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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - print a remark when an llvm.assume has a false param"
href="https://llvm.org/bugs/show_bug.cgi?id=31882">31882</a>
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<th>Summary</th>
<td>print a remark when an llvm.assume has a false param
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Filing this based on Daniel's example in:
<a href="https://reviews.llvm.org/D29404">https://reviews.llvm.org/D29404</a>
declare void @llvm.assume(i1) nounwind
define i32 @assume_false(i32 %p) {
entry:
%cmp = icmp eq i32 %p, 42
call void @llvm.assume(i1 %cmp)
br i1 %cmp, label %bb2, label %bb3
bb2:
%cmp3 = icmp eq i32 %p, 43
call void @llvm.assume(i1 %cmp3)
ret i32 15
bb3:
ret i32 17
}
-----------------------------------------------------------------------
-instsimplify tells us that the 2nd icmp is false via isKnownNonEqual() which
calls computeKnownBits() which calls computeKnownBitsFromAssume():
$ ./opt -instsimplify badass.ll -S
...
declare void @llvm.assume(i1) #0
define i32 @assume_false(i32 %p) {
entry:
%cmp = icmp eq i32 %p, 42
call void @llvm.assume(i1 %cmp)
br i1 %cmp, label %bb2, label %bb3
bb2:
call void @llvm.assume(i1 false) <--- oops...alternative facts!
ret i32 15
bb3:
ret i32 17
}
--------------------------------------------------------------------------
As mentioned in D29404, we can't print a strong warning, but we can at least
print a weasel-worded remark because it's likely that either the program or the
compiler has a bug when a condition that was supposed to be true was proven
false.</pre>
</div>
</p>
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