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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - deduce icmp result from llvm.assume"
href="https://llvm.org/bugs/show_bug.cgi?id=31596">31596</a>
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<th>Summary</th>
<td>deduce icmp result from llvm.assume
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Transformation Utilities
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Filing this based on a question from Daniel Berlin:
<a href="http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170109/418024.html">http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170109/418024.html</a>
declare void @llvm.assume(i1)
define i1 @deduce_icmp_from_assume(i8 %a, i8 %b) {
%eqcond = icmp eq i8 %a, %b
call void @llvm.assume(i1 %eqcond)
%ltcond = icmp slt i8 %a, %b
ret i1 %ltcond
}
------------------------------------------------------------------------------
Some pass (don't know which one yet because the line between analysis and the
compile-time cost of that analysis is not clear) should be able to simplify
this to return 'false', but nothing in the -O2 pipeline as of r291443 handles
this.
FWIW, we already call computeKnownBits in InstCombiner::visitReturnInst() for
the stated purpose of checking assumptions:
// There might be assume intrinsics dominating this return that completely
// determine the value. If so, constant fold it.
...so this particular case could be handled by adding to the pattern-matching
logic in computeKnownBitsFromAssume() ( see <a href="https://reviews.llvm.org/D28485">https://reviews.llvm.org/D28485</a> ),
but of course that's not the most general solution.
More typically, we'd expect the 2nd icmp result to be used in a branch or
select, so calling value tracking (should computeKnownBitsFromAssume be raised
into the value tracking API?) when visiting those insts in InstCombine might be
a better way. I have a patch proposal for the select variant here:
<a href="https://reviews.llvm.org/D28337">https://reviews.llvm.org/D28337</a>
There may be no good solution because the design of the llvm.assume intrinsic
itself just isn't ideal ( <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - refactor metadata propagation for consistency and completeness"
href="show_bug.cgi?id=31518#c10">bug 31518, comment 10</a> ). Alternative suggestions are
that we should convert it to a form more like llvm.expect:
<a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missed simplification from llvm.assume"
href="show_bug.cgi?id=31512#c4">bug 31512, comment 4</a>
That could lead to a lower-to-metadata pass such as "-lower-expect", so we
wouldn't have the problem of optimization intrinsics interfering with
optimization as noted by Eli here:
<a href="https://reviews.llvm.org/D27114">https://reviews.llvm.org/D27114</a></pre>
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