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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed optimization with struct only memcpy'ed"
href="http://llvm.org/bugs/show_bug.cgi?id=16948">16948</a>
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<th>Summary</th>
<td>Missed optimization with struct only memcpy'ed
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>sharparrow1@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>chandlerc@gmail.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Given the following (simplified) C++ code:
#include <new>
struct X { void* a; long b; void* c; X() : a(0), b(0) {} };
void f(struct X *a) { X x; new (a) X(x); }
It currently compiles to the following IR:
define void @_Z1fP1X(%struct.X* %a) #0 {
entry:
%x = alloca %struct.X, align 8
%x.0.cast = bitcast %struct.X* %x to i8*
call void @llvm.memset.p0i8.i64(i8* %x.0.cast, i8 0, i64 16, i32 8, i1 false)
%new.isnull = icmp eq %struct.X* %a, null
br i1 %new.isnull, label %new.cont, label %new.notnull
new.notnull: ; preds = %entry
%0 = bitcast %struct.X* %a to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %x.0.cast, i64 24, i32 8, i1
false), !tbaa.struct !0
br label %new.cont
new.cont: ; preds = %new.notnull,
%entry
ret void
}
I think SROA should be able to eliminate the alloca and the memcpy; I'm not
sure why it doesn't.</pre>
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