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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 --- - branch condition should be known from llvm.assume"
href="https://llvm.org/bugs/show_bug.cgi?id=31929">31929</a>
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</tr>
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<th>Summary</th>
<td>branch condition should be known from llvm.assume
</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>
<tr>
<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>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<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 the request in:
<a href="https://reviews.llvm.org/D28204">https://reviews.llvm.org/D28204</a>
These are purposely minimized to be instcombine tests. If we decide that the
functionality doesn't belong there, then the tests may require additional
instructions to thwart other transformations (phi -> select) and/or trigger
from a different pass (LICM?).
-----------------------------------------------------------------------------
declare void @llvm.assume(i1)
define i8 @assume_guarantees_notnull(i8* %x) {
entry:
%notnull = icmp ne i8* %x, null
tail call void @llvm.assume(i1 %notnull)
br i1 %notnull, label %if, label %endif <--- condition is known true
if:
%ld = load i8, i8* %x
br label %endif
endif:
%sel = phi i8 [ 0, %entry ], [ %ld, %if ]
ret i8 %sel
}
Depending on how we want to solve this, it may be relevant to use a non-pointer
value in the icmp:
define i8 @assume_guarantees_notnull(i8 %y, i8* %x) {
entry:
%notnull = icmp ne i8 %y, 0 <--- type shouldn't matter?
tail call void @llvm.assume(i1 %notnull)
br i1 %notnull, label %if, label %endif
if:
%ld = load i8, i8* %x, align 1
br label %endif
endif:
%sel = phi i8 [ 0, %entry ], [ %ld, %if ]
ret i8 %sel
}
----------------------------------------------------------------------------
For reference, we handle the 'select' variant of these examples in instcombine
by calling computeKnownBits when we know there might be an assumption that
defines the condition. That was <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missed simplification from llvm.assume"
href="show_bug.cgi?id=31512">bug 31512</a> / <a href="https://reviews.llvm.org/rL291915">https://reviews.llvm.org/rL291915</a> .</pre>
</div>
</p>
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