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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong codegen for i1 vector truncating stores"
href="https://bugs.llvm.org/show_bug.cgi?id=35520">35520</a>
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<th>Summary</th>
<td>Wrong codegen for i1 vector truncating stores
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>Other
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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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Common Code Generator Code
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>uweigand@de.ibm.com
</td>
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<th>CC</th>
<td>llvm-bugs@justinbogner.com, llvm-bugs@lists.llvm.org, Matthew.Arsenault@amd.com, paulsson@linux.vnet.ibm.com
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<pre>The following simple test case generates completely broken code on SystemZ (run
with -mcpu=z13 to enable the vector ISA):
define i16 @test(<16 x i1> %src)
{
%res = bitcast <16 x i1> %src to i16
ret i16 %res
}
What happens is that in the ABI, the <16 x i1> is passed as <16 x i8>, so the
code would need to truncate to a real 16-bit vector, and reinterpret the result
as i16. For some reason, the code generator attempts to implement this as a
truncating store of a <16 x i8> in register to a <16 x i1> in memory, and then
loads that memory location as i16.
So far still OK, but the truncating store generates completely broken code: it
simply repeatedly stores all 16 bytes of the source vector to the same byte in
memory. Looking for the code that does this, I found this in
VectorLegalizer::ExpandStore (added by arsenm):
// FIXME: This is completely broken and inconsistent with ExpandLoad
// handling.
// For sub-byte element sizes, this ends up with 0 stride between elements,
// so the same element just gets re-written to the same location. There
seem
// to be tests explicitly testing for this broken behavior though. tests
// for this broken behavior.
which in fact matches exactly what I'm seeing.
What's going on here? Does this not happen on other platforms? Should I be
doing something different in the back-end, or should we try to fix this common
code issue after all? Any suggestions welcome ...</pre>
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