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[flang][OpenMP] Allocate reduction init temps on the stack for GPUs #146667

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Jul 4, 2025
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[flang][OpenMP] Allocate reduction init temps on the stack for GPUs #146667

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860bd59
[flang][OpenMP] Allocate `reduction` init temps on the stack for GPUs
ergawy Jul 2, 2025
4cdd257
add todo
ergawy Jul 3, 2025
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47 changes: 31 additions & 16 deletions flang/lib/Lower/Support/PrivateReductionUtils.cpp
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Expand Up @@ -502,22 +502,37 @@ void PopulateInitAndCleanupRegionsHelper::initAndCleanupBoxedArray(

// Allocating on the heap in case the whole reduction/privatization is nested
// inside of a loop
auto [temp, needsDealloc] = createTempFromMold(loc, builder, source);
// if needsDealloc isn't statically false, add cleanup region. Always
// do this for allocatable boxes because they might have been re-allocated
// in the body of the loop/parallel region

std::optional<int64_t> cstNeedsDealloc = fir::getIntIfConstant(needsDealloc);
assert(cstNeedsDealloc.has_value() &&
"createTempFromMold decides this statically");
if (cstNeedsDealloc.has_value() && *cstNeedsDealloc != false) {
mlir::OpBuilder::InsertionGuard guard(builder);
createCleanupRegion(converter, loc, argType, cleanupRegion, sym,
isDoConcurrent);
} else {
assert(!isAllocatableOrPointer &&
"Pointer-like arrays must be heap allocated");
}
auto temp = [&]() {
bool shouldAllocateOnStack = false;

// On the GPU, always allocate on the stack since heap allocatins are very
// expensive.
if (auto offloadMod = llvm::dyn_cast<mlir::omp::OffloadModuleInterface>(
*builder.getModule()))
shouldAllocateOnStack = offloadMod.getIsGPU();

if (shouldAllocateOnStack)
return createStackTempFromMold(loc, builder, source);

auto [temp, needsDealloc] = createTempFromMold(loc, builder, source);
// if needsDealloc isn't statically false, add cleanup region. Always
// do this for allocatable boxes because they might have been re-allocated
// in the body of the loop/parallel region

std::optional<int64_t> cstNeedsDealloc =
fir::getIntIfConstant(needsDealloc);
assert(cstNeedsDealloc.has_value() &&
"createTempFromMold decides this statically");
if (cstNeedsDealloc.has_value() && *cstNeedsDealloc != false) {
mlir::OpBuilder::InsertionGuard guard(builder);
createCleanupRegion(converter, loc, argType, cleanupRegion, sym,
isDoConcurrent);
} else {
assert(!isAllocatableOrPointer &&
"Pointer-like arrays must be heap allocated");
}
return temp;
}();

// Put the temporary inside of a box:
// hlfir::genVariableBox doesn't handle non-default lower bounds
Expand Down
166 changes: 88 additions & 78 deletions flang/test/Lower/OpenMP/parallel-reduction-array.f90
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@@ -1,5 +1,8 @@
! RUN: bbc -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s
! RUN: %flang_fc1 -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s
! RUN: bbc -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s --check-prefix=CPU
! RUN: %flang_fc1 -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s --check-prefix=CPU

! RUN: bbc -emit-hlfir -fopenmp -fopenmp-is-target-device -fopenmp-is-gpu -o - %s 2>&1 | FileCheck %s --check-prefix=GPU
! RUN: %flang_fc1 -triple amdgcn-amd-amdhsa -emit-hlfir -fopenmp -fopenmp-is-target-device -o - %s 2>&1 | FileCheck %s --check-prefix=GPU

program reduce
integer, dimension(3) :: i = 0
Expand All @@ -13,81 +16,88 @@ program reduce
print *,i
end program

