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<div class="moz-cite-prefix">On 8/15/2018 12:21 PM, Alexandre Isoard
via llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CANLM5Lc30_y9s-z=_aAuvG7kanBKnAUfXqUXp1qkhPoeUJbXFg@mail.gmail.com">
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<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"> </span>Exits:
(-1 + (zext i32 %n to i64))<span style="white-space:pre"> </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"> </span>Exits: (zext
i32 %n to i64)<span style="white-space:pre"> </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="moz-txt-link-freetext" href="https://reviews.llvm.org/D28536">https://reviews.llvm.org/D28536</a> .<br>
<br>
-Eli<br>
<pre class="moz-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>
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