<div dir="ltr"><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Oct 24, 2016 at 1:26 PM, Daniel Berlin <span dir="ltr"><<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="gmail-h5">On Sun, Oct 23, 2016 at 9:30 AM, Nuno Lopes <span dir="ltr"><<a href="mailto:nuno.lopes@ist.utl.pt" target="_blank">nuno.lopes@ist.utl.pt</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span>> So just to be clear (mainly so i can go back to letting y'all hash this out), in value numbering:<br>
> 1. I can value number all freeze operations on the same operand to the same value<br>
> *as long as*<br>
> 2. I replace all freeze values of the same operand with a single one.<br>
><br>
> Or am i misunderstanding?<br>
> If i've got it right, the issue i see with #2 is whether i can hoist freezes or not.<br>
><br>
> If not, it may not be possible to cover all uses.<br>
><br>
> IE<br>
> if (a) {<br>
> %1 = freeze (%x)<br>
> } else if (b) {<br>
> %2 = freeze (%x)<br>
> } else if (c){<br>
> %3 = freeze (%x)<br>
> }<br>
><br>
> I cannot replace these with a single freeze def without placing a freeze above the if block.<br>
<br>
</span>As Sanjoy said, hoisting freeze is fine, but movement is not without constraints. For example, you cannot duplicate execution of freeze.<br>
e.g. the following is wrong (assuming loop body may execute multiple times):<br>
<br>
%x = freeze %y<br>
while(...) { use %x }<br>
=><br>
while(...) {<br>
%x = freeze %y<br>
use %x<br>
}<br>
<br>
So Sanjoy saying that freeze can be thought as an atomic/volatile operation seems like a good intuition.<br>
<br>
<br>
> *If* i need to replace them all with the same freeze def in order to be able to consider them the same value, (...)<br>
<br>
Right, so just to make it very clear, the following would be wrong:<br>
%x = freeze %y<br>
%w = freeze %y<br>
use_1 %x<br>
use_2 %w<br>
use_3 %x<br>
=><br>
%x = freeze %y<br>
%w = freeze %y<br>
use_1 %w<br>
use_2 %w<br>
use_3 %x<br>
<br>
If you replace one use of %x, all uses have to be replaced, since different freezes of the same operand *may* return different values.<br>
I guess that since we are never (err, most of the times) sure of whether a value may come out of freeze, this rule needs to be applied to any value.<br>
Daniel: is that fine for GVN, or would it make things too complicated?<br></blockquote><div><br></div></div></div><div>Current GVN will definitely get this wrong an d would be hard to fix because of how it eliminates (IE it may cause performance regression)</div></div></div></div></blockquote><div><br></div><div>To be clear, i mean: "If you make current GVN understand freeze x has the same value everywhere, it will do the wrong thing. If you make it think freeze x has a different value everywhere, it will never eliminate anything, which is a regression"</div><div><br></div><div>But again, just to make sure i understand, is the following legal:<br><span style="color:rgb(80,0,80)">%x = freeze %y</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">%w = freeze %y</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_1 %x</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_2 %w</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_3 %x</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">=></span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">%x = freeze %y</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_1 %x</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_2 %x</span><br style="color:rgb(80,0,80)"><span style="color:rgb(80,0,80)">use_3 %x</span><br></div><div> <br></div><div><br></div><div>(assuming use_2 is the only use of %w)<br><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div></div><div>We can make newgvn get it right.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<span class="gmail-m_-780788523212706778HOEnZb"><font color="#888888"><br>
Nuno<br>
<br>
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