[mlir][emitc] Add a structured for operation

Add an emitc.for op to the EmitC dialect as a lowering target for
scf.for, replacing its current direct translation to C; The translator
now handles emitc.for instead.

Author: aniragil

mlir/docs/Dialects/emitc.md

@@ -31,8 +31,5 @@ translating the following operations:
     *   `func.constant`
     *   `func.func`
     *   `func.return`
-*   'scf' Dialect
-    *   `scf.for`
-    *   `scf.yield`
 *   'arith' Dialect
     *   `arith.constant`

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

@@ -246,6 +246,67 @@ def EmitC_DivOp : EmitC_BinaryOp<"div", []> {
   let results = (outs FloatIntegerIndexOrOpaqueType);
 }
 
+def EmitC_ForOp : EmitC_Op<"for",
+      [AllTypesMatch<["lowerBound", "upperBound", "step"]>,
+       SingleBlockImplicitTerminator<"emitc::YieldOp">,
+       RecursiveMemoryEffects]> {
+  let summary = "for operation";
+  let description = [{
+    The `emitc.for` operation represents a C loop of the following form:
+
+    ```c++
+    for (T i = lb; i < ub; i += step) { /* ... */ } // where T is typeof(lb)
+    ```
+
+    The operation takes 3 SSA values as operands that represent the lower bound,
+    upper bound and step respectively, and defines an SSA value for its
+    induction variable. It has one region capturing the loop body. The induction
+    variable is represented as an argument of this region. This SSA value is a
+    signless integer or index. The step is a value of same type.
+
+    This operation has no result. The body region must contain exactly one block
+    that terminates with `emitc.yield`. Calling ForOp::build will create such a
+    region and insert the terminator implicitly if none is defined, so will the
+    parsing even in cases when it is absent from the custom format. For example:
+
+    ```mlir
+    // Index case.
+    emitc.for %iv = %lb to %ub step %step {
+      ... // body
+    }
+    ...
+    // Integer case.
+    emitc.for %iv_32 = %lb_32 to %ub_32 step %step_32 : i32 {
+      ... // body
+    }
+    ```
+  }];
+  let arguments = (ins IntegerIndexOrOpaqueType:$lowerBound,
+                       IntegerIndexOrOpaqueType:$upperBound,
+                       IntegerIndexOrOpaqueType:$step);
+  let results = (outs);
+  let regions = (region SizedRegion<1>:$region);
+
+  let skipDefaultBuilders = 1;
+  let builders = [
+    OpBuilder<(ins "Value":$lowerBound, "Value":$upperBound, "Value":$step,
+      CArg<"function_ref<void(OpBuilder &, Location, Value)>", "nullptr">)>
+  ];
+
+  let extraClassDeclaration = [{
+    using BodyBuilderFn =
+        function_ref<void(OpBuilder &, Location, Value)>;
+    Value getInductionVar() { return getBody()->getArgument(0); }
+    void setLowerBound(Value bound) { getOperation()->setOperand(0, bound); }
+    void setUpperBound(Value bound) { getOperation()->setOperand(1, bound); }
+    void setStep(Value step) { getOperation()->setOperand(2, step); }
+  }];
+
+  let hasCanonicalizer = 1;
+  let hasCustomAssemblyFormat = 1;
+  let hasRegionVerifier = 1;
+}
+
 def EmitC_IncludeOp
     : EmitC_Op<"include", [HasParent<"ModuleOp">]> {
   let summary = "Include operation";
@@ -430,7 +491,8 @@ def EmitC_AssignOp : EmitC_Op<"assign", []> {
   let assemblyFormat = "$value `:` type($value) `to` $var `:` type($var) attr-dict";
 }
 
