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[flang][OpenMP] Upstream do concurrent loop-nest detection.
Upstreams the next part of `do concurrent` to OpenMP mapping pass (from AMD's ROCm implementation). See #126026 for more context. This PR add loop nest detection logic. This enables us to discover muli-range `do concurrent` loops and then map them as "collapsed" loop nests to OpenMP.
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‎flang/docs/DoConcurrentConversionToOpenMP.md

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@@ -53,6 +53,79 @@ that:
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* It has been tested in a very limited way so far.
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* It has been tested mostly on simple synthetic inputs.
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### Loop nest detection
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On the `FIR` dialect level, the following loop:
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```fortran
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do concurrent(i=1:n, j=1:m, k=1:o)
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a(i,j,k) = i + j + k
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end do
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```
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is modelled as a nest of `fir.do_loop` ops such that an outer loop's region
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contains **only** the following:
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1. The operations needed to assign/update the outer loop's induction variable.
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1. The inner loop itself.
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So the MLIR structure for the above example looks similar to the following:
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```
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fir.do_loop %i_idx = %34 to %36 step %c1 unordered {
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%i_idx_2 = fir.convert %i_idx : (index) -> i32
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fir.store %i_idx_2 to %i_iv#1 : !fir.ref<i32>
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fir.do_loop %j_idx = %37 to %39 step %c1_3 unordered {
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%j_idx_2 = fir.convert %j_idx : (index) -> i32
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fir.store %j_idx_2 to %j_iv#1 : !fir.ref<i32>
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fir.do_loop %k_idx = %40 to %42 step %c1_5 unordered {
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%k_idx_2 = fir.convert %k_idx : (index) -> i32
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fir.store %k_idx_2 to %k_iv#1 : !fir.ref<i32>
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... loop nest body goes here ...
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}
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}
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}
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```
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This applies to multi-range loops in general; they are represented in the IR as
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a nest of `fir.do_loop` ops with the above nesting structure.
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Therefore, the pass detects such "perfectly" nested loop ops to identify multi-range
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loops and map them as "collapsed" loops in OpenMP.
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#### Further info regarding loop nest detection
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Loop nest detection is currently limited to the scenario described in the previous
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section. However, this is quite limited and can be extended in the future to cover
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more cases. At the moment, for the following loop nest, even though both loops are
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perfectly nested, only the outer loop is parallelized:
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```fortran
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do concurrent(i=1:n)
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do concurrent(j=1:m)
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a(i,j) = i * j
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end do
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end do
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```
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Similarly, for the following loop nest, even though the intervening statement `x = 41`
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does not have any memory effects that would affect parallelization, this nest is
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not parallelized either (only the outer loop is).
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```fortran
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do concurrent(i=1:n)
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x = 41
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do concurrent(j=1:m)
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a(i,j) = i * j
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end do
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end do
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```
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The above also has the consequence that the `j` variable will **not** be
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privatized in the OpenMP parallel/target region. In other words, it will be
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treated as if it was a `shared` variable. For more details about privatization,
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see the "Data environment" section below.
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See `flang/test/Transforms/DoConcurrent/loop_nest_test.f90` for more examples
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of what is and is not detected as a perfect loop nest.
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<!--
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More details about current status will be added along with relevant parts of the
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implementation in later upstreaming patches.
@@ -63,6 +136,17 @@ implementation in later upstreaming patches.
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This section describes some of the open questions/issues that are not tackled yet
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even in the downstream implementation.
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### Separate MLIR op for `do concurrent`
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At the moment, both increment and concurrent loops are represented by one MLIR
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op: `fir.do_loop`; where we differentiate concurrent loops with the `unordered`
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attribute. This is not ideal since the `fir.do_loop` op support only single
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iteration ranges. Consequently, to model multi-range `do concurrent` loops, flang
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emits a nest of `fir.do_loop` ops which we have to detect in the OpenMP conversion
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pass to handle multi-range loops. Instead, it would better to model multi-range
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concurrent loops using a separate op which the IR more representative of the input
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Fortran code and also easier to detect and transform.
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### Delayed privatization
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So far, we emit the privatization logic for IVs inline in the parallel/target
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- [x] Command line options for `flang` and `bbc`.
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- [x] Conversion pass skeleton (no transormations happen yet).
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- [x] Status description and tracking document (this document).
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- [x] Loop nest detection to identify multi-range loops.
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- [ ] Basic host/CPU mapping support.
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- [ ] Basic device/GPU mapping support.
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- [ ] More advanced host and device support (expaned to multiple items as needed).

