[mlir][emitc] Deferred emission as expressions

The translator currently implements deferred emission for certain ops.
Like expressions, these ops are emitted as part of their users but
unlike expressions, this is mandatory. Besides complicating the code
with a second inlining mechanism, deferred emission's inlining is
limited as it's not recursive.

This patch extends EmitC's expressions to deferred emission ops by (a)
marking them as CExpressions, (b) extending expression interface to
mark ops as always-inline and (c) support inlining of always-inline
CExpressions even when not packed of an `emitc.expression` op,
retaining current behavior.

Author: aniragil

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

@@ -507,7 +507,7 @@ def EmitC_ExpressionOp
 
   let arguments = (ins Variadic<AnyTypeOf<[EmitCType, EmitC_LValueType]>>:$defs,
       UnitAttr:$do_not_inline);
-  let results = (outs EmitCType:$result);
+  let results = (outs AnyTypeOf<[EmitCType, EmitC_LValueType]>:$result);
   let regions = (region SizedRegion<1>:$region);
 
   let hasVerifier = 1;
@@ -873,7 +873,7 @@ def EmitC_IncludeOp
   let hasCustomAssemblyFormat = 1;
 }
 
-def EmitC_LiteralOp : EmitC_Op<"literal", [Pure]> {
+def EmitC_LiteralOp : EmitC_Op<"literal", [Pure, CExpressionInterface]> {
   let summary = "Literal operation";
   let description = [{
     The `emitc.literal` operation produces an SSA value equal to some constant
@@ -896,6 +896,15 @@ def EmitC_LiteralOp : EmitC_Op<"literal", [Pure]> {
 
   let hasVerifier = 1;
   let assemblyFormat = "$value attr-dict `:` type($result)";
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
 def EmitC_LogicalAndOp : EmitC_BinaryOp<"logical_and", []> {
@@ -1062,7 +1071,7 @@ def EmitC_SubOp : EmitC_BinaryOp<"sub", []> {
   let hasVerifier = 1;
 }
 
-def EmitC_MemberOp : EmitC_Op<"member"> {
+def EmitC_MemberOp : EmitC_Op<"member", [CExpressionInterface]> {
   let summary = "Member operation";
   let description = [{
     With the `emitc.member` operation the member access operator `.` can be
@@ -1083,9 +1092,18 @@ def EmitC_MemberOp : EmitC_Op<"member"> {
     EmitC_LValueOf<[EmitC_OpaqueType]>:$operand
   );
   let results = (outs AnyTypeOf<[EmitC_ArrayType, EmitC_LValueType]>);
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
-def EmitC_MemberOfPtrOp : EmitC_Op<"member_of_ptr"> {
+def EmitC_MemberOfPtrOp : EmitC_Op<"member_of_ptr", [CExpressionInterface]> {
   let summary = "Member of pointer operation";
   let description = [{
     With the `emitc.member_of_ptr` operation the member access operator `->`
@@ -1108,6 +1126,15 @@ def EmitC_MemberOfPtrOp : EmitC_Op<"member_of_ptr"> {
     EmitC_LValueOf<[EmitC_OpaqueType,EmitC_PointerType]>:$operand
   );
   let results = (outs AnyTypeOf<[EmitC_ArrayType, EmitC_LValueType]>);
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
 def EmitC_ConditionalOp : EmitC_Op<"conditional",
@@ -1277,8 +1304,10 @@ def EmitC_GlobalOp : EmitC_Op<"global", [Symbol]> {
   let hasVerifier = 1;
 }
 
-def EmitC_GetGlobalOp : EmitC_Op<"get_global",
-    [Pure, DeclareOpInterfaceMethods<SymbolUserOpInterface>]> {
+def EmitC_GetGlobalOp
+    : EmitC_Op<"get_global", [Pure,
+                              DeclareOpInterfaceMethods<SymbolUserOpInterface>,
+                              CExpressionInterface]> {
   let summary = "Obtain access to a global variable";
   let description = [{
      The `emitc.get_global` operation retrieves the lvalue of a
@@ -1296,6 +1325,15 @@ def EmitC_GetGlobalOp : EmitC_Op<"get_global",
   let arguments = (ins FlatSymbolRefAttr:$name);
   let results = (outs AnyTypeOf<[EmitC_ArrayType, EmitC_LValueType]>:$result);
   let assemblyFormat = "$name `:` type($result) attr-dict";
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
 def EmitC_VerbatimOp : EmitC_Op<"verbatim"> {
@@ -1406,7 +1444,8 @@ def EmitC_YieldOp : EmitC_Op<"yield",
     value is yielded.
   }];
 