! CHECK-LABEL: omp.declare_reduction @add_reduction_byref_box_3xi32 : !fir.ref<!fir.box<!fir.array<3xi32>>> alloc {
! CHECK: %[[VAL_8:.*]] = fir.alloca !fir.box<!fir.array<3xi32>>
! CHECK: omp.yield(%[[VAL_8]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CHECK-LABEL: } init {
! CHECK: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>, %[[ALLOC:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CHECK: %[[VAL_2:.*]] = arith.constant 0 : i32
! CHECK: %[[VAL_3:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_4:.*]] = arith.constant 3 : index
! CHECK: %[[VAL_5:.*]] = fir.shape %[[VAL_4]] : (index) -> !fir.shape<1>
! CHECK: %[[VAL_1:.*]] = fir.allocmem !fir.array<3xi32> {bindc_name = ".tmp", uniq_name = ""}
! CHECK: %[[VAL_6:.*]]:2 = hlfir.declare %[[VAL_1]](%[[VAL_5]]) {uniq_name = ".tmp"} : (!fir.heap<!fir.array<3xi32>>,
! CHECK: %[[TRUE:.*]] = arith.constant true
! CPU-LABEL: omp.declare_reduction @add_reduction_byref_box_3xi32 : !fir.ref<!fir.box<!fir.array<3xi32>>> alloc {
! CPU: %[[VAL_8:.*]] = fir.alloca !fir.box<!fir.array<3xi32>>
! CPU: omp.yield(%[[VAL_8]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CPU-LABEL: } init {
! CPU: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>, %[[ALLOC:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CPU: %[[VAL_2:.*]] = arith.constant 0 : i32
! CPU: %[[VAL_3:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_4:.*]] = arith.constant 3 : index
! CPU: %[[VAL_5:.*]] = fir.shape %[[VAL_4]] : (index) -> !fir.shape<1>
! CPU: %[[VAL_1:.*]] = fir.allocmem !fir.array<3xi32> {bindc_name = ".tmp", uniq_name = ""}
! CPU: %[[VAL_6:.*]]:2 = hlfir.declare %[[VAL_1]](%[[VAL_5]]) {uniq_name = ".tmp"} : (!fir.heap<!fir.array<3xi32>>,
! CPU: %[[TRUE:.*]] = arith.constant true
!fir.shape<1>) -> (!fir.heap<!fir.array<3xi32>>, !fir.heap<!fir.array<3xi32>>)
! CHECK: %[[C0:.*]] = arith.constant 0 : index
! CHECK: %[[DIMS:.*]]:3 = fir.box_dims %[[VAL_3]], %[[C0]] : (!fir.box<!fir.array<3xi32>>, index) -> (index, index, index)
! CHECK: %[[SHIFT:.*]] = fir.shape_shift %[[DIMS]]#0, %[[DIMS]]#1 : (index, index) -> !fir.shapeshift<1>
! CHECK: %[[VAL_7:.*]] = fir.embox %[[VAL_6]]#0(%[[SHIFT]]) : (!fir.heap<!fir.array<3xi32>>, !fir.shapeshift<1>) -> !fir.box<!fir.array<3xi32>>
! CHECK: hlfir.assign %[[VAL_2]] to %[[VAL_7]] : i32, !fir.box<!fir.array<3xi32>>
! CHECK: fir.store %[[VAL_7]] to %[[ALLOC]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: omp.yield(%[[ALLOC]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CHECK: } combiner {
! CHECK: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>, %[[VAL_1:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CHECK: %[[VAL_2:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_3:.*]] = fir.load %[[VAL_1]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[C1:.*]] = arith.constant 1 : index
! CHECK: %[[C3:.*]] = arith.constant 3 : index
! CHECK: %[[SHAPE_SHIFT:.*]] = fir.shape_shift %[[C1]], %[[C3]] : (index, index) -> !fir.shapeshift<1>
! CHECK: %[[C1_0:.*]] = arith.constant 1 : index
! CHECK: fir.do_loop %[[VAL_8:.*]] = %[[C1_0]] to %[[C3]] step %[[C1_0]] unordered {
! CHECK: %[[VAL_9:.*]] = fir.array_coor %[[VAL_2]](%[[SHAPE_SHIFT]]) %[[VAL_8]] : (!fir.box<!fir.array<3xi32>>, !fir.shapeshift<1>, index) -> !fir.ref<i32>
! CHECK: %[[VAL_10:.*]] = fir.array_coor %[[VAL_3]](%[[SHAPE_SHIFT]]) %[[VAL_8]] : (!fir.box<!fir.array<3xi32>>, !fir.shapeshift<1>, index) -> !fir.ref<i32>
! CHECK: %[[VAL_11:.*]] = fir.load %[[VAL_9]] : !fir.ref<i32>