‎flang/lib/Optimizer/OpenMP/DoConcurrentConversion.cpp

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#include "flang/Optimizer/Dialect/FIROps.h"
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#include "flang/Optimizer/OpenMP/Passes.h"
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#include "flang/Optimizer/OpenMP/Utils.h"
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#include "mlir/Analysis/SliceAnalysis.h"
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#include "mlir/Dialect/OpenMP/OpenMPDialect.h"
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#include "mlir/Transforms/DialectConversion.h"
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#include "mlir/Transforms/RegionUtils.h"
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namespace flangomp {
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#define GEN_PASS_DEF_DOCONCURRENTCONVERSIONPASS
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#define DBGS() (llvm::dbgs() << "[" DEBUG_TYPE << "]: ")
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namespace {
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namespace looputils {
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using LoopNest = llvm::SetVector<fir::DoLoopOp>;
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/// Loop \p innerLoop is considered perfectly-nested inside \p outerLoop iff
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/// there are no operations in \p outerloop's body other than:
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///
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/// 1. the operations needed to assign/update \p outerLoop's induction variable.
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/// 2. \p innerLoop itself.
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///
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/// \p return true if \p innerLoop is perfectly nested inside \p outerLoop
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/// according to the above definition.
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bool isPerfectlyNested(fir::DoLoopOp outerLoop, fir::DoLoopOp innerLoop) {
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mlir::ForwardSliceOptions forwardSliceOptions;
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forwardSliceOptions.inclusive = true;
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// The following will be used as an example to clarify the internals of this
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// function:
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// ```
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// 1. fir.do_loop %i_idx = %34 to %36 step %c1 unordered {
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// 2. %i_idx_2 = fir.convert %i_idx : (index) -> i32
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// 3. fir.store %i_idx_2 to %i_iv#1 : !fir.ref<i32>
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//
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// 4. fir.do_loop %j_idx = %37 to %39 step %c1_3 unordered {
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// 5. %j_idx_2 = fir.convert %j_idx : (index) -> i32
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// 6. fir.store %j_idx_2 to %j_iv#1 : !fir.ref<i32>
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// ... loop nest body, possible uses %i_idx ...
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// }
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// }
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// ```
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// In this example, the `j` loop is perfectly nested inside the `i` loop and
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// below is how we find that.
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// We don't care about the outer-loop's induction variable's uses within the
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// inner-loop, so we filter out these uses.
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//
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// This filter tells `getForwardSlice` (below) to only collect operations
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// which produce results defined above (i.e. outside) the inner-loop's body.
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//
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// Since `outerLoop.getInductionVar()` is a block argument (to the
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// outer-loop's body), the filter effectively collects uses of
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// `outerLoop.getInductionVar()` inside the outer-loop but outside the
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// inner-loop.
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forwardSliceOptions.filter = [&](mlir::Operation *op) {
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return mlir::areValuesDefinedAbove(op->getResults(), innerLoop.getRegion());
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};
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llvm::SetVector<mlir::Operation *> indVarSlice;
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// The forward slice of the `i` loop's IV will be the 2 ops in line 1 & 2
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// above. Uses of `%i_idx` inside the `j` loop are not collected because of
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// the filter.
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mlir::getForwardSlice(outerLoop.getInductionVar(), &indVarSlice,
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forwardSliceOptions);
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llvm::DenseSet<mlir::Operation *> indVarSet(indVarSlice.begin(),
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indVarSlice.end());
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llvm::DenseSet<mlir::Operation *> outerLoopBodySet;
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// The following walk collects ops inside `outerLoop` that are **not**:
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// * the outer-loop itself,
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// * or the inner-loop,
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// * or the `fir.result` op (the outer-loop's terminator).
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//
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// For the above example, this will also populate `outerLoopBodySet` with ops
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// in line 1 & 2 since we skip the `i` loop, the `j` loop, and the terminator.
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outerLoop.walk<mlir::WalkOrder::PreOrder>([&](mlir::Operation *op) {
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if (op == outerLoop)
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return mlir::WalkResult::advance();
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if (op == innerLoop)
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return mlir::WalkResult::skip();
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if (mlir::isa<fir::ResultOp>(op))
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return mlir::WalkResult::advance();
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outerLoopBodySet.insert(op);
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return mlir::WalkResult::advance();
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});
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// If `outerLoopBodySet` ends up having the same ops as `indVarSet`, then
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// `outerLoop` only contains ops that setup its induction variable +
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// `innerLoop` + the `fir.result` terminator. In other words, `innerLoop` is
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// perfectly nested inside `outerLoop`.
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bool result = (outerLoopBodySet == indVarSet);
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mlir::Location loc = outerLoop.getLoc();
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LLVM_DEBUG(DBGS() << "Loop pair starting at location " << loc << " is"
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<< (result ? "" : " not") << " perfectly nested\n");
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return result;
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}
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/// Starting with `currentLoop` collect a perfectly nested loop nest, if any.
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/// This function collects as much as possible loops in the nest; it case it
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/// fails to recognize a certain nested loop as part of the nest it just returns
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/// the parent loops it discovered before.
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mlir::LogicalResult collectLoopNest(fir::DoLoopOp currentLoop,
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LoopNest &loopNest) {
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assert(currentLoop.getUnordered());
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while (true) {
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loopNest.insert(currentLoop);
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llvm::SmallVector<fir::DoLoopOp> unorderedLoops;
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for (auto nestedLoop : currentLoop.getRegion().getOps<fir::DoLoopOp>())
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if (nestedLoop.getUnordered())
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unorderedLoops.push_back(nestedLoop);
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if (unorderedLoops.empty())
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break;
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// Having more than one unordered loop means that we are not dealing with a
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// perfect loop nest (i.e. a mulit-range `do concurrent` loop); which is the
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// case we are after here.
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if (unorderedLoops.size() > 1)
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return mlir::failure();
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fir::DoLoopOp nestedUnorderedLoop = unorderedLoops.front();
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if (!isPerfectlyNested(currentLoop, nestedUnorderedLoop))
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return mlir::failure();
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currentLoop = nestedUnorderedLoop;
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}
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return mlir::success();
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}
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} // namespace looputils
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class DoConcurrentConversion : public mlir::OpConversionPattern<fir::DoLoopOp> {
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public:
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using mlir::OpConversionPattern<fir::DoLoopOp>::OpConversionPattern;
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mlir::LogicalResult
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matchAndRewrite(fir::DoLoopOp doLoop, OpAdaptor adaptor,
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mlir::ConversionPatternRewriter &rewriter) const override {
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looputils::LoopNest loopNest;
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bool hasRemainingNestedLoops =
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failed(looputils::collectLoopNest(doLoop, loopNest));
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if (hasRemainingNestedLoops)
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mlir::emitWarning(doLoop.getLoc(),
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"Some `do concurent` loops are not perfectly-nested. "
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"These will be serialized.");
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// TODO This will be filled in with the next PRs that upstreams the rest of
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// the ROCm implementaion.
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return mlir::success();

