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I don't want to get into the debate w.r.t. which IR style is better
- ask me over beer if you care about my opinions - but as an FYI,
there are serious proposals being worked on to introduce some notion
of memory def-use edges to help in analysing memory operations. I
don't think we've settled on a concrete proposal yet, but I wouldn't
be surprised to see something in the form of an analysis pass which
produces 'def-use' information for memory operations. <br>
<br>
Philip<br>
<br>
<div class="moz-cite-prefix">On 02/22/2015 07:47 PM, Jeehoon Kang
wrote:<br>
</div>
<blockquote
cite="mid:CAPunEiLvKcR7ZUGHTaD=i1inuMLsHrQCOeeSvCwP3dQCNen6qw@mail.gmail.com"
type="cite">
<div dir="ltr">Thank you David and Jeremy!
<div><br>
</div>
<div>I am quite convinced that LLVM IR in SSA form already
expresses data dependence quite well, as said David and
Jeremy. Expressing IR in DAG may enable more optimizations on
memory operations, but the benefit seems to be not so much.</div>
<div><br>
</div>
<div>Furthermore, I strongly agree with Jeremy in that
instruction orders should be preserved for -O1 for debugging
purposes.</div>
<div><br>
</div>
<div>Thank you,<br>
</div>
<div>Jeehoon</div>
<div><br>
</div>
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<div class="gmail_extra"><br>
<div class="gmail_quote">2015-02-21 21:41 GMT+09:00 Jeremy
Lakeman <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:Jeremy.Lakeman@gmail.com" target="_blank">Jeremy.Lakeman@gmail.com</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr"><br>
<div class="gmail_extra"><br>
<div class="gmail_quote">
<div>
<div class="h5">On Sat, Feb 21, 2015 at 6:38 PM,
David Chisnall <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:David.Chisnall@cl.cam.ac.uk"
target="_blank">David.Chisnall@cl.cam.ac.uk</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0
0 .8ex;border-left:1px #ccc
solid;padding-left:1ex"><span><br>
> On 21 Feb 2015, at 05:59, Jeehoon Kang
<<a moz-do-not-send="true"
href="mailto:jeehoon.kang@sf.snu.ac.kr"
target="_blank">jeehoon.kang@sf.snu.ac.kr</a>>
wrote:<br>
><br>
> this is Jeehoon Kang, a CS PhD student
and a newbie to LLVM.<br>
><br>
> I am wondering why LLVM IR's basic block
consists of a list of instructions,<br>
> rather than a DAG of instruction as in
the low level (ISelectionDAG).<br>
<br>
</span>SSA form is implicitly a DAG, defined by
the uses relation (registers in LLVM can be
thought of as edges between instructions). It
is not *solely* a DAG, however. For example, in
some places the order of two loads matters -
particularly when they are atomic operations -
it's only side-effect-free operations that can
be represented entirely as a DAG. In general,
optimisations that work best with a DAG
representation deal with use-def chains and are
not explicitly aware of the sequence of
instructions in the basic blocks unless they
need to be.<br>
<br>
</blockquote>
</div>
</div>
<div>The order of loads is still essentially a
directed graph. Currently that information is
implicit in the basic block order, and optimisations
need to know if it is safe to screw around with
them. Perhaps these relationships would be better
represented explicitly instead, in which case the
order of instructions in a block would be less
relevant. <br>
<br>
</div>
<div>Though of course machine instructions need to be
ordered, -O0 shouldn't mess with the order of
operations for debugging purposes, and you do need
some deterministic way to iterate over instructions.
So I'm not certain there'd be much benefit in trying
to remove the current ordering of instructions. If
you want to walk the instructions as a DAG you can,
if you want to walk them in execution order you can
do that too.<br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
David<br>
<br>
<br>
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</blockquote>
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-- <br>
<div class="gmail_signature">
<div dir="ltr"><a moz-do-not-send="true"
href="http://sf.snu.ac.kr/jeehoon.kang" target="_blank">Jeehoon
Kang (Ph.D. student)</a>
<div><a moz-do-not-send="true" href="http://sf.snu.ac.kr"
target="_blank">Software Foundations Laboratory</a>
<div><a moz-do-not-send="true" href="http://www.snu.ac.kr"
target="_blank">Seoul National University</a></div>
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