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[flang][OpenACC] Generalize cross-region GOTO exit handling for all ACC region ops (#187613)

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When a `GoTo` inside an ACC region (`acc.loop`, `acc.data`,
`acc.parallel`, etc.) targets a label outside that region, the lowering
generated an illegal cross-region `cf.br`. This caused MLIR verification
failures or stack overflows in `runRegionDCE`'s recursive
`propagateLiveness`.

This patch addresses the issue with a generalized approach:

- Add `genOpenACCRegionExitBranch` helper that detects cross-region
branches from any ACC region op and generates the appropriate terminator
(`acc.yield` for compute/loop ops, `acc.terminator` for data ops). The
helper verifies that `parentOp` is an ACC operation, so it does not
interfere with branches inside `scf.execute_region` or other non-ACC
regions.
- In `genBranch`, when a cross-region exit from an ACC region is
detected, store a unique exit ID into a selector variable and generate
the region terminator. After the ACC op, a jump table dispatches to the
correct target based on the selector. This correctly handles GOTOs that
skip intermediate code between the loop end and the target label.
- Emit a TODO diagnostic for GOTOs that cross multiple nested ACC region
boundaries.
- Fix `acc.data` creation when the construct has no data clauses but
contains unstructured control flow: skip the early return in
`genACCDataOp` so the `acc.data` region is created and blocks are
properly managed.
This commit is contained in:
khaki3 2026-03-30 06:35:08 -07:00 committed by GitHub
parent 1e99c9e4c7
commit 13cd7a29e8
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5 changed files with 275 additions and 1 deletions
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8
flang/include/flang/Lower/OpenACC.h
View File

@ -138,6 +138,14 @@ void setInsertionPointAfterOpenACCLoopIfInside(fir::FirOpBuilder &);
void genEarlyReturnInOpenACCLoop(fir::FirOpBuilder &, mlir::Location);
/// If \p targetBlock is outside the ACC region containing the current
/// insertion point, generate the appropriate region terminator
/// (acc.terminator or acc.yield) instead of a cross-region branch.
/// Returns true if the exit was handled, false if no ACC region boundary
/// is crossed.
bool genOpenACCRegionExitBranch(fir::FirOpBuilder &, mlir::Location,
mlir::Block *targetBlock);
/// Generates an OpenACC loop from a do construct in order to
/// properly capture the loop bounds, parallelism determination mode,
/// and to privatize the loop variables.

54
flang/lib/Lower/Bridge.cpp
View File

@ -1609,6 +1609,20 @@ private:
void genBranch(mlir::Block *targetBlock) {
assert(targetBlock && "missing unconditional target block");
if (!accRegionExitStack.empty()) {
mlir::Region *curRegion = builder->getBlock()->getParent();
if (targetBlock->getParent() != curRegion) {
auto &exitInfo = accRegionExitStack.back();
int exitId = exitInfo.nextId++;
exitInfo.exits.push_back({exitId, targetBlock});
mlir::Value id = builder->createIntegerConstant(
toLocation(), builder->getI32Type(), exitId);
fir::StoreOp::create(*builder, toLocation(), id, exitInfo.selector);
Fortran::lower::genOpenACCRegionExitBranch(*builder, toLocation(),
targetBlock);
return;
}
}
mlir::cf::BranchOp::create(*builder, toLocation(), targetBlock);
}
@ -3575,6 +3589,19 @@ private:
if (!isCacheConstruct)
localSymbols.pushScope();
// Allocate exit selector for GOTO jump table if the construct is
// unstructured (may contain GOTOs that exit the ACC region).
bool needsExitSelector = getEval().lowerAsUnstructured();
if (needsExitSelector) {
AccRegionExitInfo exitInfo;
exitInfo.selector =
builder->createTemporary(toLocation(), builder->getI32Type());
mlir::Value zero = builder->createIntegerConstant(
toLocation(), builder->getI32Type(), 0);
fir::StoreOp::create(*builder, toLocation(), zero, exitInfo.selector);
accRegionExitStack.push_back(std::move(exitInfo));
}
mlir::Value exitCond = genOpenACCConstruct(
*this, bridge.getSemanticsContext(), getEval(), acc, localSymbols);
@ -3677,6 +3704,25 @@ private:
localSymbols.popScope();
builder->restoreInsertionPoint(insertPt);
// Generate jump table for GOTO exits from the ACC region.
if (needsExitSelector) {
auto exitInfo = accRegionExitStack.pop_back_val();
if (!exitInfo.exits.empty()) {
mlir::Location loc = toLocation();
mlir::Value sel = fir::LoadOp::create(*builder, loc, exitInfo.selector);
for (auto &[id, target] : exitInfo.exits) {
mlir::Value idVal =
builder->createIntegerConstant(loc, builder->getI32Type(), id);
mlir::Value cmp = mlir::arith::CmpIOp::create(
*builder, loc, mlir::arith::CmpIPredicate::eq, sel, idVal);
mlir::Block *nextBlock =
builder->getBlock()->splitBlock(builder->getBlock()->end());
mlir::cf::CondBranchOp::create(*builder, loc, cmp, target, nextBlock);
builder->setInsertionPointToEnd(nextBlock);
}
}
}
if (accLoop && exitCond) {
Fortran::lower::pft::FunctionLikeUnit *funit =
getEval().getOwningProcedure();
@ -7213,6 +7259,14 @@ private:
// Stack to manage object deallocation and finalization at construct exits.
llvm::SmallVector<ConstructContext> activeConstructStack;
/// Track GOTO exits from ACC regions for jump table generation.
struct AccRegionExitInfo {
mlir::Value selector{}; // alloca i32 for exit selector
llvm::SmallVector<std::pair<int, mlir::Block *>> exits; // {id, target}
int nextId = 1;
};
llvm::SmallVector<AccRegionExitInfo> accRegionExitStack;
/// BLOCK name mangling component map
int blockId = 0;
Fortran::lower::mangle::ScopeBlockIdMap scopeBlockIdMap;

