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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong code generated from instcombine"
href="https://bugs.llvm.org/show_bug.cgi?id=46940">46940</a>
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</tr>
<tr>
<th>Summary</th>
<td>Wrong code generated from instcombine
</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>Linux
</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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kazu@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I'm seeing what appears to be miscompilation after:
<a href="https://github.com/llvm/llvm-project/commit/00472067c34ccbceb2fad4b905524f3c780bb7d5">https://github.com/llvm/llvm-project/commit/00472067c34ccbceb2fad4b905524f3c780bb7d5</a>
Consider:
target triple = "x86_64-grtev4-linux-gnu"
define i1 @foo(i32 %arg) {
entry:
%cmp1 = icmp eq i32 %arg, 37
br i1 %cmp1, label %bb_then, label %bb_else
bb_then:
call void @bar()
br label %bb_exit
bb_else:
%cmp2 = icmp slt i32 %arg, 17
br label %bb_exit
bb_exit:
%phi1 = phi i1 [ true, %bb_else ], [ false, %bb_then ]
%phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
%and1 = and i1 %phi2, %phi1
ret i1 %and1
}
declare void @bar()
Note that for every control flow, the return value is well defined.
arg == 37 -> return false
arg < 17 -> return true
otherwise -> return false
Then
$ ./build/bin/opt -instcombine -S ~/undef2.ll -o ~/undef2-opt.ll
modifies bb_exit like so:
bb_exit: ; preds = %bb_else, %bb_then
%phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
ret i1 %phi2
Notice undef in the phi node.
FWIW, immediately after SimplifyUsingControlFlow and replaceInstUsesWith
run on phi1, we have:
bb_exit: ; preds = %bb_else, %bb_then
%phi1 = phi i1 [ true, %bb_else ], [ false, %bb_then ]
%phi2 = phi i1 [ %cmp2, %bb_else ], [ undef, %bb_then ]
%0 = xor i1 %cmp1, true
%and1 = and i1 %phi2, %0
ret i1 %and1</pre>
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