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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Teach instruction scheduling and other reordering passes about LFENCE speculation barrier semantics"
href="https://bugs.llvm.org/show_bug.cgi?id=45167">45167</a>
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<th>Summary</th>
<td>Teach instruction scheduling and other reordering passes about LFENCE speculation barrier semantics
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<th>Product</th>
<td>new-bugs
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>zbrid@google.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
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<pre>Problem
We added speculation barrier semantics to LFENCE, but haven't taught
instruction scheduling (or other reordering passes) about that semantic.
The compiler may reorder LFENCEs in a basic block in a way that may not be
correct, whether reordering branch instructions and LFENCEs or loads/stores and
LFENCEs.
Possible directions to go in for a test case:
A MIR basic block with LFENCEs interleaved with multiple `jCC` instructions --
it is not valid to sink or hoist the LFENCEs.
Another posible testing strategy would be to interleave stores with an LFENCE.
For example, store -> LFENCE -> load: historically changing this to LFENCE ->
store -> load is okay, but from the perspective of speculative execution there
is a difference between the two orderings.</pre>
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