div>
<div class=""><br class="">
</div>
<div class="">Stepping back a bit, I’m also surprised by the
decision to move away from synthetic testing when there’s still
so much low-hanging fruit to pick using that technique. The
example from <a href="https://reviews.llvm.org/D81939" class="" moz-do-not-send="true">https://reviews.llvm.org/D81939</a> illustrates
this perfectly: in this case it’s not necessary to invent a new
testing technique to uncover the bug, because simply running
`./bin/llvm-lit -Dopt="opt -debugify-each"
test/Transforms/DeadArgElim` finds the same issue.</div>
<div class=""><br class="">
</div>
<div class="">In D81939, you discuss finding the new tool useful
when responding to bug reports about optimized-out variables or
missing locations. We sorely do need something better than
-opt-bisect-limit, but why not start with something simple?
-check-debugify already knows how to report when & where a
location is dropped, it would be simple to teach it to emit a
report when a variable is fully optimized-out.</div>
<div class=""><br class="">
</div>
<div class="">
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On Jun 17, 2020, at 2:10 AM, Djordje <<a href="mailto:djordje.todorovic@syrmia.com" class="" moz-do-not-send="true">djordje.todorovic@syrmia.com</a>>
wrote:</div>
<div class="">
<div class=""><br class="">
I am sharing the proposal [0] which gives a brief
introduction for the implementation of the LLVM DI
Checker utility. On a very high level, it is a pair of
LLVM (IR) Passes that check the preservation of the
original debug info in the optimizations. There are
options controlling the passes, that could be invoked
from ``clang`` as well as from ``opt`` level.<br class="">
<br class="">
By testing the utility on the GDB 7.11 project (using it
as a testbed), it has found a certain number of
potential issues regarding the DILocations (using it on
LLVM project build itself, it has found one bug
regarding DISubprogram metadata). Please take a look
into the final report (on the GDB 7.11 testbed)
generated from the script that collects the data at [1].
By looking at these data, it looks that the utility like
this could be useful when trying to detect the real
issues related to debug info production by the compiler.<br class="">
</div>
</div>
</blockquote>
<div class=""><br class="">
</div>
<div class="">Thanks for sharing these results. The data here is older
(from the 2018 debug info BoF) and from a different project
(sqlite3), but we saw some similar patterns: <a href="https://llvm.org/devmtg/2018-10/slides/Prantl-Kumar-debug-info-bof-2018.pdf" class="" moz-do-not-send="true">https://llvm.org/devmtg/2018-10/slides/Prantl-Kumar-debug-info-bof-2018.pdf</a></div>
<div class=""><br class="">
</div>
<div class="">best</div>
<div class="">vedant</div>
</div>
</div>
</blockquote>
</div>
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