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<div class="moz-cite-prefix">On 08/01/2017 11:54 AM, Adrian Prantl
wrote:<br>
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<div class="">On Aug 1, 2017, at 9:42 AM, Hal Finkel <<a
moz-do-not-send="true" href="mailto:hfinkel@anl.gov"
class="">hfinkel@anl.gov</a>> wrote:</div>
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<div class="moz-cite-prefix">On 08/01/2017 11:28 AM,
Adrian Prantl via llvm-commits wrote:<br class="">
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<div class="">On Aug 1, 2017, at 9:24 AM, David
Blaikie <<a moz-do-not-send="true"
href="mailto:dblaikie@gmail.com" class="">dblaikie@gmail.com</a>>
wrote:</div>
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<div dir="ltr" class="">On Tue, Aug 1, 2017 at
9:04 AM Adrian Prantl via Phabricator <<a
moz-do-not-send="true"
href="mailto:reviews@reviews.llvm.org"
class="">reviews@reviews.llvm.org</a>>
wrote:<br class="">
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rgb(204, 204, 204); padding-left: 1ex;">aprantl
added inline comments.<br class="">
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================<br class="">
Comment at:
lib/Transforms/IPO/GlobalOpt.cpp:1577<br
class="">
+ for(auto *GV : GVs)<br class="">
+ NewGV->addDebugInfo(GV);<br class="">
+<br class="">
----------------<br class="">
NikolaPrica wrote:<br class="">
> aprantl wrote:<br class="">
> > I'm not familiar with this
transformation: Do we need to add a
DIExpression to mask out all but the first
bit (i.e. can multiple bools be packed into
the same uint32_t such that it could confuse
debuggers)?<br class="">
> The debug info which is provided here
with addDebugInfo later generates address of
the variable (DW_OP_addr: xxxx) in
DW_AT_location. If we provide here metadata
which is for 1byte variable the debugger
would get confused because the enum type is
written as 4-byte and he would try to read 4
bytes. This is just temporary fix until
proper way to handle this is found.<br
class="">
If I understood you correctly then the best
way to represent this would be a
`DW_OP_LLVM_fragment /*offset*/0
/*bitsize*/1 (or 8?)`<br class="">
expression. This will get lowered into a
DW_OP_bit_piece to tell the debugger that
this location is describing of the first n
bits of the variable.<br class="">
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A slight problem with this is that at least
GDB won't print a single value if it's
partially unavailable (eg: if it's a struct
and the fragment describes one member but
not the othe,r I think that's OK - but if
it's a single int and only one out of 4
bytes are described - well, the value is
known/unknowable).<br class="">
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<div class="">A debugger would have to print the value
as something like 0x??????01 to indicate that pieces
are missing. But no debugger I'm aware of does that.</div>
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<div class="">If this optimization is really
based on the proof that the other bytes are
unused, never written to or read, then a
fragment describing the other bytes as
constant zero would be good to have as well.<br
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No, it is more complicated than that. See
TryToShrinkGlobalToBoolean in
lib/Transforms/IPO/GlobalOpt.cpp. It is looking for cases
where the global is only set to one initialized value and
one other value. In that case, it can make a Boolean
global and replace uses of that Global with selects over
that value. I think you actually want to generate a DWARF
expression for these.<br class="">
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Are you saying the two values may be arbitrary values and not
only 0 and 1?</div>
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Yes, I believe that's correct.<br>
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-Hal<br>
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<div>-- adrian<br class="">
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-Hal<br class="">
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Agreed, that would be the most practical solution — if
we can prove that it is correct.</div>
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<div class="">-- adrian<br class="">
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But I doubt that's the case - no doubt if we
can see all the reads and writes, we
optimize away the writes if we know they
won't be read, so we may end up with only
the one byte. C'est la vie.<br class="">
<br class="">
Just mention it to check/understand what the
optimization is/isn't doing, etc.<br
class="">
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<a moz-do-not-send="true"
href="https://reviews.llvm.org/D35994"
rel="noreferrer" target="_blank" class="">https://reviews.llvm.org/D35994</a></blockquote>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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