[mlir][emitc] Refactor brackets in expressions (llvm#168267)

This patch is a minor NFC-intended refactoring to the way emitting
redundant parentheses is prevented.
The current implementation pushes and later pops a fake low precedence
into the precedence stack when emitting function calls. The new
implementation adds a boolean argument to `emitOperand()` that explicity
guarantees that the operand is being emitted between some kind of
brackets, exempting the method from enforcing correct evaluation order
w.r.t precedence and associativity up the expression tree.

Author: aniragil

mlir/lib/Target/Cpp/TranslateToCpp.cpp

@@ -173,8 +173,11 @@ struct CppEmitter {
   /// Emits the operands of the operation. All operands are emitted in order.
   LogicalResult emitOperands(Operation &op);
 
-  /// Emits value as an operands of an operation
-  LogicalResult emitOperand(Value value);
+  /// Emits value as an operand of some operation. Unless \p isInBrackets is
+  /// true, operands emitted as sub-expressions will be parenthesized if needed
+  /// in order to enforce correct evaluation based on precedence and
+  /// associativity.
+  LogicalResult emitOperand(Value value, bool isInBrackets = false);
 
   /// Emit an expression as a C expression.
   LogicalResult emitExpression(ExpressionOp expressionOp);
@@ -1578,18 +1581,20 @@ LogicalResult CppEmitter::emitExpression(ExpressionOp expressionOp) {
   return success();
 }
 
-LogicalResult CppEmitter::emitOperand(Value value) {
+LogicalResult CppEmitter::emitOperand(Value value, bool isInBrackets) {
   if (isPartOfCurrentExpression(value)) {
     Operation *def = value.getDefiningOp();
     assert(def && "Expected operand to be defined by an operation");
     FailureOr<int> precedence = getOperatorPrecedence(def);
     if (failed(precedence))
       return failure();
 
-    // Sub-expressions with equal or lower precedence need to be parenthesized,
-    // as they might be evaluated in the wrong order depending on the shape of
-    // the expression tree.
-    bool encloseInParenthesis = precedence.value() <= getExpressionPrecedence();
+    // Unless already in brackets, sub-expressions with equal or lower
+    // precedence need to be parenthesized as they might be evaluated in the
+    // wrong order depending on the shape of the expression tree.
+    bool encloseInParenthesis =
+        !isInBrackets && precedence.value() <= getExpressionPrecedence();
+
     if (encloseInParenthesis)
       os << "(";
     pushExpressionPrecedence(precedence.value());
@@ -1628,15 +1633,9 @@ LogicalResult CppEmitter::emitOperand(Value value) {
 
 LogicalResult CppEmitter::emitOperands(Operation &op) {
   return interleaveCommaWithError(op.getOperands(), os, [&](Value operand) {
-    // If an expression is being emitted, push lowest precedence as these
-    // operands are either wrapped by parenthesis.
-    if (getEmittedExpression())
-      pushExpressionPrecedence(lowestPrecedence());
-    if (failed(emitOperand(operand)))
-      return failure();
-    if (getEmittedExpression())
-      popExpressionPrecedence();
-    return success();
+    // Emit operand under guarantee that if it's part of an expression then it
+    // is being emitted within brackets.
+    return emitOperand(operand, /*isInBrackets=*/true);
   });
 }