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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - instcombine forms illegal <2 x i64> multiply with trunc/zext"
href="https://bugs.llvm.org/show_bug.cgi?id=40032">40032</a>
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<th>Summary</th>
<td>instcombine forms illegal <2 x i64> multiply with trunc/zext
</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>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>efriedma@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>husseydevin@gmail.com, llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
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<p>
<div>
<pre>C testcase:
#include <arm_neon.h>
typedef int64x2_t U64x2;
typedef int32x2_t U32x2;
typedef int32x4_t U32x4;
U64x2 f(U64x2 top, U64x2 bot) {
U32x2 d2 = vmovn_u64(bot);
U32x2 d5 = vmovn_u64(top);
d5 = vmla_u32(d5, d2, d2);
return vshll_n_u32(d5, 0);
}
On ARM, we currently end up scalarizing the multiply, instead of using the
vector operation written in the source. (Even if we improve the expansion of
general <2 x i64 multiplies on ARM, it'll still be more expensive than the
original code. Other targets have similar issues.)
I think instcombine is being too aggressive here; I'm not sure the backend can
reasonably recover the original multiply in general.
IR testcase (for opt -instcombine):
define <2 x i64> @f(<2 x i64> %top, <2 x i64> %bot) {
entry:
%vmovn.i = trunc <2 x i64> %bot to <2 x i32>
%vmovn.i9 = trunc <2 x i64> %top to <2 x i32>
%mul.i = mul <2 x i32> %vmovn.i, %vmovn.i
%add.i = add <2 x i32> %mul.i, %vmovn.i9
%0 = zext <2 x i32> %add.i to <2 x i64>
ret <2 x i64> %0
}
Filing based on discussion in <a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang generates slow U64x2 multiply code for NEON"
href="show_bug.cgi?id=39967">bug 39967</a>.</pre>
</div>
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