Add -Wbitwise-conditional-parentheses to warn on mixing '|' and '&' with "?:"

Extend -Wparentheses to cover mixing bitwise-and and bitwise-or with the
conditional operator. There's two main cases seen with this:

unsigned bits1 = 0xf0 | cond ? 0x4 : 0x1;
unsigned bits2 = cond1 ? 0xf0 : 0x10 | cond2 ? 0x5 : 0x2;

// Intended order of evaluation:
unsigned bits1 = 0xf0 | (cond ? 0x4 : 0x1);
unsigned bits2 = (cond1 ? 0xf0 : 0x10) | (cond2 ? 0x5 : 0x2);

// Actual order of evaluation:
unsigned bits1 = (0xf0 | cond) ? 0x4 : 0x1;
unsigned bits2 = cond1 ? 0xf0 : ((0x10 | cond2) ? 0x5 : 0x2);

Differential Revision: https://reviews.llvm.org/D66043



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375326 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -61,6 +61,9 @@ Improvements to Clang's diagnostics
   operation and a constant.  The group also has the new warning which diagnoses
   when a bitwise-or with a non-negative value is converted to a bool, since
   that bool will always be true.
+- -Wbitwise-conditional-parentheses will warn on operator precedence issues
+  when mixing bitwise-and (&) and bitwise-or (|) operator with the
+  conditional operator (?:).
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

include/clang/Basic/DiagnosticGroups.td

@@ -296,6 +296,7 @@ def ExitTimeDestructors : DiagGroup<"exit-time-destructors">;
 def FlexibleArrayExtensions : DiagGroup<"flexible-array-extensions">;
 def FourByteMultiChar : DiagGroup<"four-char-constants">;
 def GlobalConstructors : DiagGroup<"global-constructors">;
+def BitwiseConditionalParentheses: DiagGroup<"bitwise-conditional-parentheses">;
 def BitwiseOpParentheses: DiagGroup<"bitwise-op-parentheses">;
 def LogicalOpParentheses: DiagGroup<"logical-op-parentheses">;
 def LogicalNotParentheses: DiagGroup<"logical-not-parentheses">;
@@ -737,6 +738,7 @@ def ParenthesesOnEquality : DiagGroup<"parentheses-equality">;
 def Parentheses : DiagGroup<"parentheses",
                             [LogicalOpParentheses,
                              LogicalNotParentheses,
+                             BitwiseConditionalParentheses,
                              BitwiseOpParentheses,
                              ShiftOpParentheses,
                              OverloadedShiftOpParentheses,

include/clang/Basic/DiagnosticSemaKinds.td

@@ -5745,6 +5745,9 @@ def note_precedence_silence : Note<
 def warn_precedence_conditional : Warning<
   "operator '?:' has lower precedence than '%0'; '%0' will be evaluated first">,
   InGroup<Parentheses>;
+def warn_precedence_bitwise_conditional : Warning<
+  "operator '?:' has lower precedence than '%0'; '%0' will be evaluated first">,
+  InGroup<BitwiseConditionalParentheses>;
 def note_precedence_conditional_first : Note<
   "place parentheses around the '?:' expression to evaluate it first">;
 

lib/Sema/SemaExpr.cpp

@@ -7620,7 +7620,12 @@ static void SuggestParentheses(Sema &Self, SourceLocation Loc,
 static bool IsArithmeticOp(BinaryOperatorKind Opc) {
   return BinaryOperator::isAdditiveOp(Opc) ||
          BinaryOperator::isMultiplicativeOp(Opc) ||
-         BinaryOperator::isShiftOp(Opc);
+         BinaryOperator::isShiftOp(Opc) || Opc == BO_And || Opc == BO_Or;
+  // This only checks for bitwise-or and bitwise-and, but not bitwise-xor and
+  // not any of the logical operators.  Bitwise-xor is commonly used as a
+  // logical-xor because there is no logical-xor operator.  The logical
+  // operators, including uses of xor, have a high false positive rate for
+  // precedence warnings.
 }
 
 /// IsArithmeticBinaryExpr - Returns true if E is an arithmetic binary
@@ -7710,7 +7715,11 @@ static void DiagnoseConditionalPrecedence(Sema &Self,
   // The condition is an arithmetic binary expression, with a right-
   // hand side that looks boolean, so warn.
 
-  Self.Diag(OpLoc, diag::warn_precedence_conditional)
+  unsigned DiagID = BinaryOperator::isBitwiseOp(CondOpcode)
+                        ? diag::warn_precedence_bitwise_conditional
+                        : diag::warn_precedence_conditional;
+
+  Self.Diag(OpLoc, DiagID)
       << Condition->getSourceRange()
       << BinaryOperator::getOpcodeStr(CondOpcode);
 

test/Sema/parentheses.c

@@ -93,6 +93,28 @@ void conditional_op(int x, int y, _Bool b, void* p) {
 
   (void)(x + y > 0 ? 1 : 2); // no warning
   (void)(x + (y > 0) ? 1 : 2); // expected-warning {{operator '?:' has lower precedence than '+'}} expected-note 2{{place parentheses}}
+
+  (void)(b ? 0xf0 : 0x10 | b ? 0x5 : 0x2); // expected-warning {{operator '?:' has lower precedence than '|'}} expected-note 2{{place parentheses}}
+
+  (void)((b ? 0xf0 : 0x10) | (b ? 0x5 : 0x2));  // no warning, has parentheses
+  (void)(b ? 0xf0 : (0x10 | b) ? 0x5 : 0x2);  // no warning, has parentheses
+
+  (void)(x | b ? 1 : 2); // expected-warning {{operator '?:' has lower precedence than '|'}} expected-note 2{{place parentheses}}
+  (void)(x & b ? 1 : 2); // expected-warning {{operator '?:' has lower precedence than '&'}} expected-note 2{{place parentheses}}
+
+  (void)((x | b) ? 1 : 2);  // no warning, has parentheses
+  (void)(x | (b ? 1 : 2));  // no warning, has parentheses
+  (void)((x & b) ? 1 : 2);  // no warning, has parentheses
+  (void)(x & (b ? 1 : 2));  // no warning, has parentheses
+
+  // Only warn on uses of the bitwise operators, and not the logical operators.
+  // The bitwise operators are more likely to be bugs while the logical
+  // operators are more likely to be used correctly.  Since there is no
+  // explicit logical-xor operator, the bitwise-xor is commonly used instead.
+  // For this warning, treat the bitwise-xor as if it were a logical operator.
+  (void)(x ^ b ? 1 : 2);  // no warning, ^ is often used as logical xor
+  (void)(x || b ? 1 : 2);  // no warning, logical operator
+  (void)(x && b ? 1 : 2);  // no warning, logical operator
 }
 
 // RUN: not %clang_cc1 -fsyntax-only -Wparentheses -Werror -fdiagnostics-show-option %s 2>&1 | FileCheck %s -check-prefix=CHECK-FLAG