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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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<br>
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