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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [OpenCL] Invalid optimization of subgroup intrinsics"
href="https://bugs.llvm.org/show_bug.cgi?id=46199">46199</a>
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<th>Summary</th>
<td>[OpenCL] Invalid optimization of subgroup intrinsics
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>OpenCL
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>piotr.fusik@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>anastasia.stulova@arm.com, llvm-bugs@lists.llvm.org
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<div>
<pre>The following OpenCL code:
kernel void test(global int *data)
{
uint id = (uint) get_global_id(0);
if (id < 4)
data[id] = sub_group_elect();
else
data[id] = sub_group_elect();
}
when compiled with the current master
(f2c97656644e783622a6e60fe452b41ffe0f1d18) as follows: clang -cl-std=CL2.0
-include opencl-c.h -S -emit-llvm sub_group_elect_opt.cl
has an invalid optimization of combining the branches:
; Function Attrs: convergent norecurse nounwind uwtable
define dso_local spir_kernel void @test(i32* nocapture %data)
local_unnamed_addr #0 !kernel_arg_addr_space !4 !kernel_arg_access_qual !5
!kernel_arg_type !6 !kernel_arg_base_type !6 !kernel_arg_type_qual !7 {
entry:
%call = tail call i64 @"?get_global_id@@$$J0YAKI@Z"(i32 0) #3
%call2 = tail call i32 @"?sub_group_elect@@$$J0YAHXZ"() #4
%idxprom = and i64 %call, 4294967295
%arrayidx = getelementptr inbounds i32, i32* %data, i64 %idxprom
store i32 %call2, i32* %arrayidx, align 4, !tbaa !8
ret void
}
sub_group_elect is one of the many functions added in
<a href="https://reviews.llvm.org/D79781">https://reviews.llvm.org/D79781</a> that perform implicit communication within a
subgroup, i.e. threads implemented as SIMD on GPU. All the added functions are
affected.
<a href="https://reviews.llvm.org/D68994">https://reviews.llvm.org/D68994</a> is a proposal of addressing this problem with
the `convergent` attribute.</pre>
</div>
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