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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 - Invalid code generation (aliasing between scalar and vector type)"
href="https://bugs.llvm.org/show_bug.cgi?id=32056">32056</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Invalid code generation (aliasing between scalar and vector type)
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>wenzel.jakob@epfl.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the simple snippet of C++ code below. It is a somewhat contrived
reduced example from a much larger codebase, where this issue was first
detected. In a situation where a vector type and a normal scalar type alias
(but where this is made very explicit to the compiler), Clang's aliasing
optimization generates incorrect code.
The class "A" is a double array with 8 entries, which is initialized to [0..8].
The entries can be accessed using Intel-specific vector types (__m256d) and
double variables. The begin() and end() functions expose a C++ iterator
interface to step through the values, and the main() function then prints them.
When compiled on my machine (with "clang++ test.cpp -std=c++11 -O3 -mavx -o
test"), I observe the following output
0.000000 1.000000 2.000000 3.000000 4.000000 0.000000 6.000000 7.000000
(^ note the zero value here, which
should be 5)
If compiled with -fno-strict-aliasing, the fifth entry is equal to 5.000000,
confirming that this is indeed an aliasing optimization-related issue.
In case this is an issue with the C++ code, I would be curious how I can signal
to Clang that an array type may alias with its underlying scalar type (it
appears to me that this is a fairly essential requirement in all sorts of
situations)
Thanks,
Wenzel
======= C++ snippet =========
#include <immintrin.h>
#include <stdio.h>
struct A {
A () {
a = _mm256_setr_pd(0.0, 1.0, 2.0, 3.0);
b = _mm256_setr_pd(4.0, 5.0, 6.0, 7.0);
}
const double *begin() { return c; }
const double *end() { return c+8; }
union {
struct { __m256d a, b; };
double c[8];
};
};
int main(int argc, char *argv[]) {
A a;
for (double value : a)
printf("%f ", value);
return 0;
}
==== LLVM IR Disassembly =====
(note the *undef* in the line
%12 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double undef)
)
define i32 @main(i32, i8** nocapture readnone) local_unnamed_addr #0 {
%3 = alloca %struct.Test, align 32
%4 = bitcast %struct.Test* %3 to i8*
call void @llvm.lifetime.start(i64 64, i8* nonnull %4) #3
%5 = getelementptr inbounds %struct.Test, %struct.Test* %3, i64 0, i32 0, i32
0, i32 0
store <4 x double> <double 0.000000e+00, double 1.000000e+00, double
2.000000e+00, double 3.000000e+00>, <4 x double>* %5, align 32, !tbaa !2
%6 = getelementptr inbounds %struct.Test, %struct.Test* %3, i64 0, i32 0, i32
0, i32 1
store <4 x double> <double 4.000000e+00, double 5.000000e+00, double
6.000000e+00, double 7.000000e+00>, <4 x double>* %6, align 32, !tbaa !6
%7 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double 0.000000e+00)
%8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double 1.000000e+00)
%9 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double 2.000000e+00)
%10 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double 3.000000e+00)
%11 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double 4.000000e+00)
%12 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double undef)
%13 = getelementptr inbounds %struct.Test, %struct.Test* %3, i64 0, i32 0,
i32 0, i32 0, i64 6
%14 = load double, double* %13, align 16, !tbaa !7
%15 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double %14)
%16 = getelementptr inbounds %struct.Test, %struct.Test* %3, i64 0, i32 0,
i32 0, i32 0, i64 7
%17 = load double, double* %16, align 8, !tbaa !7
%18 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str, i64 0, i64 0), double %17)
call void @llvm.lifetime.end(i64 64, i8* nonnull %4) #3
ret i32 0
}</pre>
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