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<div class="moz-cite-prefix">On 10/10/2017 02:49 PM, Alina Sbirlea
wrote:<br>
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<div>Sigh</div>
<div>I should have taken the time to give a better
example.</div>
<div>The must-alias part is irrelevant to an example
(it only requires read-onlyness)</div>
<div><br>
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<div>You said "LICM doesn't move calls, so we'd
never really care about must-alias for promotion".
I was just pointing out other things move calls
any may want to know.</div>
<div> </div>
<div>If you want an example where the must-alias
part would matter:</div>
<div><br>
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<div>*a = something</div>
<div>foo(a)</div>
<div>b = *a<br>
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<div>If foo mustalias a (and only a) not only can
you move foo with a, you can actually clone foo
here, change it to be pass-by-value, and promote
the argument inside of it (if you wanted to).</div>
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<div>So you can use this info to, for example, do
interprocedural promotion.</div>
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<div>Are we instead looking to set a MRI_Must
bit, disjunct of MRI_Mod, and test for
MRI_Ref&MRI_Must or
MRI_Mod&MRI_Must?</div>
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<div>Yes.</div>
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<div>I didn't mean to pick on the example, sorry if that's
how it came through.</div>
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<div>Since the consensus is to expose the Must info in
ModRefInfo, I'm trying to figure out how to add it in a
way that makes sense to me.</div>
<div>The way I see ModRefInfo is designed right now is to
lower the lattice to NoModRef as fast as possible (start
with ModRef as top, get to NoModRef as bottom). The
implementation is based on having either Mod or Ref and
masking out everything else. <br>
</div>
<div>Introducing a Must bit, means setting it occasionally
(since May is conservative) and then preserving it, so the
opposite: start lattice at bottom, set to top.</div>
<div><br>
</div>
What I was trying, that *somewhat* made sense:<br>
enum ModRefInfo {<br>
MRI_NoModRef = 0,<br>
MRI_Ref = 1,<br>
MRI_Mod = 2,<br>
MRI_ModRef = MRI_Ref | MRI_Mod,<br>
MRI_Must = 4,</div>
<div class="gmail_quote"> MRI_MustRef = MRI_Ref | MRI_Must,</div>
<div class="gmail_quote"> MRI_MustMod = MRI_Mod | MRI_Must,</div>
<div class="gmail_quote"> MRI_MustModRef = MRI_ModRef |
MRI_Must<br>
};</div>
// + shift values in FunctionModRefLocation to 8, 16, 32.</div>
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<div class="gmail_extra">Recursive masking of MRI_Ref/MRI_Mod
would get replaced by MRI_MustRef/MRI_MustMod.</div>
<div class="gmail_extra">But the top of the lattice is still
MRI_ModRef.</div>
<div class="gmail_extra">While the implementation details *may*
be ok to resolve, there are calls checking for equality to
MRI_Ref or MRI_Mod (not &), so adding the occasional Must
bit seems bad.</div>
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<br>
I don't see this as a major problem. Please feel free to fix these
places by replacing the equality checks with mask checks.<br>
<br>
-Hal<br>
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<div class="gmail_extra">So I guess my question is, what's the
right approach here? I feel like I'm not on the right path.</div>
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<div dir="ltr">In getModRefInfo(CS, Loc), the
MRI_Must bit would then be set if
doesAccessArgPointees and ArgAlias ==
MustAlias for all Args, which seems correct.</div>
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<div>alias == MustAlias for Loc, not for all args.</div>
<div>(IE It it returns a singular result about Loc,
not a result about all args)</div>
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<div>To get the set answer for all args, we'd have
to query further.</div>
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<div>Yes, that's what I meant. In getModRefInfo(CS, Loc)
there is a loop over all args, it checks alias() for each
one.</div>
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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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