! CHECK: %[[VAL_12:.*]] = fir.load %[[VAL_10]] : !fir.ref<i32>
! CHECK: %[[VAL_13:.*]] = arith.addi %[[VAL_11]], %[[VAL_12]] : i32
! CHECK: fir.store %[[VAL_13]] to %[[VAL_9]] : !fir.ref<i32>
! CHECK: }
! CHECK: omp.yield(%[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CHECK: } cleanup {
! CHECK: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CHECK: %[[VAL_1:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_2:.*]] = fir.box_addr %[[VAL_1]] : (!fir.box<!fir.array<3xi32>>) -> !fir.ref<!fir.array<3xi32>>
! CHECK: %[[VAL_3:.*]] = fir.convert %[[VAL_2]] : (!fir.ref<!fir.array<3xi32>>) -> i64
! CHECK: %[[VAL_4:.*]] = arith.constant 0 : i64
! CHECK: %[[VAL_5:.*]] = arith.cmpi ne, %[[VAL_3]], %[[VAL_4]] : i64
! CHECK: fir.if %[[VAL_5]] {
! CHECK: %[[VAL_6:.*]] = fir.convert %[[VAL_2]] : (!fir.ref<!fir.array<3xi32>>) -> !fir.heap<!fir.array<3xi32>>
! CHECK: fir.freemem %[[VAL_6]] : !fir.heap<!fir.array<3xi32>>
! CHECK: }
! CHECK: omp.yield
! CHECK: }
! CPU: %[[C0:.*]] = arith.constant 0 : index
! CPU: %[[DIMS:.*]]:3 = fir.box_dims %[[VAL_3]], %[[C0]] : (!fir.box<!fir.array<3xi32>>, index) -> (index, index, index)
! CPU: %[[SHIFT:.*]] = fir.shape_shift %[[DIMS]]#0, %[[DIMS]]#1 : (index, index) -> !fir.shapeshift<1>
! CPU: %[[VAL_7:.*]] = fir.embox %[[VAL_6]]#0(%[[SHIFT]]) : (!fir.heap<!fir.array<3xi32>>, !fir.shapeshift<1>) -> !fir.box<!fir.array<3xi32>>
! CPU: hlfir.assign %[[VAL_2]] to %[[VAL_7]] : i32, !fir.box<!fir.array<3xi32>>
! CPU: fir.store %[[VAL_7]] to %[[ALLOC]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: omp.yield(%[[ALLOC]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CPU: } combiner {
! CPU: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>, %[[VAL_1:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CPU: %[[VAL_2:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_3:.*]] = fir.load %[[VAL_1]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[C1:.*]] = arith.constant 1 : index
! CPU: %[[C3:.*]] = arith.constant 3 : index
! CPU: %[[SHAPE_SHIFT:.*]] = fir.shape_shift %[[C1]], %[[C3]] : (index, index) -> !fir.shapeshift<1>
! CPU: %[[C1_0:.*]] = arith.constant 1 : index
! CPU: fir.do_loop %[[VAL_8:.*]] = %[[C1_0]] to %[[C3]] step %[[C1_0]] unordered {
! CPU: %[[VAL_9:.*]] = fir.array_coor %[[VAL_2]](%[[SHAPE_SHIFT]]) %[[VAL_8]] : (!fir.box<!fir.array<3xi32>>, !fir.shapeshift<1>, index) -> !fir.ref<i32>
! CPU: %[[VAL_10:.*]] = fir.array_coor %[[VAL_3]](%[[SHAPE_SHIFT]]) %[[VAL_8]] : (!fir.box<!fir.array<3xi32>>, !fir.shapeshift<1>, index) -> !fir.ref<i32>
! CPU: %[[VAL_11:.*]] = fir.load %[[VAL_9]] : !fir.ref<i32>
! CPU: %[[VAL_12:.*]] = fir.load %[[VAL_10]] : !fir.ref<i32>
! CPU: %[[VAL_13:.*]] = arith.addi %[[VAL_11]], %[[VAL_12]] : i32
! CPU: fir.store %[[VAL_13]] to %[[VAL_9]] : !fir.ref<i32>
! CPU: }
! CPU: omp.yield(%[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CPU: } cleanup {
! CPU: ^bb0(%[[VAL_0:.*]]: !fir.ref<!fir.box<!fir.array<3xi32>>>):
! CPU: %[[VAL_1:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_2:.*]] = fir.box_addr %[[VAL_1]] : (!fir.box<!fir.array<3xi32>>) -> !fir.ref<!fir.array<3xi32>>
! CPU: %[[VAL_3:.*]] = fir.convert %[[VAL_2]] : (!fir.ref<!fir.array<3xi32>>) -> i64
! CPU: %[[VAL_4:.*]] = arith.constant 0 : i64
! CPU: %[[VAL_5:.*]] = arith.cmpi ne, %[[VAL_3]], %[[VAL_4]] : i64
! CPU: fir.if %[[VAL_5]] {
! CPU: %[[VAL_6:.*]] = fir.convert %[[VAL_2]] : (!fir.ref<!fir.array<3xi32>>) -> !fir.heap<!fir.array<3xi32>>
! CPU: fir.freemem %[[VAL_6]] : !fir.heap<!fir.array<3xi32>>
! CPU: }
! CPU: omp.yield
! CPU: }

