<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Aug 1, 2017, at 10:20 AM, Hal Finkel <<a href="mailto:hfinkel@anl.gov" class="">hfinkel@anl.gov</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class="moz-cite-prefix">On 08/01/2017 11:54 AM, David Blaikie
wrote:<br class="">
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<div dir="ltr" class="">On Tue, Aug 1, 2017 at 9:43 AM Hal Finkel <<a moz-do-not-send="true" href="mailto:hfinkel@anl.gov" class="">hfinkel@anl.gov</a>>
wrote:<br class="">
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<div class="m_838875341229329508moz-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" target="_blank" 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" target="_blank" class="">reviews@reviews.llvm.org</a>>
wrote:<br class="">
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<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color: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 class="">
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<div bgcolor="#FFFFFF" text="#000000" class=""> 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.</div>
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Oh, wait - you mean a variable with value 42 and 87 gets
collapsed to a boolean where true/1 is used to represent 42
and false/0 is used to represent 87?<br class="">
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Yes. The code does this:<br class="">
<br class="">
// Change the load into a load of bool then a select.<br class="">
LoadInst *LI = cast<LoadInst>(UI);<br class="">
LoadInst *NLI = new LoadInst(NewGV, LI->getName()+".b",
false, 0,<br class="">
LI->getOrdering(),
LI->getSyncScopeID(), LI);<br class="">
Value *NSI;<br class="">
if (IsOneZero)<br class="">
NSI = new ZExtInst(NLI, LI->getType(), "", LI);<br class="">
else<br class="">
NSI = SelectInst::Create(NLI, OtherVal, InitVal, "", LI);<br class="">
NSI->takeName(LI);<br class="">
LI->replaceAllUsesWith(NSI);<br class=""></div></div></blockquote><div><br class=""></div><div>Whoa — this is going to be fun! A DWARF expression for that example might look like:</div><div><br class=""></div><div>DW_OP_constu 42 DW_OP_mul DW_OP_dup DW_OP_bra +2 DW_OP_skip +<end-1> DW_OP_constu 87 DW_OP_constu DW_OP_stack_value </div><div><br class=""></div><div>roughly: if (Val * 42) return Val * 42; else return 87;</div><div><br class=""></div><div>-- adrian</div><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
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-Hal<br class="">
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Oh. Yeah. That's a bit more involved. (but nice that it
means we can represent the values exactly - hopefully)<br class="">
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Thanks!<br class="">
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<div bgcolor="#FFFFFF" text="#000000" class=""> I think you actually
want to generate a DWARF expression for these.<br class="">
<br class="">
-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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<div bgcolor="#FFFFFF" text="#000000" class=""> <br class="">
<pre class="m_838875341229329508moz-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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