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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - llvm does not narrow overly-wide memcpys with !tbaa.struct metadata"
href="https://bugs.llvm.org/show_bug.cgi?id=37023">37023</a>
</td>
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<th>Summary</th>
<td>llvm does not narrow overly-wide memcpys with !tbaa.struct metadata
</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>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Testcase:
struct alignas(256) X { int n; };
X f(X x) { return x; }
Clang generates:
%struct.X = type { i32, [252 x i8] }
define dso_local void @_Z1f1X(%struct.X* noalias nocapture sret %agg.result,
%struct.X* byval nocapture readonly align 256 %x) local_unnamed_addr #0 {
entry:
%0 = bitcast %struct.X* %agg.result to i8*
%1 = bitcast %struct.X* %x to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 256 %0, i8* nonnull align 256
%1, i64 256, i1 false), !tbaa.struct !2
ret void
}
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture
readonly, i64, i1) #1
attributes #0 = { nounwind uwtable
"correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
"no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "no-trapping-math"="false"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = { argmemonly nounwind }
and (eg) the x86 backend then produces a 256-byte memcpy. This cannot be the
fastest way of copying 4 bytes with 252 bytes of padding :)</pre>
</div>
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