[mlir][acc] Add ACCImplicitData pass for implicit data attributes (llvm#166472)

This change adds the ACCImplicitData pass which implements the OpenACC
specification for "Variables with Implicitly Determined Data Attributes"
(OpenACC 3.4 spec, section 2.6.2).

The pass automatically generates data clause operations (copyin,
copyout, present, firstprivate, etc.) for variables used within OpenACC
compute constructs (parallel, kernels, serial) that do not already have
explicit data clauses.

Key features:
- Respects default(none) and default(present) clauses
- Handles scalar vs. aggregate variables with different semantics:
  * Aggregates: present clause (if default(present)) or copy clause
  * Scalars: copy clause (kernels) or firstprivate (parallel/serial)
- Generates privatization recipes when needed for firstprivate clauses
- Performs alias analysis to avoid redundant data mappings
- Ensures proper data clause ordering for partial entity access
- Generates exit operations (copyout, delete) to match entry operations

Requirements:
- Types must implement acc::MappableType and/or acc::PointerLikeType
interfaces to be considered candidates.
- Operations accessing partial entities or creating subviews should
implement acc::PartialEntityAccess and/or mlir::ViewLikeOpInterface for
proper clause ordering.
- Optionally pre-register acc::OpenACCSupport and mlir::AliasAnalysis if
custom alias analysis, variable name determination, or error reporting
is needed.

Author: razvanlupusoru

mlir/include/mlir/Dialect/OpenACC/Transforms/Passes.h

@@ -9,6 +9,9 @@
 #ifndef MLIR_DIALECT_OPENACC_TRANSFORMS_PASSES_H
 #define MLIR_DIALECT_OPENACC_TRANSFORMS_PASSES_H
 
+#include "mlir/Dialect/Arith/IR/Arith.h"
+#include "mlir/Dialect/MemRef/IR/MemRef.h"
+#include "mlir/Dialect/OpenACC/OpenACC.h"
 #include "mlir/Pass/Pass.h"
 
 namespace mlir {

mlir/include/mlir/Dialect/OpenACC/Transforms/Passes.td

@@ -27,4 +27,40 @@ def LegalizeDataValuesInRegion : Pass<"openacc-legalize-data-values", "mlir::fun
   ];
 }
 
+def ACCImplicitData : Pass<"acc-implicit-data", "mlir::ModuleOp"> {
+  let summary = "Generate implicit data attributes for OpenACC compute constructs";
+  let description = [{
+    This pass implements the OpenACC specification for "Variables with
+    Implicitly Determined Data Attributes" (OpenACC 3.4 spec, section 2.6.2).
+
+    The pass automatically generates data clause operations for variables used
+    within OpenACC compute constructs (parallel, kernels, serial) that do not
+    already have explicit data clauses. The semantics follow these rules:
+
+    1. If there is a default(none) clause visible, no implicit data actions
+       apply.
+
+    2. An aggregate variable (arrays, derived types, etc.) will be treated as:
+       - In a present clause when default(present) is visible.
+       - In a copy clause otherwise.
+
+    3. A scalar variable will be treated as if it appears in:
+       - A copy clause if the compute construct is a kernels construct.
+       - A firstprivate clause otherwise (parallel, serial).
+  }];
+  let dependentDialects = ["mlir::acc::OpenACCDialect",
+      "mlir::memref::MemRefDialect",
+      "mlir::arith::ArithDialect"];
+  let options = [
+    Option<"enableImplicitReductionCopy", "enable-implicit-reduction-copy",
+           "bool", "true",
+           "Enable applying implicit copy in lieu of implicit firstprivate for "
+           "reduction variables. This allows uniform treatment of reduction "
+           "variables between combined constructs (e.g., 'parallel loop') and "
+           "separate constructs (e.g., 'parallel' followed by 'loop'), where "
+           "the OpenACC spec requires copy semantics for the former but "
+           "firstprivate would normally apply for the latter.">
+  ];
+}
+
 #endif // MLIR_DIALECT_OPENACC_TRANSFORMS_PASSES