[Mlir-commits] [mlir] [mlir][emitc] Add a structured for operation (PR #68206)

[Marius Brehler]

================
@@ -37,7 +37,99 @@ struct SCFToEmitCPass : public impl::SCFToEmitCBase<SCFToEmitCPass> {
   void runOnOperation() override;
 };
 
-// Lower scf::if to emitc::if, implementing return values as emitc::variable's
+// Lower scf::for to emitc::for, implementing result values using
+// emitc::variable's updated within the loop body.
+struct ForLowering : public OpRewritePattern<ForOp> {
+  using OpRewritePattern<ForOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(ForOp forOp,
+                                PatternRewriter &rewriter) const override;
+};
+
+// Create an uninitialized emitc::variable op for each result of the given op.
+template <typename T>
+static SmallVector<Value> createVariablesForResults(T op,
+                                                    PatternRewriter &rewriter) {
+  SmallVector<Value> resultVariables;
+
+  if (!op.getNumResults())
+    return resultVariables;
+
+  Location loc = op->getLoc();
+  MLIRContext *context = op.getContext();
+
+  OpBuilder::InsertionGuard guard(rewriter);
+  rewriter.setInsertionPoint(op);
+
+  for (OpResult result : op.getResults()) {
+    Type resultType = result.getType();
+    auto noInit = emitc::OpaqueAttr::get(context, "");
+    auto var = rewriter.create<emitc::VariableOp>(loc, resultType, noInit);
+    resultVariables.push_back(var);
+  }
+
+  return resultVariables;
+}
+
+// Create a series of assign ops assigning given values to given variables at
+// the current insertion point of given rewriter.
+static void assignValues(ValueRange values, SmallVector<Value> &variables,
+                         PatternRewriter &rewriter, Location loc) {
+  for (auto [value, var] : llvm::zip(values, variables))
+    rewriter.create<emitc::AssignOp>(loc, var, value);
+}
+
+static void lowerYield(SmallVector<Value> &resultVariables,
+                       PatternRewriter &rewriter, scf::YieldOp yield) {
+  Location loc = yield.getLoc();
+  ValueRange operands = yield.getOperands();
+
+  OpBuilder::InsertionGuard guard(rewriter);
+  rewriter.setInsertionPoint(yield);
+
+  assignValues(operands, resultVariables, rewriter, loc);
+
+  rewriter.create<emitc::YieldOp>(loc);
+  rewriter.eraseOp(yield);
+}
+
+LogicalResult ForLowering::matchAndRewrite(ForOp forOp,
+                                           PatternRewriter &rewriter) const {
+  Location loc = forOp.getLoc();
+
+  // Create an emitc::variable op for each result. These variables will be
+  // assigned to by emitc::assign ops within the loop body.
+  SmallVector<Value> resultVariables =
+      createVariablesForResults(forOp, rewriter);
+  SmallVector<Value> iterArgsVariables =
+      createVariablesForResults(forOp, rewriter);
+
+  assignValues(forOp.getInits(), iterArgsVariables, rewriter, loc);
+
+  auto loweredFor = rewriter.create<emitc::ForOp>(
----------------
marbre wrote:

```suggestion
  emitc::ForOp loweredFor = rewriter.create<emitc::ForOp>(
```

https://github.com/llvm/llvm-project/pull/68206