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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi Stefanus,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I had actually tried that already, but my project kept crashing.
However, after giving it a second try I noticed that the crash was happening
elsewhere this time. After disabling the GVN optimization pass everything
worked fine. Somewhere between revision 67979 and 67996 the bug that was
causing GVN to fail was fixed.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>So thanks for tracking this down!<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I agree that option (2) is a good solution for now. But in my
humble opinion the whole idea of storing other information in pointers is not
very robust. One alternative would be to have a global store of all Value
objects. They can then be identified with just an index instead of a pointer.
This index can be stored in a bitfield that is smaller than 32-bit, leaving
bits available for additional data…<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Cheers,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Nicolas<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu] <b>On Behalf
Of </b>Stefanus Du Toit<br>
<b>Sent:</b> zaterdag 2 mei 2009 0:41<br>
<b>To:</b> LLVM Developers Mailing List<br>
<b>Subject:</b> Re: [LLVMdev] PointerIntPair causing trouble<o:p></o:p></span></p>
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</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Hi Nicolas,<o:p></o:p></p>
<div>
<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal>Looks like Preston and I have found the cause of the
problem. The issue is with PointerLikeTypeTraits<T*>::NumLowBitsAvailable.
This is set to 3, which basically assumes that unless the traits are
specialized for a particular pointer type, objects of that type are allocated
with malloc() and aligned to 8 bytes.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>While PointerLikeTypeTraits is overloaded for Use*, it is
not overloaded for User*. lib/VMCore/Use.cpp uses a PointerIntPair<User*, 1,
Tag>, and things go bad. Users are typically allocated in an array after a
bunch of Uses, which are 12 bytes in size, so they may actually only be aligned
to 4 bytes.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>The attached patch (which I don't intend to commit, it's
just a workaround) works around this simply by dropping this down to 2 bits,
which gets us past our problem on Windows. I would appreciate if you could
verify that this works for you as well.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>To actually solve this properly I see two main options:<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>(1) we could specialize PointerLikeTypeTraits for User*, and
leave the default value of NumLowBitsAvailable at 3.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>(2) we could drop the default NumLowBitsAvailable to 2 (or
even use _alignof and similar compiler-specific extensions to determine it),
and allow classes that assert that they are only ever allocated through malloc
to relax it back up to 3.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
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<div>
<p class=MsoNormal>My preference would be for option (2), due to the difficulty
of tracking this bug down, and the risk of future similar bugs happening.
However, I'd appreciate some feedback from the LLVM developer community on
this. Please let me know what you think, and I'll be happy to prepare a patch.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>I'm still wondering why this wasn't an issue on other
platforms. One possibility is that Use is being bumped up to 16 bytes in size,
and thus Users never get placed at less than an 8-byte boundary. However, in
that case, the whole point of the "use diet" optimization is lost!
I'll investigate and try to find out.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>I'm also still not sure why the assertion in
PointerIntPair::setPointer() did not get triggered by the
User::allocHungOffUses() implementation in Use.cpp. Perhaps the assertion is
wrong (it looks reasonable, though) or perhaps there is something else going on
I haven't seen yet. I'll look into this some more as well.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Let me know if the workaround works for you, and I'd
appreciate feedback from anyone (Chris? Gabor?) as to how best to proceed.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Thanks!<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>Stefanus<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
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