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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Bitcast of <4 x i1> to i4 gives unexpected result"
href="http://llvm.org/bugs/show_bug.cgi?id=15215">15215</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Bitcast of <4 x i1> to i4 gives unexpected result
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>3.2
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</td>
</tr>
<tr>
<th>OS</th>
<td>other
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kai.trojahner@rtt.ag
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=9985" name="attach_9985" title="Demonstrator">attachment 9985</a> <a href="attachment.cgi?id=9985&action=edit" title="Demonstrator">[details]</a></span>
Demonstrator
On x86, the following two LLVM IR functions give different results.
define i32 @bad(<4 x i32> %input) {
entry:
%0 = trunc <4 x i32> %input to <4 x i1>
%1 = bitcast <4 x i1> %0 to i4
%2 = zext i4 %1 to i32
ret i32 %2
}
define i32 @good(<4 x i32> %input) {
entry:
%0 = trunc <4 x i32> %input to <4 x i1>
%1 = extractelement <4 x i1> %0, i32 0
%e1 = select i1 %1, i32 1, i32 0
%2 = extractelement <4 x i1> %0, i32 1
%e2 = select i1 %2, i32 2, i32 0
%3 = extractelement <4 x i1> %0, i32 2
%e3 = select i1 %3, i32 4, i32 0
%4 = extractelement <4 x i1> %0, i32 3
%e4 = select i1 %4, i32 8, i32 0
%5 = or i32 %e1, %e2
%6 = or i32 %5, %e3
%7 = or i32 %6, %e4
ret i32 %7
}
A demonstrator program illustrates the effect:
#include <emmintrin.h>
#include <stdio.h>
extern int bad(__m128i input);
extern int good(__m128i input);
int main() {
int i;
__m128i input[16];
input[0] = _mm_set_epi32(0, 0, 0, 0);
input[1] = _mm_set_epi32(0, 0, 0, -1);
input[2] = _mm_set_epi32(0, 0, -1, 0);
input[3] = _mm_set_epi32(0, 0, -1, -1);
input[4] = _mm_set_epi32(0, -1, 0, 0);
input[5] = _mm_set_epi32(0, -1, 0, -1);
input[6] = _mm_set_epi32(0, -1, -1, 0);
input[7] = _mm_set_epi32(0, -1, -1, -1);
input[8] = _mm_set_epi32(-1, 0, 0, 0);
input[9] = _mm_set_epi32(-1, 0, 0, -1);
input[10]= _mm_set_epi32(-1, 0, -1, 0);
input[11]= _mm_set_epi32(-1, 0, -1, -1);
input[12]= _mm_set_epi32(-1, -1, 0, 0);
input[13]= _mm_set_epi32(-1, -1, 0, -1);
input[14]= _mm_set_epi32(-1, -1, -1, 0);
input[15]= _mm_set_epi32(-1, -1, -1, -1);
for (i=0; i<16; i++) {
printf("permutation %i, good: %x: bad: %x\n",
i, good(input[i]), bad(input[i]));
}
return 0;
}
On my machine (Xeon 5160, SSE2, Windows 7, Cygwin, demonstrator compiled with
MSVC), the program yields:
permutation 0, good: 0: bad: 0
permutation 1, good: 1: bad: 0
permutation 2, good: 2: bad: 1
permutation 3, good: 3: bad: 1
permutation 4, good: 4: bad: 0
permutation 5, good: 5: bad: 0
permutation 6, good: 6: bad: 1
permutation 7, good: 7: bad: 1
permutation 8, good: 8: bad: 0
permutation 9, good: 9: bad: 0
permutation 10, good: a: bad: 1
permutation 11, good: b: bad: 1
permutation 12, good: c: bad: 0
permutation 13, good: d: bad: 0
permutation 14, good: e: bad: 1
permutation 15, good: f: bad: 1
Even more bizarrely, on a different machine (Xeon W3565, SSE4.1, Linux,
demonstrator compiled with GCC), the program yields:
permutation 0, good: 0: bad: 0
permutation 1, good: 1: bad: 1
permutation 2, good: 2: bad: 0
permutation 3, good: 3: bad: 1
permutation 4, good: 4: bad: 0
permutation 5, good: 5: bad: 1
permutation 6, good: 6: bad: 0
permutation 7, good: 7: bad: 1
permutation 8, good: 8: bad: 0
permutation 9, good: 9: bad: 1
permutation 10, good: a: bad: 0
permutation 11, good: b: bad: 1
permutation 12, good: c: bad: 0
permutation 13, good: d: bad: 1
permutation 14, good: e: bad: 0
permutation 15, good: f: bad: 1</pre>
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