-  let arguments = (ins Optional<EmitCType>:$result);
+  let arguments =
+      (ins Optional<AnyTypeOf<[EmitCType, EmitC_LValueType]>>:$result);
   let builders = [OpBuilder<(ins), [{ /* nothing to do */ }]>];
 
   let hasVerifier = 1;
@@ -1477,7 +1516,7 @@ def EmitC_IfOp : EmitC_Op<"if",
   let hasCustomAssemblyFormat = 1;
 }
 
-def EmitC_SubscriptOp : EmitC_Op<"subscript", []> {
+def EmitC_SubscriptOp : EmitC_Op<"subscript", [CExpressionInterface]> {
   let summary = "Subscript operation";
   let description = [{
     With the `emitc.subscript` operation the subscript operator `[]` can be applied
@@ -1525,6 +1564,15 @@ def EmitC_SubscriptOp : EmitC_Op<"subscript", []> {
 
   let hasVerifier = 1;
   let assemblyFormat = "$value `[` $indices `]` attr-dict `:` functional-type(operands, results)";
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
 def EmitC_SwitchOp : EmitC_Op<"switch", [RecursiveMemoryEffects,
@@ -1707,8 +1755,9 @@ def EmitC_FieldOp : EmitC_Op<"field", [Symbol]> {
 }
 
 def EmitC_GetFieldOp
-    : EmitC_Op<"get_field", [Pure, DeclareOpInterfaceMethods<
-                                       SymbolUserOpInterface>]> {
+    : EmitC_Op<"get_field", [Pure,
+                             DeclareOpInterfaceMethods<SymbolUserOpInterface>,
+                             CExpressionInterface]> {
   let summary = "Obtain access to a field within a class instance";
   let description = [{
      The `emitc.get_field` operation retrieves the lvalue of a
@@ -1725,6 +1774,15 @@ def EmitC_GetFieldOp
   let results = (outs EmitCType:$result);
   let assemblyFormat = "$field_name `:` type($result) attr-dict";
   let hasVerifier = 1;
+
+  let extraClassDeclaration = [{
+    bool hasSideEffects() {
+      return false;
+    }
+    bool alwaysInline() {
+      return true; // C doesn't support variable references.
+    }
+  }];
 }
 
 #endif // MLIR_DIALECT_EMITC_IR_EMITC

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

@@ -21,8 +21,8 @@ def CExpressionInterface : OpInterface<"CExpressionInterface"> {
   }];
 
   let cppNamespace = "::mlir::emitc";
-  let methods = [
-    InterfaceMethod<[{
+  let methods =
+      [InterfaceMethod<[{
       Check whether operation has side effects that may affect the expression
       evaluation.
 
@@ -38,9 +38,28 @@ def CExpressionInterface : OpInterface<"CExpressionInterface"> {
       };
       ```
     }],
-      "bool", "hasSideEffects", (ins), /*methodBody=*/[{}],
-       /*defaultImplementation=*/[{
+                       "bool", "hasSideEffects", (ins), /*methodBody=*/[{}],
+                       /*defaultImplementation=*/[{
         return true;
+    }]>,
+       InterfaceMethod<[{
+      Check whether operation must be inlined into all its users.
+
+      By default operation is not marked as always inlined.
+
+      ```c++
+      class ConcreteOp ... {
+      public:
+        bool alwaysInline() {
+          // That way we can override the default implementation.
+          return true;
+        }
+      };
+      ```
+    }],
+                       "bool", "alwaysInline", (ins), /*methodBody=*/[{}],
+                       /*defaultImplementation=*/[{
+        return false;
     }]>,
   ];
 }

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

@@ -483,7 +483,8 @@ LogicalResult ExpressionOp::verify() {
     Operation *op = worklist.back();
     worklist.pop_back();
     if (visited.contains(op)) {
-      if (cast<CExpressionInterface>(op).hasSideEffects())
+      auto cExpr = cast<CExpressionInterface>(op);
+      if (!cExpr.alwaysInline() && cExpr.hasSideEffects())
         return emitOpError(
             "requires exactly one use for operations with side effects");
     }
@@ -494,6 +495,12 @@ LogicalResult ExpressionOp::verify() {
       }
   }
 
+  if (getDoNotInline() &&
+      cast<emitc::CExpressionInterface>(rootOp).alwaysInline()) {
+    return emitOpError("root operation must be inlined but expression is marked"
+                       " do-not-inline");
+  }
+
   return success();
 }
 
@@ -980,6 +987,10 @@ LogicalResult emitc::YieldOp::verify() {
   if (!result && containingOp->getNumResults() != 0)
     return emitOpError() << "does not yield a value to be returned by parent";
 
+  if (result && isa<emitc::LValueType>(result.getType()) &&
+      !isa<ExpressionOp>(containingOp))
+    return emitOpError() << "yielding lvalues is not supported for this op";
+
   return success();
 }