‎flang/test/Transforms/DoConcurrent/loop_nest_test.f90

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! Tests loop-nest detection algorithm for do-concurrent mapping.
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! REQUIRES: asserts
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! RUN: %flang_fc1 -emit-hlfir -fopenmp -fdo-concurrent-to-openmp=host \
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! RUN: -mmlir -debug %s -o - 2> %t.log || true
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! RUN: FileCheck %s < %t.log
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program main
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implicit none
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contains
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subroutine foo(n)
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implicit none
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integer :: n, m
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integer :: i, j, k
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integer :: x
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integer, dimension(n) :: a
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integer, dimension(n, n, n) :: b
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is perfectly nested
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do concurrent(i=1:n, j=1:bar(n*m, n/m))
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a(i) = n
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end do
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is perfectly nested
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do concurrent(i=bar(n, x):n, j=1:bar(n*m, n/m))
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a(i) = n
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end do
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is not perfectly nested
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do concurrent(i=bar(n, x):n)
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do concurrent(j=1:bar(n*m, n/m))
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a(i) = n
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end do
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end do
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is not perfectly nested
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do concurrent(i=1:n)
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x = 10
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do concurrent(j=1:m)
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b(i,j,k) = i * j + k
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end do
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end do
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is not perfectly nested
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do concurrent(i=1:n)
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do concurrent(j=1:m)
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b(i,j,k) = i * j + k
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end do
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x = 10
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end do
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is not perfectly nested
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do concurrent(i=1:n)
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do concurrent(j=1:m)
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b(i,j,k) = i * j + k
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x = 10
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end do
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end do
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! Verify the (i,j) and (j,k) pairs of loops are detected as perfectly nested.
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!
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 3]]:{{.*}}) is perfectly nested
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! CHECK: Loop pair starting at location
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! CHECK: loc("{{.*}}":[[# @LINE + 1]]:{{.*}}) is perfectly nested
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do concurrent(i=bar(n, x):n, j=1:bar(n*m, n/m), k=1:bar(n*m, bar(n*m, n/m)))
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a(i) = n
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end do
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end subroutine
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pure function bar(n, m)
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implicit none
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integer, intent(in) :: n, m
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integer :: bar
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bar = n + m
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end function
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end program main

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