-def EmitC_YieldOp : EmitC_Op<"yield", [Pure, Terminator, ParentOneOf<["IfOp"]>]> {
+def EmitC_YieldOp : EmitC_Op<"yield",
+      [Pure, Terminator, ParentOneOf<["IfOp", "ForOp"]>]> {
   let summary = "block termination operation";
   let description = [{
     "yield" terminates blocks within EmitC control-flow operations. Since

mlir/lib/Conversion/SCFToEmitC/SCFToEmitC.cpp

@@ -37,7 +37,100 @@ 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();
+    emitc::OpaqueAttr noInit = emitc::OpaqueAttr::get(context, "");
+    emitc::VariableOp 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);
+
+  emitc::ForOp loweredFor = rewriter.create<emitc::ForOp>(
+      loc, forOp.getLowerBound(), forOp.getUpperBound(), forOp.getStep());
+
+  Block *loweredBody = loweredFor.getBody();
+
+  // Erase the auto-generated terminator for the lowered for op.
+  rewriter.eraseOp(loweredBody->getTerminator());
+
+  SmallVector<Value> replacingValues;
+  replacingValues.push_back(loweredFor.getInductionVar());
+  replacingValues.append(iterArgsVariables.begin(), iterArgsVariables.end());
+
+  rewriter.mergeBlocks(forOp.getBody(), loweredBody, replacingValues);
+  lowerYield(iterArgsVariables, rewriter,
+             cast<scf::YieldOp>(loweredBody->getTerminator()));
+
+  // Copy iterArgs into results after the for loop.
+  assignValues(iterArgsVariables, resultVariables, rewriter, loc);
+
+  rewriter.replaceOp(forOp, resultVariables);
+  return success();
+}
+
+// Lower scf::if to emitc::if, implementing result values as emitc::variable's
 // updated within the then and else regions.
 struct IfLowering : public OpRewritePattern<IfOp> {
   using OpRewritePattern<IfOp>::OpRewritePattern;
@@ -52,20 +145,10 @@ LogicalResult IfLowering::matchAndRewrite(IfOp ifOp,
                                           PatternRewriter &rewriter) const {
   Location loc = ifOp.getLoc();
 
-  SmallVector<Value> resultVariables;
-
   // Create an emitc::variable op for each result. These variables will be
   // assigned to by emitc::assign ops within the then & else regions.
-  if (ifOp.getNumResults()) {
-    MLIRContext *context = ifOp.getContext();
-    rewriter.setInsertionPoint(ifOp);
-    for (OpResult result : ifOp.getResults()) {
-      Type resultType = result.getType();
-      auto noInit = emitc::OpaqueAttr::get(context, "");
-      auto var = rewriter.create<emitc::VariableOp>(loc, resultType, noInit);
-      resultVariables.push_back(var);
-    }
-  }
+  SmallVector<Value> resultVariables =
+      createVariablesForResults(ifOp, rewriter);
 
   // Utility function to lower the contents of an scf::if region to an emitc::if
   // region. The contents of the scf::if regions is moved into the respective
@@ -76,16 +159,7 @@ LogicalResult IfLowering::matchAndRewrite(IfOp ifOp,
                                                    Region &loweredRegion) {
     rewriter.inlineRegionBefore(region, loweredRegion, loweredRegion.end());
     Operation *terminator = loweredRegion.back().getTerminator();
-    Location terminatorLoc = terminator->getLoc();
-    ValueRange terminatorOperands = terminator->getOperands();
-    rewriter.setInsertionPointToEnd(&loweredRegion.back());
-    for (auto value2Var : llvm::zip(terminatorOperands, resultVariables)) {
-      Value resultValue = std::get<0>(value2Var);
-      Value resultVar = std::get<1>(value2Var);
-      rewriter.create<emitc::AssignOp>(terminatorLoc, resultVar, resultValue);
-    }
-    rewriter.create<emitc::YieldOp>(terminatorLoc);
-    rewriter.eraseOp(terminator);
+    lowerYield(resultVariables, rewriter, cast<scf::YieldOp>(terminator));
   };
 
   Region &thenRegion = ifOp.getThenRegion();
@@ -109,6 +183,7 @@ LogicalResult IfLowering::matchAndRewrite(IfOp ifOp,
 }
 
 void mlir::populateSCFToEmitCConversionPatterns(RewritePatternSet &patterns) {
+  patterns.add<ForLowering>(patterns.getContext());
   patterns.add<IfLowering>(patterns.getContext());
 }
 
