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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - extractelement lowering and LangRef don't match"
href="https://bugs.llvm.org/show_bug.cgi?id=50382">50382</a>
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<th>Summary</th>
<td>extractelement lowering and LangRef don't match
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Target Description Classes
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nunoplopes@sapo.pt
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<th>CC</th>
<td>florian_hahn@apple.com, fwage73@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, regehr@cs.utah.edu, spatel+llvm@rotateright.com
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<p>
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<pre>LangRef says that extractelement with an OOB index yields poison.
However, it seems all targets lower an OOB index operation to a crash:
<a href="https://gcc.godbolt.org/z/9jxd9esnE">https://gcc.godbolt.org/z/9jxd9esnE</a>
This lowering would correspond to an UB semantics rather than poison on OOB.
define i32 @f(<4 x i32>* %x, i64 %idx) {
%lv = load <4 x i32>, <4 x i32>* %x
%r = extractelement <4 x i32> %lv, i64 %idx
ret i32 %r
}
x86:
mov eax, dword ptr [rdi + 4*rsi]
ret
We have 2 options:
1) Change LangRef so that extractelement gives UB on OOB index
2) Mask the index when lowering (unless it's known to be inbounds)
I don't know what the implications of 1) are. i.e., which front-ends produce it
and from which code and which guarantees those front-ends need.</pre>
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