! CPU-LABEL: func.func @_QQmain()
! CPU: %[[VAL_0:.*]] = fir.address_of(@_QFEi) : !fir.ref<!fir.array<3xi32>>
! CPU: %[[VAL_1:.*]] = arith.constant 3 : index
! CPU: %[[VAL_2:.*]] = fir.shape %[[VAL_1]] : (index) -> !fir.shape<1>
! CPU: %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_0]](%[[VAL_2]]) {uniq_name = "_QFEi"} : (!fir.ref<!fir.array<3xi32>>, !fir.shape<1>) -> (!fir.ref<!fir.array<3xi32>>, !fir.ref<!fir.array<3xi32>>)
! CPU: %[[VAL_4:.*]] = fir.embox %[[VAL_3]]#0(%[[VAL_2]]) : (!fir.ref<!fir.array<3xi32>>, !fir.shape<1>) -> !fir.box<!fir.array<3xi32>>
! CPU: %[[VAL_5:.*]] = fir.alloca !fir.box<!fir.array<3xi32>>
! CPU: fir.store %[[VAL_4]] to %[[VAL_5]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: omp.parallel reduction(byref @add_reduction_byref_box_3xi32 %[[VAL_5]] -> %[[VAL_6:.*]] : !fir.ref<!fir.box<!fir.array<3xi32>>>) {
! CPU: %[[VAL_7:.*]]:2 = hlfir.declare %[[VAL_6]] {uniq_name = "_QFEi"} : (!fir.ref<!fir.box<!fir.array<3xi32>>>) -> (!fir.ref<!fir.box<!fir.array<3xi32>>>, !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CPU: %[[VAL_8:.*]] = arith.constant 1 : i32
! CPU: %[[VAL_9:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_10:.*]] = arith.constant 1 : index
! CPU: %[[VAL_11:.*]] = hlfir.designate %[[VAL_9]] (%[[VAL_10]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CPU: hlfir.assign %[[VAL_8]] to %[[VAL_11]] : i32, !fir.ref<i32>
! CPU: %[[VAL_12:.*]] = arith.constant 2 : i32
! CPU: %[[VAL_13:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_14:.*]] = arith.constant 2 : index
! CPU: %[[VAL_15:.*]] = hlfir.designate %[[VAL_13]] (%[[VAL_14]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CPU: hlfir.assign %[[VAL_12]] to %[[VAL_15]] : i32, !fir.ref<i32>
! CPU: %[[VAL_16:.*]] = arith.constant 3 : i32
! CPU: %[[VAL_17:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CPU: %[[VAL_18:.*]] = arith.constant 3 : index
! CPU: %[[VAL_19:.*]] = hlfir.designate %[[VAL_17]] (%[[VAL_18]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CPU: hlfir.assign %[[VAL_16]] to %[[VAL_19]] : i32, !fir.ref<i32>
! CPU: omp.terminator
! CPU: }