32
flang/lib/Lower/OpenACC.cpp
View File

@ -2768,7 +2768,8 @@ static void genACCDataOp(Fortran::lower::AbstractConverter &converter,
addOperands(operands, operandSegments, waitOperands);
addOperands(operands, operandSegments, dataClauseOperands);
if (dataClauseOperands.empty() && !hasDefaultNone && !hasDefaultPresent)
if (dataClauseOperands.empty() && !hasDefaultNone && !hasDefaultPresent &&
!eval.lowerAsUnstructured())
return;
auto dataOp = createRegionOp<mlir::acc::DataOp, mlir::acc::TerminatorOp>(
@ -4566,6 +4567,35 @@ void Fortran::lower::genEarlyReturnInOpenACCLoop(fir::FirOpBuilder &builder,
mlir::acc::YieldOp::create(builder, loc, yieldValue);
}
bool Fortran::lower::genOpenACCRegionExitBranch(fir::FirOpBuilder &builder,
mlir::Location loc,
mlir::Block *targetBlock) {
mlir::Block *currentBlock = builder.getBlock();
if (!currentBlock || !targetBlock)
return false;
mlir::Region *curRegion = currentBlock->getParent();
mlir::Region *targetRegion = targetBlock->getParent();
if (curRegion == targetRegion)
return false;
// Check all regions between the current region and the target for ACC ops.
for (mlir::Region *r = curRegion; r && r != targetRegion;
r = r->getParentRegion()) {
mlir::Operation *op = r->getParentOp();
if (op &&
mlir::isa<ACC_COMPUTE_CONSTRUCT_OPS, ACC_DATA_CONSTRUCT_STRUCTURED_OPS,
mlir::acc::LoopOp>(op)) {
if (targetRegion == r->getParentRegion()) {
genOpenACCTerminator(builder, op, loc);
return true;
}
TODO(loc, "GOTO exiting OpenACC region");
}
}
return false;
}
uint64_t Fortran::lower::getLoopCountForCollapseAndTile(
const Fortran::parser::AccClauseList &clauseList) {
uint64_t collapseLoopCount = getCollapseSizeAndForce(clauseList).first;

44
flang/test/Lower/OpenACC/Todo/acc-goto-multi-level-exit.f90 Normal file
View File

@ -0,0 +1,44 @@
! GOTO exits through two nested ACC regions. The branch crosses
! two ACC region boundaries, requiring multi-level exit handling.
! RUN: split-file %s %t
! RUN: %not_todo_cmd bbc -fopenacc -emit-hlfir %t/nested_data.f90 -o - 2>&1 | FileCheck %s --check-prefix=DATA
! RUN: %not_todo_cmd bbc -fopenacc -emit-hlfir %t/nested_loop.f90 -o - 2>&1 | FileCheck %s --check-prefix=LOOP
!--- nested_data.f90
subroutine nested_data_exit(a, n)
integer :: n, i, j
real :: a(*)
!$acc data copy(a(1:n))
!$acc data copyout(a(1:n))
do i = 1, n
do j = 1, n
if (a(j) > 0.0) goto 888
end do
end do
!$acc end data
!$acc end data
888 continue
end subroutine
! DATA: not yet implemented: GOTO exiting OpenACC region
!--- nested_loop.f90
subroutine nested_loop_exit(A, B, N)
implicit real*8 (a-h, o-z)
!$acc routine seq
dimension A(*), B(*)
!$acc loop seq
do 100 i = 1, N
!$acc loop seq
do 10 j = 1, N
if (A(j) .gt. B(j)) goto 200
10 continue
100 continue
200 continue
end subroutine
! LOOP: not yet implemented: GOTO exiting OpenACC region

