Subgroup broadcast of uniform value becomes a NOP (intel#59)

Author: AmyCodeplay

modules/compiler/vecz/source/transform/packetizer.cpp

@@ -1258,10 +1258,20 @@ Value *Packetizer::Impl::packetizeSubgroupBroadcast(Instruction *I) {
 
   IRBuilder<> B(buildAfter(CI, F));
 
-  auto *const idx = CI->getArgOperand(1);
+  auto *const src = CI->getArgOperand(0);
 
-  auto op = packetize(CI->getArgOperand(0));
+  auto op = packetize(src);
   PACK_FAIL_IF(!op);
+
+  // If the source operand happened to be a broadcast value already, we can use
+  // it directly.
+  if (op.info->numInstances == 0) {
+    IC.deleteInstructionLater(CI);
+    CI->replaceAllUsesWith(src);
+    return src;
+  }
+
+  auto *const idx = CI->getArgOperand(1);
   Value *val = nullptr;
   // Optimize the constant fixed-vector case, where we can choose the exact
   // subpacket to extract from directly.

modules/compiler/vecz/test/lit/llvm/subgroup_broadcast.ll

@@ -0,0 +1,46 @@
+; Copyright (C) Codeplay Software Limited
+;
+; Licensed under the Apache License, Version 2.0 (the "License") with LLVM
+; Exceptions; you may not use this file except in compliance with the License.
+; You may obtain a copy of the License at
+;
+;     https://github.com/codeplaysoftware/oneapi-construction-kit/blob/main/LICENSE.txt
+;
+; Unless required by applicable law or agreed to in writing, software
+; distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+; WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+; License for the specific language governing permissions and limitations
+; under the License.
+;
+; SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+; RUN: veczc -vecz-simd-width=4 -S < %s | FileCheck %s 
+
+target triple = "spir64-unknown-unknown"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare spir_func i32 @_Z16get_sub_group_idv()
+declare spir_func i32 @_Z22get_sub_group_local_idv()
+declare spir_func i32 @_Z19sub_group_broadcastij(i32, i32)
+
+; It makes sure broadcast still works when its source operand is uniform
+define spir_kernel void @sub_group_broadcast(i32 addrspace(1)* %in, i32 addrspace(1)* %out) {
+  %call = tail call spir_func i32 @_Z16get_sub_group_idv()
+  %arrayidx = getelementptr inbounds i32, i32 addrspace(1)* %in, i32 %call
+  %v = load i32, i32 addrspace(1)* %arrayidx, align 4
+  %broadcast = call spir_func i32 @_Z19sub_group_broadcastij(i32 %v, i32 0)
+  %idx = tail call spir_func i32 @_Z22get_sub_group_local_idv()
+  %arrayidx2 = getelementptr inbounds i32, i32 addrspace(1)* %out, i32 %idx
+  store i32 %broadcast, i32 addrspace(1)* %arrayidx2, align 4
+  ret void
+}
+
+; CHECK-LABEL: define spir_kernel void @__vecz_v4_sub_group_broadcast(
+; CHECK: [[LD:%.+]] = load i32, ptr addrspace(1) %{{.+}}, align 4
+; CHECK: [[INS:%.+]] = insertelement <4 x i32> poison, i32 [[LD]], i64 0
+; CHECK: [[BCAST:%.+]] = shufflevector <4 x i32> [[INS]], <4 x i32> poison, <4 x i32> zeroinitializer
+; CHECK: store <4 x i32> [[BCAST]], ptr addrspace(1) %out, align 4
+
+!opencl.ocl.version = !{!0}
+
+!0 = !{i32 3, i32 0}