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Hi Jeremy,</div>
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Thanks a lot for your feedback.</div>
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<blockquote style="border-left: 3px solid rgb(200, 200, 200); border-top-color: rgb(200, 200, 200); border-right-color: rgb(200, 200, 200); border-bottom-color: rgb(200, 200, 200); padding-left: 1ex; margin-left: 0.8ex; color: rgb(102, 102, 102);">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">For
the example in<span> </span></span><a href="https://reviews.llvm.org/D85012" target="_blank" rel="noopener noreferrer" data-auth="NotApplicable" style="margin: 0px; font-size: 14.6667px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; background-color: rgb(255, 255, 255)">https://reviews.llvm.org/D85012</a><span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important"><span> </span>,
I'm not sure that</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">just
using an entry value is correct. The reason why the dbg.values</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">for
arguments are set to undef is not because the value can't be</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">described,
it's because deadargelim changes all the call sites to pass</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">in
'undef', which I believe makes the value unrecoverable even with</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">entry
values. Can call sites like that be described by DWARF? If so, I</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">guess
a combination of entry-value variable locations and salvaging</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<div><span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">the
call-site arguments would work.</span></div>
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Using entry-values ('callee' side <span style="font-family: Calibri, Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255); display: inline !important">
of the feature</span>) is not enough in any case. It is always connected to the call-site-param (function arguments but we call it call-site-params; 'caller' side of the feature) debug info. I believe that there are call-site-params that could be expressed
in terms of DWARF for the cases we face within deadargelim. GCC does perform correct output for both caller and callee sides for unused params.</div>
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<br>
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<blockquote style="border-left: 3px solid rgb(200, 200, 200); border-top-color: rgb(200, 200, 200); border-right-color: rgb(200, 200, 200); border-bottom-color: rgb(200, 200, 200); padding-left: 1ex; margin-left: 0.8ex; color: rgb(102, 102, 102);">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">As
you say, we could speculatively produce entry value expressions as</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">a
"backup". I reckon this will work fine, although there'll be some</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">un-necessary
work involved due to it being speculative. I kind of have</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">a
different vision for how this could be done though, and not just for</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">entry
values. It hinges on the instruction referencing variable</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">location
stuff I've been working on: I believe we can use that to</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">connect
variable values at the end of compilation back to LLVM-IR</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">Values.
If that's achievable, we'll have:</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important"> *
Information about the set of LLVM-IR Values that are live, and what</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">physical
registers they're in,</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important"> *
The IR itself, which contains the target-independent relationships</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">between
Values,</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">After
which we will have enough information at the end of compilation</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">to
directly answer the question "How can we describe this variable</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">value
using only Values that are live?", effectively doing very late</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">salvaging.
We can consider entry values to be always available, and</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">should
be able to recover any value that _can_ be described in terms</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">of
entry values.</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">That's
assuming that the instruction referencing stuff works out. I</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255); display: inline !important">can
elaborate and give some examples if wanted.</span><br>
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Please share that work when you are ready.</div>
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<br>
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<blockquote style="border-left: 3px solid rgb(200, 200, 200); border-top-color: rgb(200, 200, 200); border-right-color: rgb(200, 200, 200); border-bottom-color: rgb(200, 200, 200); padding-left: 1ex; margin-left: 0.8ex; color: rgb(102, 102, 102);">
