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<div class="moz-cite-prefix">On 12/07/2014 07:57 PM, Benjamin
Poulain wrote:<br>
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<blockquote cite="mid:548521B8.8080503@webkit.org" type="cite">
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I looked into this today. <br>
<br>
Following your advice, I made something local to loop unrolling
(patch attached). What this does is look if the previous block
already checks for a trip count of zero.<br>
</blockquote>
Given the canonicalization introduced by loop rotation, this might
be sufficient for many interesting cases. You're basically taking
advantage of the fact you know this is being inserted in a loop
preheader. <br>
<blockquote cite="mid:548521B8.8080503@webkit.org" type="cite"> <br>
This seems a bit hackish, it works for basic for() and while()
loops. Is there an analysis pass I could use to track properties
of values? I thought ScalarEvolution could help but it seems to
only track that value inside the loop.<br>
</blockquote>
There is LazyValueInfo which is used by JumpThreading, but I suspect
that's too expensive for this case. There's also isKnownNonNull in
ValueTracking, but that's not path sensitive. (It doesn't hurt to
leverage that anyway.)<br>
<br>
What you have seems like a reasonable starting point. If you want
to put it up on phabricator, I have a couple of small comments, but
that's about it. Opening a new thread is also a good way to get
other eyes on the revised patch. <br>
<br>
Structure wise, I think the following would be clearer for the
actual test:<br>
Value *LCmp = B.CreateIsNotNull(ModVal, "lcmp.mod");<br>
Value *OverflowCheck = IsKnownNonZero(TripCount, PH) ?
ConstantInt::getFalse(i1) : B.CreateIsNull(TripCount,
"lcmp.overflow");<br>
Value *BranchVal = B.CreateOr(OverflowCheck, LCmp, "lcmp.or");<br>
<br>
<br>
<blockquote cite="mid:548521B8.8080503@webkit.org" type="cite"> <br>
Benjamin<br>
<br>
<div class="moz-cite-prefix">On 12/2/14, 3:42 PM, Philip Reames
wrote:<br>
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<blockquote cite="mid:547E4E64.5080100@philipreames.com"
type="cite">
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Benjamin,<br>
<br>
This doesn't really feel like the right approach to the
problem. Adding another run of GVN after every unrolling
opportunity is a fairly heavy weight hammer. Have you looked to
see if there's a simple change you could make to the unroller
itself to handle this case? I haven't looked at the code, but
my suspicion is there would be.<br>
<br>
Philip<br>
<br>
<div class="moz-cite-prefix">On 11/29/2014 12:59 PM, Benjamin
Poulain wrote:<br>
</div>
<blockquote cite="mid:547A33AB.1040105@webkit.org" type="cite">Hi,
<br>
<br>
I noticed the optimized code generated for loops often include
redundant checks that the size is not zero. <br>
<br>
For example, code like this: <br>
unsigned total = 0; <br>
for (unsigned i = 0; i < size; ++i) { ... <br>
<br>
when optimized, generate the following prologue: <br>
entry: <br>
%cmp4 = icmp eq i32 %size, 0 <br>
br i1 %cmp4, label %for.end, label %for.body.lr.ph <br>
<br>
for.body.lr.ph: ; preds =
%entry <br>
%0 = add i32 %size, -1 <br>
%xtraiter = and i32 %size, 3 <br>
%lcmp.mod = icmp ne i32 %xtraiter, 0 <br>
%lcmp.overflow = icmp eq i32 %size, 0 <br>
%lcmp.or = or i1 %lcmp.overflow, %lcmp.mod <br>
br i1 %lcmp.or, label %for.body.prol, label
%for.body.lr.ph.split <br>
<br>
Notice the redundant test for "icmp eq i32 %size, 0". When
compiled to target, we have one redundant check per loop that
can never be true. <br>
<br>
The extra check for size==0 comes from LoopUnroll. It is never
optimized out because LoopUnrollPass is run after the passes
that could eliminate redundant conditions. <br>
<br>
I have attached a patch that fixes the problem. For every pass
of LoopUnroll, I added a GVN pass to remove every redundant
conditions and branches. My patch is without test, I need
guidance on how to properly test this. <br>
<br>
Benjamin <br>
<br>
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