<div dir="ltr">The loop exits iff
{2,+,1}<nuw><%for.body> ==
(zext i32 %n to i64)<div><br></div><div>The nuw marking on the "induction variable" should be sufficient to deduce a max loop trip count of 2^32.</div><div>But I do not know how we compute it (we build a database and it is contrived to follow, at least to me).</div><div><br></div><div>I saw that we annotate it with <nsw> (which is correct and can be (and probably has been) deduced from the ranges) and following our discussion, we can't annotate it with <nuw> as per limitation of our unification.</div><div><br></div><div>So, I am kind of in a bind here...</div></div><br><div class="gmail_quote"><div dir="ltr">On Thu, Aug 16, 2018 at 4:34 PM Friedman, Eli <<a href="mailto:efriedma@codeaurora.org">efriedma@codeaurora.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div class="m_-645509329432923076moz-cite-prefix">In general, the backedge-taken count is
an unsigned value; it's possible to write a loop with a trip count
of 2^64 using a 64-bit induction variable. To prove your loop has
a "small" trip count, you have to use either the guard or the
nsw/nuw markings on the induction variable.<br>
<br>
-Eli<br>
<br>
On 8/16/2018 4:09 PM, Alexandre Isoard wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Ok.
<div><br>
</div>
<div>To go back to the original issue, would it be meaningful to
add a SCEVUMax(0, BTC) on the final BTC computed by SCEV?</div>
<div><br>
</div>
<div>So that it does not use "negative values"?</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Aug 15, 2018 at 2:40 PM Friedman, Eli
<<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div class="m_-645509329432923076m_6547919799618152574moz-cite-prefix">On
8/15/2018 2:27 PM, Alexandre Isoard wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">I'm not sure I understand the
poison/undef/UB distinctions.
<div><br>
</div>
<div>But on this example:</div>
<div><br>
</div>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">define i32
@func(i1 zeroext %b, i32 %x, i32 %y) {<br>
entry:<br>
%adds = add nsw i32 %x, %y<br>
%addu = add nuw i32 %x, %y<br>
%cond = select i1 %b, i32 %adds, i32 %addu<br>
ret i32 %cond<br>
}</blockquote>
</div>
<div><br>
</div>
<div>It is important to not propagate the nsw/nuw
between the two SCEV expressions (which unification
would do today, can I consider that a bug or is it a
feature?).</div>
</div>
</blockquote>
<br>
It's an intentional design choice.<br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div>So we work-around it by not informing SCEV of the
flags:</div>
<div><br>
</div>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Printing analysis
'Scalar Evolution Analysis' for function 'func':<br>
Classifying expressions for: @func<br>
%adds = add nsw i32 %x, %y<br>
--> (%x + %y) U: full-set S: full-set<br>
%addu = add nuw i32 %x, %y<br>
--> (%x + %y) U: full-set S: full-set<br>
%cond = select i1 %b, i32 %adds, i32 %addu<br>
--> %cond U: full-set S: full-set<br>
Determining loop execution counts for: @func</blockquote>
</div>
<div><br>
</div>
<div>Would there be problems if we properly considered
nuw/nsw flags when unifying SCEVs?</div>
</div>
</blockquote>
<br>
There would be other consequences. For example, `(%x +
%y)<nsw>` and `(%x + %y)<nuw>` wouldn't compare
equal for other simplifications, and all the places that
call setNoWrapFlags would have to be rewritten. It's
probably possible to come up with some workable design, but
nobody has actually tried it, so it's not clear how much
work it would be to implement, or whether it would improve
the generated code overall.<br>
<br>
-Eli<br>
<br>
<blockquote type="cite"><br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Aug 15, 2018 at 1:59 PM Friedman,
Eli <<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531moz-cite-prefix">On
8/15/2018 1:31 PM, Alexandre Isoard wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Is that why we do not deduce
+<nsw> from "add nsw" either?</div>
</blockquote>
<br>
Essentially, yes.<br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div>Is that an intrinsic limitation of creating
a context-invariant expressions from a Value*
or is that a limitation of our implementation
(our unification not considering the nsw
flags)?</div>
</div>
</blockquote>
<br>
It's a consequence of unification not considering
nsw. (nsw on an instruction is naturally invariant
because violating nsw produces poison, not UB.)<br>
<br>
-Eli<br>
<br>
<blockquote type="cite"><br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Aug 15, 2018 at 12:39 PM
