Emit a proper error message for CFG in workshare

Author: ivanradanov

flang/lib/Optimizer/OpenMP/LowerWorkshare.cpp

@@ -16,6 +16,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "flang/Optimizer/Builder/Todo.h"
 #include <flang/Optimizer/Builder/FIRBuilder.h>
 #include <flang/Optimizer/Dialect/FIROps.h>
 #include <flang/Optimizer/Dialect/FIRType.h>
@@ -416,8 +417,18 @@ LogicalResult lowerWorkshare(mlir::omp::WorkshareOp wsOp, DominanceInfo &di) {
 
   parallelizeRegion(wsOp.getRegion(), newOp.getRegion(), rootMapping, loc, di);
 
+  // FIXME Currently, we only support workshare constructs with structured
+  // control flow. The transformation itself supports CFG, however, once we
+  // transform the MLIR region in the omp.workshare, we need to inline that
+  // region in the parent block. We have no guarantees at this point of the
+  // pipeline that the parent op supports CFG (e.g. fir.if), thus this is not
+  // generally possible.  The alternative is to put the lowered region in an
+  // operation akin to scf.execute_region, which will get lowered at the same
+  // time when fir ops get lowered to CFG. However, SCF is not registered in
+  // flang so we cannot use it. Remove this requirement once we have
+  // scf.execute_region or an alternative operation available.
   if (wsOp.getRegion().getBlocks().size() != 1)
-    return failure();
+    TODO(wsOp->getLoc(), "omp workshare with unstructured control flow");
 
   // Inline the contents of the placeholder workshare op into its parent block.
   Block *theBlock = &newOp.getRegion().front();

flang/test/Transforms/OpenMP/lower-workshare-todo-cfg-dom.mlir

@@ -0,0 +1,23 @@
+// RUN: fir-opt --lower-workshare --allow-unregistered-dialect %s 2>&1 | FileCheck %s
+
+// CHECK: not yet implemented: omp workshare with unstructured control flow
+
+// Check that the definition of %r dominates its use post-transform
+func.func @wsfunc() {
+  %a = fir.alloca i32
+  omp.parallel {
+    omp.workshare {
+    ^bb1:
+      %c1 = arith.constant 1 : i32
+      cf.br ^bb3(%c1: i32)
+    ^bb2:
+      "test.test2"(%r) : (i32) -> ()
+      omp.terminator
+    ^bb3(%arg1: i32):
+      %r = "test.test2"(%arg1) : (i32) -> i32
+      cf.br ^bb2
+    }
+    omp.terminator
+  }
+  return
+}

flang/test/Transforms/OpenMP/lower-workshare-todo-cfg.mlir

@@ -0,0 +1,20 @@
+// RUN: fir-opt --lower-workshare --allow-unregistered-dialect %s 2>&1 | FileCheck %s
+
+// CHECK: not yet implemented: omp workshare with unstructured control flow
+
+// Check transforming a simple CFG
+func.func @wsfunc() {
+  %a = fir.alloca i32
+  omp.parallel {
+    omp.workshare {
+    ^bb1:
+      %c1 = arith.constant 1 : i32
+      cf.br ^bb3(%c1: i32)
+    ^bb3(%arg1: i32):
+      "test.test2"(%arg1) : (i32) -> ()
+      omp.terminator
+    }
+    omp.terminator
+  }
+  return
+}