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="margin: 0px; font-size: 14.6667px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; color: rgb(32, 31, 30); background-color: rgb(255, 255, 255); display: inline !important">My
overarching feeling</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="margin: 0px; font-size: 14.6667px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; color: rgb(32, 31, 30); background-color: rgb(255, 255, 255); display: inline !important">is
that it'd be great to avoid doing extra working during compilation,</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
<span style="margin: 0px; font-size: 14.6667px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; color: rgb(32, 31, 30); background-color: rgb(255, 255, 255); display: inline !important">instead
leaving things until the end when there's more information</span><br style="color: rgb(32, 31, 30); font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; font-size: 14.6667px; background-color: rgb(255, 255, 255)">
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="margin: 0px; font-size: 14.6667px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif; color: rgb(32, 31, 30); background-color: rgb(255, 255, 255); display: inline !important">available.</span></div>
</blockquote>
<div style=""><font color="#201f1e"><span style="font-size: 14.6667px;">I agree with the statement (it'd be better if possible).</span></font></div>
<div style=""><font color="#201f1e"><span style="font-size: 14.6667px;"><br>
</span></font></div>
<div style=""><font color="#201f1e"><span style="font-size: 14.6667px;">Best,</span></font></div>
<div style=""><font color="#201f1e"><span style="font-size: 14.6667px;">Djordje</span></font></div>
<div style=""><font color="#201f1e"><span style="font-size: 14.6667px;"><br>
</span></font></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Jeremy Morse <jeremy.morse.llvm@gmail.com><br>
<b>Sent:</b> Tuesday, September 8, 2020 2:25 PM<br>
<b>To:</b> Djordje Todorovic <Djordje.Todorovic@syrmia.com><br>
<b>Cc:</b> David Stenberg <david.stenberg@ericsson.com>; llvm-dev@lists.llvm.org <llvm-dev@lists.llvm.org>; Nikola Tesic <Nikola.Tesic@syrmia.com>; Petar Jovanovic <petar.jovanovic@syrmia.com>; ibaev@cisco.com <ibaev@cisco.com>; asowda@cisco.com <asowda@cisco.com><br>
<b>Subject:</b> Re: [llvm-dev] [RFC] [DebugInfo] Using DW_OP_entry_value within LLVM IR</font>
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<div class="PlainText">Hi Djordje,<br>
<br>
[Late reply as I was away, alas],<br>
<br>
For the example in <a href="https://reviews.llvm.org/D85012">https://reviews.llvm.org/D85012</a> , I'm not sure that<br>
just using an entry value is correct. The reason why the dbg.values<br>
for arguments are set to undef is not because the value can't be<br>
described, it's because deadargelim changes all the call sites to pass<br>
in 'undef', which I believe makes the value unrecoverable even with<br>
entry values. Can call sites like that be described by DWARF? If so, I<br>
guess a combination of entry-value variable locations and salvaging<br>
the call-site arguments would work.<br>
<br>
The isel example in <a href="https://reviews.llvm.org/D87233">https://reviews.llvm.org/D87233</a> is good --<br>
although dropping the variable location there is largely because of<br>
SelectionDAGs own limitations rather than the generated code. We can't<br>
describe arguments that aren't used because they're not copied to a<br>
virtual register.<br>
<br>
In general, I reckon entry values will be a key part of recovering<br>
variable locations, however I'm not sure that there are many places<br>
where it's truly necessary in LLVM-IR. deadargelim is definitely one<br>
of them; but even then, the Argument remains in the function<br>
signature. We're still able to refer to the Argument in dbg.values, in<br>
program positions where the argument will be available, and where it<br>
will be unavailable. In my opinion, entry values are probably most<br>
useful when things go out of liveness; however we don't know what's in<br>
or out of liveness until the register allocator runs.<br>
<br>
As you say, we could speculatively produce entry value expressions as<br>
a "backup". I reckon this will work fine, although there'll be some<br>
un-necessary work involved due to it being speculative. I kind of have<br>
a different vision for how this could be done though, and not just for<br>
entry values. It hinges on the instruction referencing variable<br>
location stuff I've been working on: I believe we can use that to<br>
connect variable values at the end of compilation back to LLVM-IR<br>
Values. If that's achievable, we'll have:<br>
* Information about the set of LLVM-IR Values that are live, and what<br>
physical registers they're in,<br>
* The IR itself, which contains the target-independent relationships<br>
between Values,<br>
After which we will have enough information at the end of compilation<br>
to directly answer the question "How can we describe this variable<br>
value using only Values that are live?", effectively doing very late<br>
salvaging. We can consider entry values to be always available, and<br>
should be able to recover any value that _can_ be described in terms<br>
of entry values.<br>
<br>
That's assuming that the instruction referencing stuff works out. I<br>
can elaborate and give some examples if wanted. My overarching feeling<br>
is that it'd be great to avoid doing extra working during compilation,<br>
instead leaving things until the end when there's more information<br>
available.<br>
<br>
--<br>
Thanks,<br>
Jeremy<br>
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