@@ -118,7 +193,7 @@ void SCFToEmitCPass::runOnOperation() {
 
   // Configure conversion to lower out SCF operations.
   ConversionTarget target(getContext());
-  target.addIllegalOp<scf::IfOp>();
+  target.addIllegalOp<scf::ForOp, scf::IfOp>();
   target.markUnknownOpDynamicallyLegal([](Operation *) { return true; });
   if (failed(
           applyPartialConversion(getOperation(), target, std::move(patterns))))

mlir/lib/Dialect/EmitC/IR/EmitC.cpp

@@ -189,6 +189,101 @@ LogicalResult emitc::ConstantOp::verify() {
 
 OpFoldResult emitc::ConstantOp::fold(FoldAdaptor adaptor) { return getValue(); }
 
+//===----------------------------------------------------------------------===//
+// ForOp
+//===----------------------------------------------------------------------===//
+
+void ForOp::build(OpBuilder &builder, OperationState &result, Value lb,
+                  Value ub, Value step, BodyBuilderFn bodyBuilder) {
+  result.addOperands({lb, ub, step});
+  Type t = lb.getType();
+  Region *bodyRegion = result.addRegion();
+  bodyRegion->push_back(new Block);
+  Block &bodyBlock = bodyRegion->front();
+  bodyBlock.addArgument(t, result.location);
+
+  // Create the default terminator if the builder is not provided.
+  if (!bodyBuilder) {
+    ForOp::ensureTerminator(*bodyRegion, builder, result.location);
+  } else {
+    OpBuilder::InsertionGuard guard(builder);
+    builder.setInsertionPointToStart(&bodyBlock);
+    bodyBuilder(builder, result.location, bodyBlock.getArgument(0));
+  }
+}
+
+void ForOp::getCanonicalizationPatterns(RewritePatternSet &, MLIRContext *) {}
+
+ParseResult ForOp::parse(OpAsmParser &parser, OperationState &result) {
+  Builder &builder = parser.getBuilder();
+  Type type;
+
+  OpAsmParser::Argument inductionVariable;
+  OpAsmParser::UnresolvedOperand lb, ub, step;
+
+  // Parse the induction variable followed by '='.
+  if (parser.parseOperand(inductionVariable.ssaName) || parser.parseEqual() ||
+      // Parse loop bounds.
+      parser.parseOperand(lb) || parser.parseKeyword("to") ||
+      parser.parseOperand(ub) || parser.parseKeyword("step") ||
+      parser.parseOperand(step))
+    return failure();
+
+  // Parse the optional initial iteration arguments.
+  SmallVector<OpAsmParser::Argument, 4> regionArgs;
+  SmallVector<OpAsmParser::UnresolvedOperand, 4> operands;
+  regionArgs.push_back(inductionVariable);
+
+  // Parse optional type, else assume Index.
+  if (parser.parseOptionalColon())
+    type = builder.getIndexType();
+  else if (parser.parseType(type))
+    return failure();
+
+  // Resolve input operands.
+  regionArgs.front().type = type;
+  if (parser.resolveOperand(lb, type, result.operands) ||
+      parser.resolveOperand(ub, type, result.operands) ||
+      parser.resolveOperand(step, type, result.operands))
+    return failure();
+
+  // Parse the body region.
+  Region *body = result.addRegion();
+  if (parser.parseRegion(*body, regionArgs))
+    return failure();
+
+  ForOp::ensureTerminator(*body, builder, result.location);
+
+  // Parse the optional attribute list.
+  if (parser.parseOptionalAttrDict(result.attributes))
+    return failure();
+
+  return success();
+}
+
+void ForOp::print(OpAsmPrinter &p) {
+  p << " " << getInductionVar() << " = " << getLowerBound() << " to "
+    << getUpperBound() << " step " << getStep();
+
+  p << ' ';
+  if (Type t = getInductionVar().getType(); !t.isIndex())
+    p << " : " << t << ' ';
+  p.printRegion(getRegion(),
+                /*printEntryBlockArgs=*/false,
+                /*printBlockTerminators=*/false);
+  p.printOptionalAttrDict((*this)->getAttrs());
+}
+
+LogicalResult ForOp::verifyRegions() {
+  // Check that the body defines as single block argument for the induction
+  // variable.
+  if (getInductionVar().getType() != getLowerBound().getType())
+    return emitOpError(
+        "expected induction variable to be same type as bounds and step");
+
+  return success();
+}
+
 //===----------------------------------------------------------------------===//
 // IfOp
 //===----------------------------------------------------------------------===//