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<div class="moz-cite-prefix">On 02/14/2014 05:55 PM, Philip Reames
wrote:<br>
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Splitting out a conversation which started in "make DataLayout a
mandatory part of Module" since the topic has decidedly changed.
This also relates to the email "RFC: GEP as canonical form for
pointer addressing" I just sent. <br>
<br>
<div class="moz-cite-prefix">On 02/10/2014 05:25 PM, Nick Lewycky
wrote:<br>
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<div>...<br>
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We're supposed to have the llvm.gcroots intrinsic for
this purpose, but you note that it prevents gc roots
from being in registers (they must be in memory
somewhere, usually on the stack), and that fixing it is
more work than is reasonable.<br>
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This is slightly off, but probably close to what I actually said
even if not quite what I meant. :)<br>
<br>
I'm going to skip this and respond with a fuller explanation
Monday. I'd written an explanation once, realized it was wrong,
and decided I should probably revisit when fully awake. <br>
<br>
Fundamentally, I believe that gc.roots could be made to work, even
with decent (but not optimal) performance in the end. We may even
contribute some patches towards fixing issues with the gc.root
mechanism just to make a fair comparison. I just don't believe
it's the right approach or the best way to reach the end goal.<br>
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So, not quite on Monday, but I did get around to writing up an
explanation of what's wrong with using gcroot. It turned out to be
much longer than I expected, so I turned it into a blog post:<br>
<a class="moz-txt-link-freetext" href="http://www.philipreames.com/Blog/2014/02/21/why-not-use-gcroot/">http://www.philipreames.com/Blog/2014/02/21/why-not-use-gcroot/</a><br>
<br>
The very short version: gcroot loses roots (for any GC) due to bad
interaction with the optimizer, and gcroot doesn't capture all
copies of a pointer root which fundamentally breaks collectors which
relocate roots. The only way I know to make gcroot (in its current
form) work reliably for all collectors is to insert safepoints very
early, which has highly negative performance impacts. There are
some (potentially) cheaper but ugly hacks available if you don't
need to relocate roots. <br>
<br>
There's also going to be a follow up post on implementation
problems, but that's completely separate from the fundamental
problems. <br>
<br>
Philip<br>
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