[Mlir-commits] [mlir] [mlir][emitc] Add 'emitc.while' and 'emitc.do' ops to the dialect (PR #143008)

[Gil Rapaport]

================
@@ -336,11 +336,190 @@ LogicalResult IndexSwitchOpLowering::matchAndRewrite(
   return success();
 }
 
+// Lower scf::while to emitc::do using mutable variables to maintain loop state
+// across iterations. The do-while structure ensures the condition is evaluated
+// after each iteration, matching SCF while semantics.
+struct WhileLowering : public OpConversionPattern<WhileOp> {
+  using OpConversionPattern::OpConversionPattern;
+
+  LogicalResult
+  matchAndRewrite(WhileOp whileOp, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    Location loc = whileOp.getLoc();
+    MLIRContext *context = loc.getContext();
+
+    // Create variable storage for loop-carried values to enable imperative
+    // updates while maintaining SSA semantics at conversion boundaries.
+    SmallVector<Value> variables;
+    if (failed(createInitVariables(whileOp, rewriter, variables, loc, context)))
+      return failure();
+
+    if (failed(lowerDoWhile(whileOp, variables, context, rewriter, loc)))
+      return failure();
+
+    // 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;
+    if (failed(createVariablesForResults(whileOp, getTypeConverter(), rewriter,
+                                         resultVariables))) {
+      return rewriter.notifyMatchFailure(whileOp,
+                                         "Failed to create result variables");
+    }
+
+    rewriter.setInsertionPointAfter(whileOp);
+
+    // Transfer final loop state to result variables and get final SSA results.
+    SmallVector<Value> finalResults =
+        finalizeLoopResults(resultVariables, variables, rewriter, loc);
+
+    rewriter.replaceOp(whileOp, finalResults);
+    return success();
+  }
+
+private:
+  // Initialize variables for loop-carried values to enable state updates
+  // across iterations without SSA argument passing.
+  static LogicalResult createInitVariables(WhileOp whileOp,
+                                           ConversionPatternRewriter &rewriter,
+                                           SmallVectorImpl<Value> &outVars,
+                                           Location loc, MLIRContext *context) {
+    emitc::OpaqueAttr noInit = emitc::OpaqueAttr::get(context, "");
+
+    for (Value init : whileOp.getInits()) {
+      emitc::VariableOp var = rewriter.create<emitc::VariableOp>(
+          loc, emitc::LValueType::get(init.getType()), noInit);
+      rewriter.create<emitc::AssignOp>(loc, var.getResult(), init);
+      outVars.push_back(var.getResult());
+    }
+
+    return success();
+  }
+
+  // Transition from SSA block arguments to variable-based state management by
+  // replacing argument uses with variable loads and cleaning up block
+  // interface.
+  void replaceBlockArgsWithVarLoads(Block *block, ArrayRef<Value> vars,
+                                    ConversionPatternRewriter &rewriter,
+                                    Location loc) const {
+    rewriter.setInsertionPointToStart(block);
+
+    for (auto [arg, var] : llvm::zip(block->getArguments(), vars)) {
+      Type loadedType = cast<emitc::LValueType>(var.getType()).getValueType();
+      Value load = rewriter.create<emitc::LoadOp>(loc, loadedType, var);
+      arg.replaceAllUsesWith(load);
+    }
+
+    // Remove arguments after replacement to simplify block structure.
+    block->eraseArguments(0, block->getNumArguments());
+  }
+
+  // Transfers final loop state from mutable variables to result variables,
+  // then returns the final SSA values to replace the original scf::while
+  // results.
+  static SmallVector<Value>
+  finalizeLoopResults(ArrayRef<Value> resultVariables,
+                      ArrayRef<Value> loopVariables,
+                      ConversionPatternRewriter &rewriter, Location loc) {
+    // Transfer final loop state to result variables to bridge imperative loop
+    // variables with SSA result expectations of the original op.
+    for (auto [resultVar, var] : llvm::zip(resultVariables, loopVariables)) {
+      Type loadedType = cast<emitc::LValueType>(var.getType()).getValueType();
+      Value load = rewriter.create<emitc::LoadOp>(loc, loadedType, var);
+      rewriter.create<emitc::AssignOp>(loc, resultVar, load);
+    }
+
+    // Replace op with loaded values to integrate with converted SSA graph.
+    SmallVector<Value> finalResults;
+    for (Value resultVar : resultVariables) {
+      Type loadedType =
+          cast<emitc::LValueType>(resultVar.getType()).getValueType();
+      finalResults.push_back(
+          rewriter.create<emitc::LoadOp>(loc, loadedType, resultVar));
+    }
+
+    return finalResults;
+  }
+
+  // Lower scf.while to emitc.do.
+  LogicalResult lowerDoWhile(WhileOp whileOp, ArrayRef<Value> vars,
+                             MLIRContext *context,
+                             ConversionPatternRewriter &rewriter,
+                             Location loc) const {
+    Type i1Type = IntegerType::get(context, 1);
+    auto globalCondition =
+        rewriter.create<emitc::VariableOp>(loc, emitc::LValueType::get(i1Type),
+                                           emitc::OpaqueAttr::get(context, ""));
+    Value conditionVal = globalCondition.getResult();
+
+    auto loweredDo = rewriter.create<emitc::DoOp>(loc);
+
+    // Lower before-region as body.
+    rewriter.inlineRegionBefore(whileOp.getBefore(), loweredDo.getBodyRegion(),
+                                loweredDo.getBodyRegion().end());
+
+    Block *bodyBlock = &loweredDo.getBodyRegion().front();
+    replaceBlockArgsWithVarLoads(bodyBlock, vars, rewriter, loc);
----------------
aniragil wrote:

Replacing the arguments with the loaded values can be automatically using the rewriter's `mergeBlocks` method, as done in `ForLowering::matchAndRewrite()`

https://github.com/llvm/llvm-project/pull/143008