138
flang/test/Lower/OpenACC/acc-unstructured.f90
View File

@ -84,3 +84,141 @@ subroutine test_unstructured3(a, b, c)
! CHECK: acc.yield
end subroutine
! Test that acc.data is still created when there are no data clauses but the
! construct contains unstructured control flow. Without this, the early return
! in genACCDataOp skips acc.data creation, leaving orphaned blocks.
subroutine test_unstructured4(a, n)
integer :: n, i, j
real :: a(:)
logical :: use_gpu
use_gpu = .true.
!$acc data if(use_gpu)
do i = 1, n
do j = 1, n
if (a(j) > 0.0) stop 'unstructured'
end do
end do
!$acc end data
end subroutine
! CHECK-LABEL: func.func @_QPtest_unstructured4
! CHECK: acc.data if(%{{.*}}) {
! CHECK: fir.call @_FortranAStopStatementText
! CHECK: acc.terminator
! CHECK: }
! Test that GOTO exiting acc.data (one level) generates acc.terminator
! instead of an invalid cross-region branch.
subroutine test_unstructured5(a, n)
integer :: n, i, j
real :: a(:)
logical :: use_gpu
use_gpu = .true.
!$acc data if(use_gpu)
do i = 1, n
do j = 1, n
if (a(j) > 0.0) goto 999
end do
end do
!$acc end data
999 continue
end subroutine
! CHECK-LABEL: func.func @_QPtest_unstructured5
! CHECK: acc.data if(%{{.*}}) {
! CHECK: fir.store %{{.*}} to %{{.*}} : !fir.ref<i32>
! CHECK: acc.terminator
! CHECK: acc.terminator
! CHECK: }
! CHECK: arith.cmpi eq
! CHECK: cf.cond_br
! Test that GOTO exiting acc.loop (one level) generates acc.yield
! instead of an invalid cross-region branch.
subroutine test_unstructured6(N, A, B)
implicit real*8 (a-h, o-z)
!$acc routine gang
dimension A(*), B(*)
!$acc loop gang vector
do 100 i = 1, N
!$acc loop seq
do 10 j = 1, 1000
if (A(i) .gt. B(i)) goto 20
10 continue
20 B(i) = A(i)
100 continue
end subroutine
! CHECK-LABEL: func.func @_QPtest_unstructured6
! CHECK: acc.loop gang vector
! CHECK: acc.loop
! CHECK: arith.cmpf ogt
! CHECK: fir.store %{{.*}} to %{{.*}} : !fir.ref<i32>
! CHECK: acc.yield
! CHECK: } attributes {seq = [#acc.device_type<none>], unstructured}
! Test GOTO exiting acc.loop with intermediate code between loop end and
! target. A jump table (exit selector + dispatch) skips the intermediate code.
subroutine test_unstructured7(A, B, C, N)
implicit real*8 (a-h, o-z)
!$acc routine gang
dimension A(*), B(*), C(*)
!$acc loop gang vector
do 100 i = 1, N
!$acc loop seq
do 10 j = 1, 1000
if (A(i) .gt. B(i)) goto 20
10 continue
C(i) = 999.0
20 B(i) = A(i)
100 continue
end subroutine
! CHECK-LABEL: func.func @_QPtest_unstructured7
! CHECK: acc.loop gang vector
! Inner loop stores exit selector and yields:
! CHECK: acc.loop
! CHECK: fir.store %{{.*}} to %{{.*}} : !fir.ref<i32>
! CHECK: acc.yield
! CHECK: } attributes {seq = [#acc.device_type<none>], unstructured}
! Jump table after inner loop:
! CHECK: fir.load %{{.*}} : !fir.ref<i32>
! CHECK: arith.cmpi eq
! CHECK: cf.cond_br
! Intermediate code on fall-through path:
! CHECK: arith.constant 9.990000e+02
! Test GOTO exiting acc.data with intermediate code. Jump table dispatches
! after the acc.data op.
subroutine test_unstructured8(a, n)
integer :: n, i, j
real :: a(:)
logical :: use_gpu
use_gpu = .true.
!$acc data if(use_gpu)
do i = 1, n
do j = 1, n
if (a(j) > 0.0) goto 999
end do
end do
a(1) = -1.0
!$acc end data
999 continue
end subroutine
! CHECK-LABEL: func.func @_QPtest_unstructured8
! Inside acc.data, GOTO stores exit selector and terminates:
! CHECK: acc.data if(%{{.*}}) {
! CHECK: fir.store %{{.*}} to %{{.*}} : !fir.ref<i32>
! CHECK: acc.terminator
! CHECK: acc.terminator
! CHECK: }
! Jump table after acc.data:
! CHECK: fir.load %{{.*}} : !fir.ref<i32>
! CHECK: arith.cmpi eq
! CHECK: cf.cond_br