! CHECK-LABEL: func.func @_QQmain()
! CHECK: %[[VAL_0:.*]] = fir.address_of(@_QFEi) : !fir.ref<!fir.array<3xi32>>
! CHECK: %[[VAL_1:.*]] = arith.constant 3 : index
! CHECK: %[[VAL_2:.*]] = fir.shape %[[VAL_1]] : (index) -> !fir.shape<1>
! CHECK: %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_0]](%[[VAL_2]]) {uniq_name = "_QFEi"} : (!fir.ref<!fir.array<3xi32>>, !fir.shape<1>) -> (!fir.ref<!fir.array<3xi32>>, !fir.ref<!fir.array<3xi32>>)
! CHECK: %[[VAL_4:.*]] = fir.embox %[[VAL_3]]#0(%[[VAL_2]]) : (!fir.ref<!fir.array<3xi32>>, !fir.shape<1>) -> !fir.box<!fir.array<3xi32>>
! CHECK: %[[VAL_5:.*]] = fir.alloca !fir.box<!fir.array<3xi32>>
! CHECK: fir.store %[[VAL_4]] to %[[VAL_5]] : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: omp.parallel reduction(byref @add_reduction_byref_box_3xi32 %[[VAL_5]] -> %[[VAL_6:.*]] : !fir.ref<!fir.box<!fir.array<3xi32>>>) {
! CHECK: %[[VAL_7:.*]]:2 = hlfir.declare %[[VAL_6]] {uniq_name = "_QFEi"} : (!fir.ref<!fir.box<!fir.array<3xi32>>>) -> (!fir.ref<!fir.box<!fir.array<3xi32>>>, !fir.ref<!fir.box<!fir.array<3xi32>>>)
! CHECK: %[[VAL_8:.*]] = arith.constant 1 : i32
! CHECK: %[[VAL_9:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_10:.*]] = arith.constant 1 : index
! CHECK: %[[VAL_11:.*]] = hlfir.designate %[[VAL_9]] (%[[VAL_10]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CHECK: hlfir.assign %[[VAL_8]] to %[[VAL_11]] : i32, !fir.ref<i32>
! CHECK: %[[VAL_12:.*]] = arith.constant 2 : i32
! CHECK: %[[VAL_13:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_14:.*]] = arith.constant 2 : index
! CHECK: %[[VAL_15:.*]] = hlfir.designate %[[VAL_13]] (%[[VAL_14]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CHECK: hlfir.assign %[[VAL_12]] to %[[VAL_15]] : i32, !fir.ref<i32>
! CHECK: %[[VAL_16:.*]] = arith.constant 3 : i32
! CHECK: %[[VAL_17:.*]] = fir.load %[[VAL_7]]#0 : !fir.ref<!fir.box<!fir.array<3xi32>>>
! CHECK: %[[VAL_18:.*]] = arith.constant 3 : index
! CHECK: %[[VAL_19:.*]] = hlfir.designate %[[VAL_17]] (%[[VAL_18]]) : (!fir.box<!fir.array<3xi32>>, index) -> !fir.ref<i32>
! CHECK: hlfir.assign %[[VAL_16]] to %[[VAL_19]] : i32, !fir.ref<i32>
! CHECK: omp.terminator
! CHECK: }
! GPU: omp.declare_reduction {{.*}} alloc {
! GPU: } init {
! GPU-NOT: fir.allocmem {{.*}} {bindc_name = ".tmp", {{.*}}}
! GPU: fir.alloca {{.*}} {bindc_name = ".tmp"}
! GPU: } combiner {
! GPU: }
5 changes: 5 additions & 0 deletions mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1291,6 +1291,11 @@ initReductionVars(OP op, ArrayRef<BlockArgument> reductionArgs,
mapInitializationArgs(op, moduleTranslation, reductionDecls,
reductionVariableMap, i);

// TODO In some cases (specially on the GPU), the init regions may
// contains stack alloctaions. If the region is inlined in a loop, this is
// problematic. Instead of just inlining the region, handle allocations by
// hoisting fixed length allocations to the function entry and using
// stacksave and restore for variable length ones.
if (failed(inlineConvertOmpRegions(reductionDecls[i].getInitializerRegion(),
"omp.reduction.neutral", builder,
moduleTranslation, &phis)))
Expand Down