Friedman, Eli <<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531m_-6181515349790066455m_-6236410721431341120moz-cite-prefix">On
8/15/2018 12:21 PM, Alexandre Isoard via
llvm-dev wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Hello,
<div><br>
</div>
<div>If I run clang on the following
code:</div>
<div><br>
</div>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">void
func(unsigned n) {<br>
for (unsigned long x = 1; x <
n; ++x)<br>
dummy(x);<br>
}</blockquote>
<div><br>
</div>
<div>I get the following llvm ir:</div>
<div><br>
</div>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">define
void @func(i32 %n) {<br>
entry:<br>
%conv = zext i32 %n to i64<br>
%cmp5 = icmp ugt i32 %n, 1<br>
br i1 %cmp5, label %for.body,
label %for.cond.cleanup<br>
for.cond.cleanup:
; preds =
%for.body, %entry<br>
ret void<br>
for.body:
; preds = %entry,
%for.body<br>
%x.06 = phi i64 [ %inc,
%for.body ], [ 1, %entry ]<br>
tail call void @dummy(i64 %x.06)
#2<br>
%inc = add nuw nsw i64 %x.06, 1<br>
%exitcond = icmp eq i64 %inc,
%conv<br>
br i1 %exitcond, label
%for.cond.cleanup, label %for.body<br>
}</blockquote>
</div>
<div><br>
</div>
<div>Over which, SCEV will provide the
following analysis:</div>
<div><br>
</div>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Printing
analysis 'Scalar Evolution
Analysis' for function 'func':<br>
Classifying expressions for: @func<br>
%conv = zext i32 %n to i64<br>
--> (zext i32 %n to i64) U:
[0,4294967296) S: [0,4294967296)<br>
%x.06 = phi i64 [ %inc,
%for.body ], [ 1, %entry ]<br>
-->
{1,+,1}<nuw><nsw><%for.body>
U: [1,-9223372036854775808) S:
[1,-9223372036854775808)<span style="white-space:pre-wrap"> </span>Exits:
(-1 + (zext i32 %n to i64))<span style="white-space:pre-wrap"> </span>LoopDispositions:
{ %for.body: Computable }<br>
%inc = add nuw nsw i64 %x.06, 1<br>
-->
{2,+,1}<nuw><%for.body>
U: [2,0) S: [2,0)<span style="white-space:pre-wrap"> </span>Exits:
(zext i32 %n to i64)<span style="white-space:pre-wrap"> </span>LoopDispositions:
{ %for.body: Computable }<br>
Determining loop execution counts
for: @func<br>
Loop %for.body: backedge-taken
count is (-2 + (zext i32 %n to
i64))<nsw><br>
Loop %for.body: max backedge-taken
count is -2<br>
Loop %for.body: Predicated
backedge-taken count is (-2 +
(zext i32 %n to i64))<nsw><br>
Predicates:<br>
Loop %for.body: Trip multiple is 1</blockquote>
</div>
<div><br>
</div>
<div>Now, I was surprised by the max
backedge-taken count being -2, and I
suspect it is due to the
backedge-taken count being marked as
<nsw> instead of <nuw>.</div>
<div><br>
</div>
<div>Is that on purpose, is that a
bug, or is my analysis incorrect? I
am not sure where to fix that issue.</div>
</div>
</div>
</blockquote>
<br>
The backedge-taken count isn't nuw because
nsw/nuw markings aren't flow-sensitive:
there isn't any way to mark the trip count
as nuw without marking every computation of
`(long)n-2` as nuw.<br>
<br>
There's some code in
ScalarEvolution::howFarToZero to try to
refine the max backedge-taken count in some
cases, but it isn't very general. See <a class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531m_-6181515349790066455m_-6236410721431341120moz-txt-link-freetext" href="https://reviews.llvm.org/D28536" target="_blank">https://reviews.llvm.org/D28536</a>
.<br>
<br>
-Eli<br>
<pre class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531m_-6181515349790066455m_-6236410721431341120moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
</div>
</blockquote>
</div>
<br clear="all">
<div><br>
</div>
-- <br>
<div dir="ltr" class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531m_-6181515349790066455gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr"><b>Alexandre Isoard</b><br>
</div>
</div>
</blockquote>
<p><br>
</p>
<pre class="m_-645509329432923076m_6547919799618152574m_-5001341794010270531moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
</div>
</blockquote>
</div>
<br clear="all">
<div><br>
</div>
-- <br>
<div dir="ltr" class="m_-645509329432923076m_6547919799618152574gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr"><b>Alexandre Isoard</b><br>
</div>
</div>
</blockquote>
<p><br>
</p>
<pre class="m_-645509329432923076m_6547919799618152574moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
</div>
</blockquote>
</div>
<br clear="all">
<div><br>
</div>
-- <br>
<div dir="ltr" class="m_-645509329432923076gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr"><b>Alexandre Isoard</b><br>
</div>
</div>
</blockquote>
<p><br>
</p>
<pre class="m_-645509329432923076moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
</div>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><b>Alexandre Isoard</b><br></div></div>